Accoustics tech notes 2014

Course materialfor ARC 6.6 Under the
Visveswaraya Technological University, Belgaum
Dayanand Sagar Acadamy of Technology & Management.
Udayapura, Bangalore 560 082
Architecture Acoustics
By Prof .K.S. Mukunda
Dean School of Architecture
1

14ARC 6.3 - BUILDING SERVICES - IV (ACOUSTICS AND NOISE CONTROL)
OBJECTIVE: To develop the knowledge and skills required for understanding acoustics in buildings and its integration
with architectural design.
OUTLINE
1. Introduction to the study of acoustics: Nature of Sound, basic terminology, decibel scale, threshold of audibility and pain,
masking, sound and distance, inverse square law.
2. Introduction to Room Acoustics: History of Greek, Roman theatres. Reflection, Diffusion, Diffraction, reverberation,
Absorption. Calculation of reverberation time using Sabine's and Eyring's formulae.
3. Room Acoustics defects and measurement techniques: Echoes, focusing of sound, dead spots, flutter echo. Room
resonances, small enclosures, room modes, standing waves.
4. Rooms for speech and music: Effect of RT and SNR on speech and music, AI, STI, RASTI, Speech intelligibility. Sound
reinforcement systems and background noise masking systems.
5. Acoustical Design recommendations: Halls for speech and music. Raked Seating, Use of IS code 2526 - 1963. Home
theatres, recording studios, open plan offices, speech privacy issues and sound attenuation.
6. Acoustical Materials and Corrections: Absorptive materials - NRC value, porous materials, panel absorbers, membrane
absorbers, diffusers, cavity or Helmholtz resonators. Adjustable acoustics and variable sound absorbers. Acoustical correction and
retrofits to existing spaces.
7. Design and Detailing for Acoustics of Multipurpose halls (Site visit and studio component): Case studies of acoustically
designed and treated multipurpose halls. Design of a multipurpose hall for optimum acoustics - drawings and construction details
of acoustical treatment.
8. Introduction to environmental noise control - Types of noise - indoor, outdoor noise, airborne and structure borne noise,
noise transmission, Mass Law, Transmission loss. Noise from ventilating systems.
9. Means of noise control in buildings - Maximum acceptable noise levels, Enclosures, Barriers, Sound insulation, STC ratings,
Sound Isolation. Noise measurement using SLM. (Sound level meter)
10. Constructional measures of noise control (studio component) - Construction details of composite walls, double walls,
floating floors, wood-joist floors, plenum barriers, sound locks, etc.
11. Industrial Noise - Sources of industrial noise - impact, friction, reciprocation, air turbulence and other noise. Methods of
reduction using enclosures and barriers -Case study of industrial buildings.
12. Introduction to Urban Soundscape - Introduction to Urban noise climate, Noise sources - Air traffic, Rail traffic, Road
traffic, Seashore and inland. Traffic planning against outdoor noise. Role of architects in shaping the urban sound scape.
Sustainable design strategies in building acoustics.
Note - Site visits to be arranged by studio in-charge teachers. Case study of multipurpose halls along with the measurement
details, study inferences, acoustical designs for a multipurpose hall with construction drawings and details to be produced in the
form of portfolio and a minimum of four plates to be produced for constructional measures of noise control in buildings.
REFERENCES:
1. "Architectural Acoustics" by M.David Egan 2. "Environmental Acoustics" by Leslie L. Doelle 3. "Acoustical Designing in
Architecture" by Vern O.Knudsen and Cyril M.Harris 4."Acoustics, noise and buildings" by Peter H. Parkins and H. R. Humphreys
5. "Master Handbook of Acoustics" by F.Alton Everest and Ken C. Pohlmann
2

Some Important definitions in Acoustics
Sound it is a disturbance of energy that comes through matter as a sine wave, it moves at a speed of 1100 ft per second. the speed of
sound in air is determined by the conditions of the air itself (e.g. humidity, temperature, altitude).
pitch The pitch of a sound is generally thought of as the 'highness' or 'lowness' of a sound. Together with amplitude, duration, and tone color, pitch is one
of the four basic elements of all musical sounds . it’s determined by the rate of vibration, or frequency, of the sound wave.
Frequency /Oscillations /hertz The sound fluctuation of these waves, called oscillations, can be measured by the number of wave cycles per second. referred to as the frequency of
the sound. Frequency is quantified using a unit of measurement known as hertz (abbreviated Hz), which defines the number of repeating cycles per
second
Voice The distinctive quality, pitch or condition of a person's speech produced.
Voice Box / larynx A cartilaginous structure at the top of the trachea; which contains elastic vocal cords that are the source of the vocal tone in speech produced in all
humans located in the upper portion of the wind pipe of the throat.
Audible sound The human ear can recognize the sounds of frequencies in the range of 20 Hz to 20,000 Hz.
Infrasonic sounds Sounds of frequencies less than 20 Hz are called infrasonic sounds.
Ultrasonic sounds The sounds of frequencies greater than 20,000 Hz are called ultrasonic sounds.
Amplitude of sound height of the sound wave.( our perception of loudness ) is influenced by both the frequency and timbre of a sound
Tone / colour/Timber of
sound
The unique sound or tone color produced by every instrument and voice is known as it's timbre. It is also referred to an instrument's color
Decibel The system used to measure the loudness of sounds, given the unit dB named after Alexander Graham Bell, the inventor of the telephone, the
decibel became a standard . It is also defined as the signal to the noise ratio, (SNR )often expressed in decibels
Echo In audio signal processing and acoustics, an echo (plural echoes) is a reflection of sound, arriving at the listener some time after the direct sound .
The human ear cannot distinguish an echo from the original sound if the delay is less than 1/15 of a second and it’s heard as a reverberating sound.
Sabin A unit of acoustic absorption equivalent to the absorption by a square foot of a surface that absorbs all incident sound
Hi-Fi sound High fidelity sound -Accuracy of the sound or image of its input electronic signal ( see also Musical sounds Hi-fi, Lo-fi, No-fi)
Phon A unit of subjective loudness of pure tones. the number of phon of a sound is the dB SPL of a sound at a frequency of 1 kHz that sounds just as loud.
This implies that 0 phon is the limit of perception, and inaudible sounds have negative phon levels
Phone is an instrument for Electro-acoustic transducer for converting electric signals into sounds
Octave (Latin: octavus: eighth) or perfect octave is the interval between one musical pitch and another with half or double its frequency , It is an most
important musical scale & referred to as the "basic miracle of music. The most important musical scales are typically written using eight notes, and the
interval between the first and last notes is an octave
Wi-fi A local area network that uses high frequency radio signals to transmit and receive data over distances of a few hundred feet; uses ethernet
(wireless ) protocol
3

Human Hearing Organ
Architectural Acoustics also known as Room Acoustics / Building Acoustics , it is the science of achieving a good sound condition for speech intelligibility in lecture halls,
restaurant or for enhancing the quality of music in a concert hall or for suppressing the noise in offices / houses in our living environments through Building skin
envelops like Roof, Windows , doors, etc,. Based on sound absorption & Sound Reflection properties.
Sound originates when a vibrating body pushes and
pulls on the air particles about it, thus generating
compressions and rarefactions that travel out in the
surrounding air with a large velocity of about 1100
feet per second. These waves enter the ear canal and
push and pull on the ear drum, which transmits the
motion through an effective arrangement of three
small hones to the inner ear, where the mechanical
energy is transformed into nervous energy and the
person hears.
4

In humans, sound is produced by the voice box or the larynx, which is present in the
upper portion of the wind pipe of the throat.
Sound travels as a sign wave. It needs a material medium to travel. Sound travels
through gases, liquids and solids.
The speed of sound is the maximum in solids, less in liquids and the least in gases.
Sound cannot travel through vacuum.
Attenuation of Sound Waves. When sound travels through a medium, its intensity
diminishes with distance The combined effect of scattering and absorption is called
attenuation .
Students in today’s classrooms are unable to understand 25 to 30 percent of what
their teacher said because of excessive noise and reverberation.
The Physics of SOUND is called ACCOUSTICS
There are three major components of acoustics: ambient noise, reverberation, and
the signal to noise ratio. (SNR)
Acoustic considerations in work spaces: Studies show that acoustics are an essential
consideration when designing an office for optimum performance. The work place
should provide occupants freedom from distracting noise, and enable them to work
without distracting others. Undesirable noises affects concentration, the ability to
think clearly, the ability to communicate effectively, and avoid increases error rates. It
has been estimated that productivity can increase by as much as 26% if noise is
controlled.
There has been a century of technical development Since Mr Sabine developed the
knowledge, and Acoustics has developed from an Art to a Predictable science.
African dance and music evolved a highly complex rhythmic character because it was
mostly performed outdoors. Early European music was more melodic because tribes
sought shelter in caves and later constructed increasingly large and reverberant
temples and churches. Chant grew out of the acoustical characteristics of these
cathedrals and baroque music was written to accommodate the greatly designed
churches of the time.
The definition of sound says that it is a disturbance of
energy that comes through matter as a wave, and that
humans perceive sound by the sense of hearing. It is
important to note that the speed of sound in air is
determined by the conditions of the air itself (e.g.
humidity, temperature, altitude). It is not dependent upon
the sound’s amplitude, frequency or wavelength. SOUND
moves at a speed of 1100 ft per second.
Introduction to Sound
Audible sound
Not all sound produced by vibrating bodies is audible. The human ear
can recognize the sounds of frequencies in the range of 20 Hz to
20,000 Hz. This range of frequency of sound is called audible sound.
Some animals like dogs and snakes can hear sounds of frequencies
greater than 20,000 Hz.
The unique sound or tone color produced by every instrument and
voice is known as it's timbre. It is also referred to an instrument's
color
Infrasonic sounds
Sounds of frequencies less than 20 Hz are called infrasonic sounds.
Ultrasonic sounds
The sounds of frequencies greater than 20,000 Hz are called
ultrasonic sounds.
Wave length- the distance between the layers of
compression is called Wave length. It obviously depends on
the frequency of sound. If one cycle occurs in 1/100 of a
second, then wave length =speed of sound/100 = 3.4
meters (speed of sound is340 m/sec)
5

Basic of sound
Any sound that misses your intended audience is noise. Only the Audio Spotlight system can keep sound focused specifically to your listeners, providing sound
where you want it, and quiet everywhere else. The revolutionary Audio Spotlight technology creates a tight, narrow beam of sound that can be controlled with
the same precision as light. Since the Yr 2000, Audio Spotlight systems have been installed in thousands of locations around the world to provide high-quality,
precisely targeted sound.
Frequency
It is determined by how fast the sound producing objects vibrates.
Frequency is the number of waves that move past a point in one second. In the diagram, if
the sound waves move from the speaker, through the air, and into the ear in one second,
what is the frequency of the sound? Frequency = 6 waves per second
Frequency is quantified using a unit of measurement known as hertz (abbreviated Hz),
which defines the number of repeating cycles per second. If one cycle occurs in 1/100 of a
second, then wave length =speed of sound/100 = 3.4 meters (speed of sound is 340
m/sec)
PITCH
Pitch is basically your ears’ response to the frequency of a sound.
Pitch is how high or low a sound is.
Pitch depends on the frequency of a sound
Loudness
The loudness of a wave depends on its energy. The greater the energy the louder the
sound. The greater the energy the greater the amplitude (height) of the sound wave. our
perception of loudness is influenced by both the frequency and timbre of a sound
Tone of sound: refers to the quality of a person's voice with varying pitch
We can say---"he began in a conversational tone"; "he spoke in a nervous tone of voice“.
Phons; A unit for expressing sound pressure to measure the standards of Loudness (
like Decibels) A phon is expressed in terms of Dynes/sq.cm
THE EAR IS LESS SENSITIVE TO LOWER & HIGHER FREEQUENCIES THAN AT MEDIUM FREEQUENCIES ONLY AT HIGH
SOUND PRESSURE THE EAR BECOMES SENSITIVE TO SOUNDS OF ALL FREEQUENCIES
In the above given diagram:
Wave A and B have the same frequency, but A is
louder.
Waves A and B have the same pitch.
Waves C and D have the same frequency, but C is
louder.
Waves C and D have the same pitch
Human & instrumental sounds
The QUALITY of a sound depends on the complexity of its sound waves, such as
the waves shown in Resultant tone C of ( adjacent picture). Almost all sounds
(musical and vocal
6

Quality of sound
Characteristics of Sound
• A sound can be characterized by the following three quantities:
• (i) Pitch.
• (ii) Quality.
• (iii) Loudness.
• Pitch is the frequency of a sound as perceived by human ear. A high frequency gives rise to a high pitch note and a low
frequency produces a low pitch note. Figure 2 shows the frequencies of same common sounds
• A pure tone is the sound of only one frequency, such as that given by a tuning fork or electronic signal generator.
• Musical Tone is a combination of many free tones. An integral multiple of these frequencies . The fundamental note has
the greatest amplitude and is heard predominantly because it has a larger intensity. The other frequencies such as 2fo,
3fo, 4fo, ............. are called overtones or harmonics and they determine the quality of the sound.
• Loudness is a physiological sensation. It depends mainly on sound pressure but also on the spectrum of the harmonics
and the physical duration.
The shape of a wave is directly related to its spectral content, or the particular frequencies, amplitudes and
phases of its components. Spectral content is the primary factor in our perception of timbre or tone color. We are
familiar with the fact that white light, when properly refracted, can be broken down into component colors, as in
the rainbow. So too with a complex sound wave, which is the composite shape of multiple frequencies
7

• Since a sound wave consists of a repeating pattern of high-pressure and low-pressure regions moving through a medium, it is sometimes referred to
as a pressure wave Sound waves are nothing more than pressure waves that enable the air and our eardrums to get in motion and let our
eardrums and microphones vibrate. That is the sound we hear. Sound pressure is expressed in logarithmic scale, this is the basis of decibel scale.
Engineers should consider especially the sound pressure and its effect.
Sound pressure and Sound power – Effect and Cause A sound source radiates power P and this result in a sound pressure p. Sound power is the
cause – Sound pressure is the effect. An electric heater radiates heat into a room and temperature is the effect.
Sound power is the distance independent ,Where as cause of this effect, sound pressure is the distance-dependent
Do not use the expression "intensity of sound pressure". Intensity is really not sound pressure.
Compare: Sound pressure, sound pressure level, SPL, sound intensity, sound intensity level.
How much is a twice (double, half) or three times louder sound? Sound? Which sound?
"Damping of sound levels with distance“
In a direct field or free field, the sound Preasure level (SPL) of a spherical wave decreases with doubling of the distance by (−)6 dB.
• Loudnessis as a psychological correlate of physical strength (amplitude) is also affected by parameters other than sound pressure, including
frequency, bandwidth and duration
• Children’s cognitive auditory capabilities are not fully developed until age fifteen
Sound Level L and the Distance
Distance-related decrease of sound level
The physics of sound (acoustics) is often confused with the way in which we perceive it (psychoacoustics). begins with a study of sound’s physical characteristics and
common measurements, followed by a discussion of human aural and musical perception. MIDI stands for Musical Instrument Digital Interface. The development of the
MIDI system has been a major catalyst in the recent unprecedented explosion of music technology in 1982 underlying mechanisms for converting real-world sound
into digital values,
Because of the longitudinal motion of the air particles, there are regions in the air
where the air particles are compressed together and other regions where the air
particles are spread apart. These regions are known as compressions and
rarefactions respectively
Since the particles of the medium vibrate in a longitudinal
fashion, compressions and rarefactions are created. Study
the tuning fork animation provided on the Tutorial page.
8

Properties of sound
We will look in detail at three fundamental characteristics of sound: speed,
frequency, and loudness
• The speed of sound in air actually depends on the temperature of the air. 340m/s at
15°C
• Most often we will be looking at sound waves that humans can actually hear, which
are frequencies from 20 – 20 000 Hz.
• Check out the specifications for headphones printed on the back of the package.
They’ll probably list their range from 20 – 20 000Hz, since that’s what the average
person can hear. (Threshold of hearing)
• The fluctuation of these waves, called oscillations, can be measured by the number of
wave cycles per second. It is this measurement that is referred to as the frequency of
the sound . Frequency is quantified using a unit of measurement known
as hertz (abbreviated Hz), which defines the number of repeating cycles per second.
For example, if an event happens once per second, it will have a hertz number of 1.
Therefore, the faster the oscillations of the sound waves, the higher the hertz number,
and the higher the pitch.
– 20 Hz would be very deep, low, rumbling sounds.
– 20 000 Hz would be a very high pitched, squealing sort of noise.
– (N.B. In music “pitch” means the same as frequency.
• The loudness of a sound depends on the wave’s amplitude. The system used to
measure the loudness of sounds is the decibel system, given the unit dB. The decibel
is actually a fraction of a bel, the original unit for measuring sound (1 db = 0.1 b). The
"bel" was originally named after Alexander Graham Bell, the inventor of the
telephone, the decibel became a standard. A unit of loudness, called the phon, has
been established. The number of phons of any given sound is equal to the number of
decibels of a pure 1,000-hertz tone judged by the listener to be equally loud
• One of the loudest man-made sounds is created by the space shuttle lifting off. It will
generate sounds at an incredible 215 dB!!!
• Most concerts you go to will have sound levels between 100 – 130 dB… easily into the
permanent damage range. Lot’s of old rock stars have permanent hearing loss.
• Many modern day musicians wear ear protection of some sort while in concert from
loud sounds.
Range
(dB)
Descriptio
n
Examples
0 - 30
Very
Quiet
This is the threshold of
human hearing, up to the
sound of a quiet whisper.
31 -
50
Quiet
This is an average quiet
house, with maybe the
sound of a fridge running or
someone moving around.
51 -
70
Normal
Regular daily sounds like
people talking.
71 -
90
Loud
This is the point where a
sound becomes annoying or
distracting. Vacuums or a
noisy car on a busy street are
at these levels.
91 -
110
Very Loud
Most people will try to avoid
being in areas this loud.
Prolonged exposure can
cause permanent ear
damage. Temporary effects,
like "stereo hiss", may
happen.
111 + Painful!!!
Even limited exposure to
levels this high will cause
permanent hearing loss.
The sound level heard by your ears is commonly measured in decibels. ( name derived by inventor of
telephone) When referring to sound, a decibel is used to measure the amplitude of the sound wave.
9
Sound
foci

Sound pressure levels
Sound is a longitudinal wave. Remember that longitudinal waves are made up of areas where the wave is compressed together, and other areas where it is
expanded.
This would agree with the way that humans themselves make sounds. We force air, sometimes harder, sometimes softer, through our vocal cords. In the
process the air is either squished or allowed to move freely… making the air into a longitudinal wave! Sound pressure is expressed in logarithmic scale, Scale
on which actual distances from the origin are proportional to the logarithms of the corresponding scale numbers this is the basis of decibel scale
Any intolerable and irritating sound is called noise. The word noise comes from the Latin word nausea, meaning seasickness.
Music refers to any sound that is pleasant to the ear. Sound produced by musical instruments is pleasing to the ear. But if the intensity of the sound exceeds
a certain limit, then it is intolerable and becomes noise. Undesirable sounds and disturbances cause noise pollution. Noise pollution may cause high blood
pressure, panic attacks and lack of sleep among those exposed to it. Continuous exposure to loud noise may cause temporary or even permanent hearing
impairment above 120 db.
The Threshold of Hearing is about 5db. & Threshold of Noise Pain is 130 db
10

Inverse square law (damping of sound)
In the angle shown in Figure, the same sound energy is distributed over the spherical surfaces of
increasing areas as distance is increased. The intensity of the sound is inversely proportional to the
square of the distance of the wave front from the signal source
In a free field condition, where no reflecting surfaces around the
sound source, the radiation of sound intensity is reduced by ¼ each
time the distance from the sound source is doubled
L1/L2 = d2 sq/d1sq ( Inverse square Law)
The sound intensity from a point source of sound will obey the
inverse square law if there are no reflections or reverberation.
Because of the inverse-square law described above, reverberated
sounds will eventually lose enough energy. and drop below the
level of perception
11

Building Acoustics : Ancient Greek Theater
The Greek theatre history began with festivals honoring their gods. Actors were
allowed to perform in each play. the chorus evolved into a very active part of
Greek theatre. Music was often played during the chorus' delivery of its lines.
Greek Theatre buildings were called a theatron. The theaters were large, open-
air structures constructed on the slopes of hills. They consisted of three main
elements: the orchestra, the skene, and the audience. For communicating to
the crowds. (Both audio wise & vision wise communications) this became a
very important design feature for theater building in the open air
environments. As during those times no loud speakers were there. The seating
visual angles for the arena area was very important. The actors had to use loud
voices to become more effective communicators ( see fig below) with focused
blow of sound for attention of audience.
Skene: A large rectangular building situated behind the orchestra, used as a
backstage. Actors could change their costumes and masks. Earlier the skene
was a tent or hut, later it became a permanent stone structure. These
structures were sometimes painted to serve as backdrops behind all action
sceenarios. A proscenium (is the area of a theatre surrounding the stage
opening
12

Building Acoustics ; Ancient Roman Theater’s
The Greeks were already an established culture in southern Italy
when Rome was created Greece strongly influenced Rome in many
different ways. Rome's ideas on many things were borrowed from
the Greeks, things ranging from its Gods Plays to Theatre
construction. Later on however, Theatres were began to be built
on hillsides (hill provided extra support and is easier to build on).
All over the Roman Empire theatres were erected to entertain the
masses.
The large stone theatres seated tens of thousands of Romans.
There wasn't a front curtain nor were there performances done in
the orchastra pit (unlike Greek plays)
The Roman Coliseum structure, was the realization of an
amphitheatre concept in Rome built on a flat ground with spaces
for officers of roman kingdom, located just below the seating
space for the audience of the open theater above many activity
spaces.
Thus emerged theROMAN definition of the architecture of
structure:
In it most purest form : 'Architecture is the coherent set of
constructive, operative and decorative concepts of a structure.'
In a more practical form it is the coherent (intensively consistent)
set of constructive, operative and decorative concepts that is or
will be applied onto structure.‘
13

Room acoustics
• The physics of the propagation of sound is immensely complicated, and when the assortment of materials that make up
the walls, floors and ceiling (plus any windows, doors and furniture) are added to the equation, it's very difficult to predict
what will happen to sound waves once they've left their source. What's more, every room is different, and it's not just the
dimensions that will dictate how the room will sound. "acoustic design “is the science that restores a neutral sound
balance”. Applying that science means interfering with the path of sound to control the sound energy.
• The speed of sound is not constant, The speed of sound varies because of environmental conditions such as air pressure
and humidity. An extremely loud sound of over 130 decibels can damage hearing right away. Sounds of 85 decibels or more
can cause damage if there is exposure over a period of time
An Introductory to speech intelligibility;
The fundamental purpose of an airspace with or without a sound reinforcement system is to deliver clear intelligible
speech to the listener at a comfortable volume level.
• A surprising number of spaces fail to achieve this basic goal. There can be many reasons for this, ranging from inadequate
signal to noise ratio to poor room acoustics or inappropriate choice or location of loudspeaker.
• It is the job of the acoustic and sound system designer to take these factors into account when designing a room layout /
shape / sound system and selecting devices to provide the degree of intelligibility required
• Flutter Echo ; A multiple echo in which the reflections rapidly follow each other. If two opposing reflective surfaces of a room
(parallel wall to parallel wall or floor to ceiling) there is always a possibility of flutter echoes. Successive, repetitive
reflections, equally spaced in time, can produce a perception of a pitch or timbre coloration of music and a reduction in the
speech intelligibility within the room. Flutter echo can be reduced in one of two ways, with the use of sound
absorption or sound diffusion
Room modes are the collection of resonances that exist in a room when the
room is excited by an acoustic source such as a loudspeaker / Radio etc,.
Most rooms have their fundamental resonances in the 20 Hz to 200 Hz
region, each frequency being related to one or more of the room's
dimension's or a divisor thereof. There are three types of modes in a room:
axial, tangential, and oblique
Modifying and canceling sound field by electro-acoustical approaches is called active noise control.
There are two methods for active control. First by utilizing the actuators as an acoustic source to
produce completely out of phase signals to eliminate the disturbances. second method is to use
flexible and vibro-elastic materials to radiate a sound field interfering with the disturbances and
minimize the overall intensity. The latter method is called active structural acoustic control (ASAC)
room.
14

SOUND & SURFACE
INTERACTIONS
Reflection of sound is one of the important consideration in any acoustical design problem. If
reflected sound reaches the audience after 1/15 of a sec after the direct sound from the speaker,
an ECHO is heard. If the time lap becomes less than 0.05 sec, a beneficial effect called
reinforcement of sound results . If reflected sound reaches later than 0.05 sec then the direct
sound will have a blurred effect of sound.
Shapes of Auditorium plans.
15

Ambient noise, Reverberation time etc,.
AMBIENT NOISE; Noise affects our ability to perceive speech. There is a consensus on the need for low levels of background noise for
good intelligibility. Mr Houtgast studied the effect of ambient noise on speech intelligibility in classrooms after testing the intelligibility
under a variety of noise conditions and has concluded that a +15 dB (audio) signal to noise ratio (SNR ) eliminates the detrimental effects
of interfering noise. Another important parameter which causes distortion of speech sounds through 'acoustical smearing' changing the
quality of the speech signal is excess reverberation. Some amount of reverberation is beneficial too as it improves speech levels by
increasing early reflections through reverberant energy thus supporting the direct sound. But excess reverberation apart from producing
harmful late reflection energy also hype up noise levels reducing the intelligibility. Optimal reverberation time and SNR needs to be defined
for different user groups specifically. For this we need to know how student’s ability to recognize speech as a function of SNR under
completely realistic conditions varies with age The SNR is basically how much louder the teacher’s voice is, above the other noises in the
room
Reverberation is persistence of sound in an enclosure after the source of sound has stopped The stream of continuing sound is
called reverberation. The rate of build-up of echo density is proportional to the square root of the volume of the room. The term Echo &
Resonance are sometimes used by musicians as synonym for reverberation but we mean it here as repeat of sound
Reverberation Time (RT60) is the time it takes for a sound to decay by 60dB. It is governed by the absorption characteristics for the room
In an enclosed environment sound can continue to reflect for a period of time after a source has stopped emitting sound. This
prolongation of sound is called reverberation. the sound in a room to decrease by 60 decibels after a source stops generating sound
The reverberant sound in an auditorium dies away with time as the sound energy is absorbed by multiple interactions with the surfaces of
the room. When sound dies out quickly within a space it is referred to as being an acoustically "dead" environment. An optimum
reverberation time depends highly on the use of the space. For example, speech is best understood within a "dead" environment. Music
can be enhanced within a "live" environment as the notes blend together. Different styles of music will also require different reverberation
times. Reverberation time is affected by the size of the space and the amount of reflective or absorptive surfaces within the space. A
space with highly absorptive surfaces will absorb the sound and stop it from reflecting back into the space. This would yield a space with a
short reverberation time. Reflective surfaces will reflect sound and will increase the reverberation time within a space. In general, larger
spaces have longer reverberation times than smaller spaces. Therefore, a large space will require more absorption to achieve the same
reverberation time as a smaller space.
There are several formulas for calculating reverberation time, the most common formula is the Sabine Formula, created by Wallace
Clement Sabine. The formula is based on the volume of the space and the total amount of absorption within a space. The total amount
of absorption within a space is referred to as sabins. It is important to note that the absorption and surface area must be considered for
every material within a space in order to calculate sabins.
.
16

Reverberation Time
Reverberation Time
.8 - 1.3 1.4 - 2.0 2.1 - 3.0 Optimum**
Speech Good Fair - Poor Unacceptable* 0.8 - 1.1
Contemporary
music
Fair - Good Fair Poor 1.2 - 1.4
Choral music Poor - Fair Fair - Good Good - Fair 1.8 - 2.0+
* With an adequately designed and installed sound system, speech Intelligibility concerns can be mitigated.
** The optimum reverberation time can be somewhat subjective and can shift based on numerous variables
Sabine's formula is used to predict the reverberation time,
Reverberation time can be calculated in the preliminary design stage. This is very beneficial in determining how well a space will function
for its intended use and if more or less absorption is needed within the space
Reverberation time is not the only descriptor of an acoustic environment. There are several other principles to consider. A few of the
more important considerations include: Reflections ,loudness (strength), clarity, warmth and intimacy. Questions to consider in each of
these areas:
Reflections: Does the reflection of sound within the space cause negative results such as an echo or a megaphone effect? Or are
reflective surfaces helping to benefit sound distribution? Loudness (strength): Is the volume of the sound loud enough? Is it too loud? Or
does it seem louder than it would at the same distance outdoors? Clarity: Can I hear each of the various instruments clearly? Can I
understand what is being sung by a solo vocalist, or what is being said by a speaker? Warmth: Is there a balance of sound throughout the
various frequencies? Or is the sound overpowered with too much bass or too much treble? Intimacy: Do you feel like you are a part of
the performance? Or do you feel like the music or speech is taking place in a separate environment? It is highly advisable to hire an
acoustical consultant to assist with reverberation time issues. All the treatment added only effects the high frequencies. You must
consider all the frequencies when you treat a room. The shorter reverberation time in the high end is reasonable at 0.3 sec (around 0.4 -
0.5sec is desirable) but you must take down the low end as well. The reverb time at 125Hz is around 2 sec, at 250 it's 0.92 sec,at 500 it's
down to 0.49 sec and it reaches 0.3 sec at 1000Hz and is right down to 0.21 sec at 4kHz.
Sabine / Eyrings Formula for Reverberation
Time:
RT60 = .049 V/a ( 0.16 V/a )
Where:
RT60 = Reverberation Time
V = volume of the space (Cubic feet / Cubic
meter)
a = sabins (total room absorption at given
frequency)
k is a constant that equals 0.16 when the units of
measurement are expressed in meters and 0.049
when units are expressed in feet
A room with too short a reverb time for a particular
type of music may classified as "dry" or "dead" while
one that is too alive or has too long a reverb time may
be called "muddy" or "watery".
17

Dynamics of sound
... how sound will travel to them in a performance or speech. This is just a simple
example of the math and thought put into the design of a stage, theater,
Sound-absorbing treatment Acoustically transparent material (e.g., spaced wood
slats or open metal grille) conceals actual enclosure, which can be treated with
deep sound-absorbing material to reduce reflected sound energy and creep
echoes
When dealing with audible frequencies, the human ear
cannot distinguish an echo from the original sound if the
delay is less than 1/15 of a second. Thus, since the
velocity of sound is approximately 343 m/s at a normal
room temperature of about 25 °C, the reflecting object
must be more than 11.3 m from the sound source at this
temperature for an echo to be heard by a person at the
source.
The strength of an echo is frequently measured
in dB sound pressure level SPL relative to the directly
transmitted wave. Echoes may be desirable in ship
navigation(as in sonar) to estimate depth
IN Gol Gumbaz of Bijapur, India: Any whisper, clap or
sound gets echoed repeatedly near the dome & also in the
Whispering Gallery of St Paul's Cathedral, London. Due to
multiple reflections
18

Behavior of sound in enclosed spaces
• Although people have gathered in large auditoriums and places of worship since the advent of civilization, architectural acoustics did not exist on a
scientific basis until a young professor of physics at Harvard University in 1895 to correct the abominable acoustics of the newly constructed Fogg
Lecture Hall.
• He defined a reverberation time T as the number of seconds required for the intensity of the sound to drop from a level of audibility 60 dB above the
threshold of hearing to the threshold of inaudibility. To this day reverberation time still constitutes the most important parameter for gauging the
acoustical quality of a room
• The distribution of acoustic energy, whether originating from a single or multiple sound sources in an enclosure, depends on the room size and
geometry and on the combined effects of reflection, diffraction, and absorption. From a point source the sound waves will be spherical, and the
intensity will approximate the inverse square law. Neither reflection nor diffraction occurs to interfere with the waves emanating from the source.
Because of the interaction of sound with the room boundaries and with objects within the room, the free field will be of very limited extent. If one is
close to a sound source in a large room having considerably absorbent surfaces, the sound energy will be detected predominantly from the sound
source and not from the multiple reflections from surroundings. The degree of diffusivity will be increased if the room surfaces are not parallel so
there is no preferred direction for sound propagation. Concave surfaces with radii of curvature comparable to sound wavelengths tend to cause
focusing, but convex surfaces will promote diffusion. Multiple speakers in amplifying systems auditoriums are used to achieved better diffusion, and
special baffles may be hung from ceilings to deflect sound in the appropriate directions
• Sound reflected from walls generates a reverberant field that is time dependent. When the source suddenly ceases, a sound field persists for a finite
interval as the result of multiple reflections and the low velocity of sound propagation. This residual acoustic energy constitutes the reverberant field.
The amount of acoustic energy reaching the listener’s ear by any single reflected path will be less than that of the direct sound because the reflected
path is longer than the direct source–listener distance, which results in greater divergence; and all reflected sound undergo an energy decrease due
to the absorption of even the most ideal reflectors. But indirect sound that a listener hears comes from a great number of reflection paths.
Consequently, the contribution of reflected sound to the total intensity at the listener’s ear can exceed the contribution of direct sound particularly if
the room surfaces are highly reflective.
• Every noise is different in respect of sound pressure level and frequency at which it is generated. So a thorough noise survey is conducted with 1/3
octave analysis and data evaluated to assess the annoying frequency. Great care is taken while designing the Acoustic Enclosure, not to hamper other
parameters.
Sound diffusion resulting from multiple reflections in plan & Section.
19

Sound Absorption & Diffusion : Some time down the road you will probably want to add some diffusion to your room setup.
The difference between trapping and diffusion is that trapping absorbs, and diffusors scatter. Both types of treatment can
correct the same problem, To achieve the right balance, Materials that have absorptive properties include foam and rigid
mineral-wool (see the 'DIY & Rockwool' box), and they 'soak up' the sound energy, turning it into heat, through friction.
Most effective on high-frequencies, absorption is essential for reducing flutter echoes and for taming bright-sounding or
'ringy' rooms.
Bass trapping is also a type of absorption, but is specifically designed to absorb low-frequency energy. A clever combination
of soft, hard, thick and thin materials, including air, is used to make the most efficient bass trap, and an empty gap between
the wall and the back of the trap helps to make it even more effective.
Diffusion is the scattering of sound energy using multi-faceted surfaces. Diffusers are commonly made of wood, plastic, or
even polystyrene. Jorge Castro explains: "diffusion helps in energy control and improves the sound quality in frequencies
throughout the middle and high range of the spectrum, and also improves sweet-spot image.” The 'sweet spot' is the place
between the speakers where you should be sitting to get the best stereo image
standing waves ; A standing wave is a sound wave that is the product of continuously
reflected back & forth sound wave between two parallel walls/surfaces whereby the incident
wave and the reflected wave are in phase. Sound reflections create standing waves that
produce natural resonances that can be heard as a pleasant sensation or an annoying one.
waves', where the physical length of the wave is a multiple of the room dimensions. The result is
increased volume at frequencies where the wavelengths match room dimensions, and deep
troughs or dead spots in places where the room dimension is an even factor (such as a half or
quarter) of the wavelength. Standing waves are more apparent in smaller rooms; and square and
cubeoid rooms, or rooms where one dimension is an exact multiple of another, are the worst
culprits. The wavelength of open 'E' on a guitar is around 14 feet (just over 4m), so if you've
converted a single garage into a studio, your longest wall will probably be almost exactly the
length of a waveform at that frequency. Sound Recording rooms any recordings you make of
acoustic instruments will bear all the hallmarks of the space in which you record them. Untreated
rooms have an uneven frequency response, which means that any mixing decisions you make are
being based on a sound that is 'coloured', because you can't accurately hear what's being played.
In short, you can't possibly tell how your mix will sound when played back anywhere else. It isn't
just an issue for mixing. The first thing to grasp is the outcome you want to achieve. It's
a common misconception that acoustic treatment should kill all reverberation. and that you want
a room covered floor-to-ceiling with foam tiles: this isn't what you're aiming for. You also need to
bear in mind the limitations imposed by space and budget: most home music studios are small in
comparison & don't have the FUNDS for treatment solutions.
20

Bass Traps. are acoustic energy absorbers which are designed to damp low frequency sound energy with the goal of
attaining a flatter low frequency(LF) room response by reducing LF resonances in rooms. They are commonly used in
Recording rooms ,Home theaters and other rooms built to provide a critical listening environment. Like all acoustically
absorptive devices, they function by turning sound energy into heat through friction Low frequency sound waves are
extremely long – and thus very strong – they are the toughest to control. This is true no matter whether you’re attempting
to block their transmission to a neighboring space or trying to absorb them to clean up the low frequency response within
a room. Controlling low frequency sound is harder than controlling mid or high frequency sound and generally requires
more effort and expensive.
There are generally two types of bass traps: resonating absorbers and porous absorbers. By their nature resonating
absorbers tend toward narrow band action [absorb only a narrow range of sound frequencies] and porous absorbers tend
toward broadband action [absorbing sound all the way across the audible band - low, mid, and high frequencies]
basics of room acoustics. Most of us are in smaller, residential listening spaces which are on average 12ft X 15ft X 8ft in
size. We usually have problems with the lower end or bass frequencies in these types of rooms due to bass frequencies
being longer and more powerful than treble frequencies. Acoustic treatments like bass traps can correct issues with the
lower end, as well as somewhat the mid or higher end without making it sound too dead if it’s broadband bass trapping.
You may also want to add some diffusion into your room setup, usually on the back wall or near the tracking area.
Diffusion will help with comb filtering, and flutter and slap echo, and it can also help with making your recordings sound
more spacious.
Most of us use existing rooms such as bedrooms, living rooms or basements for a listening space versus high end recording
studios that are designed with acoustics in mind. However there are ways to treat average-sized rooms to be utilized as
control rooms, home theaters or two-channel listening rooms.
Trying to get a great sound in this type of space can be difficult. But thanks to a little math, or calculating your room size,
and a little bit of acoustic treatment, we can make our rooms sound much better.
In a typical 12 X 15 X 8 foot room, we’ll encounter acoustic problems starting with the lower end. The reason the bass
response is usually more uneven than the higher end, especially in smaller rooms, is due to bass frequencies need more
time to dissipate. And because of that, there’s more of a chance of an inaccurate low end response.
You will encounter problems with mid- and upper-frequencies; but generally in a small room, low end frequencies are the
bigger challenge. Fortunately, treating your room with properly designed broadband bass traps will absorb more low-end
frequencies without over-absorbing the mid- and upper-frequencies. Tuned bass traps can also be utilized to
absorb only low frequencies while reflecting mid- and upper-frequencies to keep more ‘life’ within the room.
Some time down the road you will probably want to add some diffusion to your room setup. The difference between
trapping and diffusion is that trapping absorbs, and diffusors scatter. Both types of treatment can correct the same
problem, just via different methods. Put simply, trapping will prevent sound waves from traveling around too much which
could have caused peaks and nulls in the frequency response. Diffusion also helps with this, but the way it works is that for
example, when a powerful sound wave hits a diffusor, it scatters the wave which makes it less intense in your room.
This image shows bass trap
installed in the upper front
corners of a small mix room.
to create this look in your
room
Bass Traps ; sound absorbing technique
21

Sound Masking & reinforcement system
Masking refers to one noise on another results in less inteligiability of sound ( also known as acoustic perfume ) for improving privacy of sound in a
place. Sound masking is often confused with noise cancellation technology. Noise cancellation works well with headphones but is not applicable to use
throughout an office where there are many noise sources and many listening positions. The lack of privacy in most offices is actually due to too little
ambient background sound, making conversations and other activities more noticeable and distracting.
Typically an office without sound masking will have an ambient sound level of under 40 decibels. Conversational speech levels tend to be near 65
decibels causing conversations to be understood, and distracting to others, from up to 45 feet away. Adding sound masking to increase ambient sound
levels to around 47 decibels does not impede local conversation, but limits the radius of distraction to around 15 feet.
In most open office environments, adding sound masking acoustically triples the distance between workers. In other words, workstations would have to
be three times larger to get the same degree of privacy as can be achieved by adding sound masking.
Soft dB's Smart SMS-AMP is a DSP-based system that provides unique adjustment functions and ensures the generation of optimum sound masking
regardless of the characteristics of a room A sound masking system basically consists of a series of loudspeakers installed in a grid-like pattern in the
ceiling, as well as a method of controlling their zoning and output. The loudspeakers distribute a background sound, raising the facility’s ambient level in
a controlled fashion
Sound masking methods benefits Architectural consultants to solve acoustic problems with retrofitted spaces without sacrificing the Design ..……For
Business owners it benefits by increasing the employee productivity from 3 to 5 % & saves money for new constructions
Sound masking systems are a common part of today's interiors, from their original use in commercial offices and call centers, to relatively newer
applications ... Build a Better Workplace; In fact, the background level in most offices is so low, you can easily hear conversations and noise from up to
50 feet (15 meters) away. These distractions make it difficult to concentrate. It takes more effort to focus, which tires you out, affecting your mood and,
ultimately, your productivity.
Reduce noise distractions and protect speech privacy with direct-field sound masking ;
22

sound reinforcement systemIn many situations, to obtain adequate loudness & good distribution
of sound, its necessary to augment the natural transmission sound
from source to listener by means of a sound reinforcement system
There are two principle types of sound reinforcement systems
(a) Central System (b) Distributed System
The most preferred system is the Central system for all Auditorium in
which a cluster of loud speakers are located directly above the actual
source of sound. This system is capable of giving realism to sound as
the listener gets his sound directly from the direction of source
sound.
The distributed system uses a number of loud speakers distributed
uniformly over the audience area with loud speakers located
overhead ( much like down lighting) It covers the Hall with small
pools of sound in spaces with very high ceilings. Loud speakers can
also be installed at th back sides o seats or mounted on the desks as
seen in parliament house/ assembly halls.
Loud speakers should never be located at the two sides of the
proscenium stage nor they should be distributed along the two sides
of the hall. Or in the four corners of the large Hall, as they cause
SERIOUS PROBLEMS of sound distortion.
Loud speakers in a distributed system cover between 60 degrees to
90 degrees effectively. Unfortunately in this system the speakers
beam of high frequency sound travels rather sharply and as such
uniform intelligibility is not possible in this system The production of
artificial echo is often a problem, particularly in this type of system,
as such the time delay device is often used to reduce this problem.
Large outdoor concerts use complex and powerful
sound reinforcement systems
23

Time delay circuit / audio spot light techniques in long Auditoriums
In long auditoriums with a distributed Loud speaker system, a listener out at the rear , will hear the amplified sound almost
instantaneously from the nearest loud-speaker while the direct sounds at some later time depending on the distance to the platform.
This delay in the arrival of direct sound is sufficient to cause discrete echo
This delay of the direct sound is due to lesser speed when compared to electrical sound transmission system between microphone &
Loud speaker.
The delay is of the order of 65m/sec or more to cause Echo or if it is some what less, the echo appears as simply as a muddying effect
on the sound heard by the listener.
To resolve this problem it’s necessary to introduce the time delay mechanism in the electrical cercuit which in effect delays the loud
speeker sound, such that it arrives at approximately the same time as the origional direct sound. For a very long Hall 2 or more delay
circuits may be required
Time delay circuit ; is usually the last resort and may not ever required if the sound system design is properly understood & planned
properly in the long Auditoriums. Any sound that misses your intended audience is noise. Only the Audio Spotlight system can keep
sound focused specifically to your listeners, providing sound where you want it, and quiet everywhere else.
The revolutionary Audio Spotlight technology creates a tight, narrow beam of sound that can be controlled with the same precision as
light. Since the Yr 2000, Audio Spotlight systems have been installed in thousands of locations around the world to provide high-quality,
precisely targeted sound.
A typical sound reinforcement system consists of; input transducers (e.g., microphones), which convert sound energy into an electric
signal, signal processors which alter the signal characteristics (e.g., equalizers, compressors, etc.), amplifiers, which add power to the
signal without otherwise changing its content, and output transducers (e.g., loudspeakers), which convert the signal back into sound
energy
A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers A sound reinforcement
system may be very complex, including hundreds of microphones, complex audio mixing and signal processing systems, tens of thousands
of watts of amplifier power, and multiple loudspeaker arrays, all overseen by a team of audio engineers and technicians. Sound Masking
System Amplifier etc,. especially for the designing Music Halls
24

Sound Acoustics Treatment for Interior spaces ;
The architecture of the enclosure should contribute as much as possible to overcoming the inverse square law and the bass loss problem. When
the compressions and rarefactions are out of phase, their interaction creates a wave with a dampened or lower intensity. This is destructive
interference. When waves are interfering with each other destructively, the sound is louder in some places and softer in others. Patterns of
destructive and constructive interference may lead to "dead spots" in which both volume and clarity of sound are poor, and "live spots" which
are liked in auditorium acoustics. Explain what can happen to the energy of sound waves when the waves interact. Compare and contrast
constructive interference and destructive interference.
Almost every acoustical situation can be described in terms of a Source of sound, a path for transmission
and a receiver of sound.
Some times the source can be increased or decreased, the path can be made less or more effective and
the receiver can be made more attentive by removing distortions or he can be made more tolerant to
disturbance.
Acoustics is one of the many aspects of the environment in which we live. Sound can distract us; they can
make us happy or sad. The quality of sound we hear of the aero-plane flying overhead may interfere
with a telephone connection Laughter in an adjacent classroom may prove destructive to the students
listening to a lecture etc,.
The basic purpose of Architectural acoustics is to provide a satisfactory acoustical environment for
whatever use the space is intended or utilized. The office building the designer may wish to provide
freedom from distraction or privacy of conversation. In Library buildings he can provide a quiet
environment with tolerable noise in the background. In almost any situation one can determine just what
the environmental requirements are and then proceed to design the building to satisfy them .
A favorable acoustic environment is the one which offers facilities for good hearing conditions viz 1.
Quietness of environmental background 2. Sounds produced are sufficiently Loud 3. Uniform
distribution of sound within the enclosed space without any distortions and focusing of sound 4. The
Reverberation time long enough to give proper blending of sound and yet be short enough so that there
is no excessive overlapping of sound and confusions. These simple criteria’s if justified & satisfied , will
result in creating good hearing conditions in any place
In noisy environments it may be necessary to use a carefully designed sound reinforcement system (
Electronically controlled) but whatever the requirements whatever the space may be, Good hearing can
be achieved for any type of required use. The most important aspect is in recognizing the problems in
advance and solve them in the design stage of the project and not after its finished.
25

Acoustics & Building Design Criteria's
STAGE ISSUES
Site
Rural or industrial - planning
regulations
Transportation noise -
roads/rail/aircraft (prediction
of future levels)
Industrial noise sources
Airborne noise and/or
vibration
Building form Site planning and screening
Ventilation - natural or
mechanical
Location of plant rooms
Detaile design Room-to-room noise
Outside-to-inside noise
HVAC noise
Room acoustics
Sound insulation
Sound systems
Supervision Quality of construction
Commissioning
Compare actual noise levels
to intended levels and
criteria
Retrofit Remedial action
Acoustics and Design
What is covered:
•Basic acoustic terminology.
•Noise sources, design criteria for different buildings and spaces, assessment of noise
levels, and noise control.
•Design issues associated with acoustic performance inside buildings due to internal or
external noise sources.
What is not covered:
•Buildings where there are special acoustic constraints e.g. auditoria.
•Factories (and buildings where there is 24 hour work, e.g. hospitals) where it may be
important to assess the effect of noise generated on adjacent dwellings.
•Sound systems in buildings. These may be required for emergency warning (e.g. fire
alarm), paging system, lecture and conference rooms, sports stadia, railway stations etc.
Examples
•External environment: buildings adjacent to motorways where there may be a need for a
sealed building with mechanical ventilation; How noisy can it be before a building cannot be
naturally ventilated?
•Internal environment: Office space within factories next to noisy process plant. How can
sound levels in offices be made acceptable?
Acoustic assessments through the design process: stages of design
Assessment of Room Sound Level
To find the total sound pressure levels in a room: define individual noise sources and their
PWL, include the modifying characteristics of the transmission paths (e.g. SRI), apply the
acoustic properties of the receiving room (amount of acoustic absorption), and sum.
Outside Noise Environment
This is important, as it has implications for ventilation, and possibly glazing/constructions
e.g. near airports or busy roads. Considerations include:
external barriers around site - height is critical: note the potential impact on shading;
magnitude of noise sources by measurement, or in case of traffic, calculation based on
vehicle flow rates, speed, ratio of heavy/light vehicles, road surface, gradient, distance from
road to building, screening correction.
distance is important: with vegetation and <4m reception point, as high as 7dBA for doubling
of distance; with a hard surface or water only 3dBA for a doubling of distance.
26

Noise control in buildings
Controlling noise in buildings is an important part of an architect's responsibility. Architect has to work
for avoiding noise problems during the design and construction of new buildings, and for eliminating
noise in existing structures. The study of properties of sound absorptive materials, acoustical
characteristics of rooms, airborne sound insulation, and structure-borne sound insulation are proven
methods for dealing with noise, HVAC systems, plumbing systems, and machinery, plus information on
the design of buildings for noise control.
27

Sound absorbing materials / panels for Floors, Walls & Ceilings
Sound absorption panels trap acoustical energy (sound) and prevent it from
reflecting off of the surfaces they cover.
Sound absorption materials 1 Porus materials 2. Panel or membrane absorbents & 3. Cavity Resonators
28

Absorption and insulation
Absorption is quantified as the absorption coefficient - the proportion not reflected
Insulation is quantified as the Sound Reduction Index SRI (in dB) - a measure of the
reduction in transmission.
What does NRC and SAA stand for?
Sound absorption properties of acoustic materials can be measured in lab tests.
Specifications for materials used in sound absorption commonly include an NRC
(Noise Reduction Coefficient) for simplicity, in addition to more detailed frequency
versus amplitude charts.
The Noise Reduction Coefficient (NRC) and Sound Absorption Average (SAA) values
are both single number ratings that indicate the level of sound absorption provided
by the product being tested
An NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption
NRC is most commonly used to rate general acoustical properties of acoustic
baffels, ceiling tiles, and banners, office screens, and acoustic wall panels.
In certain applications, such as designs of musicrehearsal rooms, performance
spaces, and rooms employed for critical speech, it is usually more appropriate to
consider the sound absorption coefficients at the individual one-third octave band
frequencies, including those above and below the bands used to compute .
NRC is being replaced by the Sound Absorption average (SAA), The SAA is a single-
number rating of sound absorption properties of a material similar to NRC, except
that the sound absorption values employed in the averaging are taken at the twelve
one-third octave bands from 200 Hz to 2500 Hz, inclusive, and rounding is to the
nearest multiple of 0.01
What is an STC Rating?
STC stands for Sound Transmission Class.
Basically, STC ratings are an established way to average how much sound is stopped
by something. STC ratings are used for windows, doors, walls and most building
materials. For windows, STC ratings range from 18 to 38.
STC ratings are the ONLY way to accurately compare various noise reduction
products. An STC rating is an instrument measurement of how much noise is
stopped.
The STC rating is the average amount of noise stopped at 18 different frequencies,
measured in decibels.
What STC Ratings do Windows Have?
For single pane windows, the STC Rating is most likely between 26 and 28. The
difference is the glass thickness and how air-tight the window is. Louvered windows
can be less than STC rating 18 in many cases
Octave band calculation/measurement:
Analyzing a source on a frequency by frequency basis is possible but time
consuming. The whole frequency range is divided into set of frequencies
called bandsA frequency is said to be an octave in width when the upper band
frequency is twice the lower band frequency. A one-third octave band is
defined as a frequency band whose upper band-edge frequency (f2) is the
lower band frequency (f1) times the cube root of two.
The absorbing/insulating properties of materials vary
significantly with frequency of the sound source. Thus
measurements and calculations often need to be
undertaken in octave bands (or 1/3 octave bands for
more detailed work). A crude approximation
sometimes used for broad-band noise is that
transmission/absorption characteristics over the full
acoustic spectrum is similar to the response at 500Hz.
Note that the human ear responds to frequencies in
the range 20Hz to 20kHz approximately
29

Mass Law refers to Sound transmission loss / sound insulation materials
in energy audits, Home Energy Ratings, whole house air-sealing, and the installation of various types of insulation for both commercial and residential projects.
Whether it’s new construction or retro-fit projects expert consultants are equipped to handle your weatherization needs
According to the MASS LAW, there will be an increase in sound insulation of about 5dB if the mass/unit area is doubled. The insulation also increases by about 6dB for a
doubling of frequency. However, this is only true up to a critical frequency, beyond which there will be a dip in insulation. The critical frequency is about 100Hz for a
one-brick wall, 200Hz for a half-brick wall.
Now that the enclosure is pretty well sealed, sound can still reach you from transmitting through the enclosure walls. This is called air borne sound transmission. There
are ways of increasing this sound transmission loss. The first is mass. Mass can be increased by using a thicker wall of the same material or use a more dense material.
There is a relationship between sound transmission loss and weight of the barrier and this is called the mass law. The mass law states that for every doubling of the
weight of the material, one can expect a 6 dB increase in the transmission loss. In addition to the mass having an effect on the transmission loss, the frequency of the
sound also has a similar affect. A doubling of the frequency will create a 6-dB increase in the transmission loss. Figure 4 shows the effect of mass and frequency on
sound transmission loss.
Concrete painted walls have very low sound absorption and hence, reflect. Five inches of fluffy snow however absorbs the sound when it strikes it.
Some porous materials allow sound energy to easily enter. Acoustic porous materials can have porosity greater than 90%. Porosity is the amount of volume being just
air. Common sound absorption materials are open cell foam and polyester fiber. Sound absorption is an energy conversion process. The kinetic energy of the sound (air)
is converted to heat energy when the sound strikes the cells or fibers. This means the sound “disappears” after striking the material due to its conversion into heat.
We know that most sounds contain many different pitches or frequencies. A bass guitar plays low frequency sounds while a violin plays high frequency sounds.
Low frequency sounds have long wavelengths and high frequency sounds have short wave lengths.
Low frequency sounds pass through materials much easier than high frequency sounds. That's why one can hear the constant thud of a subwoofer through the wall of
the room next door. Because sound passes through materials differently at different frequencies, the sound absorption will typically change with frequency. Besides the
sound absorption changing with frequency, it also changes with the thickness of the material. For the same materials, thin sections will not absorb as much low
frequency sound as will thicker pieces
There are a variety of ways of producing sound transmission loss. One way is distance. We all know that noisy machines are louder at close distances versus when
farther away. When designing a school for example, distance plays a good common sense noise control solution many times. A poor school design would place a
gymnasium or cafeteria next to a library. However, in regards to machinery, distance, as a noise control solution is rarely practical
Sound is lazy and will always take the easiest path to get from point A to point B. The easiest path for sound to travel is a clear unobstructed path. If you can visually see
the noisy machine, there is nothing obstructing or reducing the noise from the maximum amount that you could receive. It is important not to only place something in
between you and the noisy machine, but also to make sure that there is no place for that sneaky noise to slip through holes and cracks. A solution for blocking the
noise from getting to you would be to place the machine in an enclosure. A well-designed enclosure will always make sound work hard to get to your ears, so it is
essential to not have any holes, gaps or cracks. If ventilation is required for combustion intake and exhaust or cooling purposes, a sound silencer or tortuous path is
necessary. The tortuous path is lined with sound absorptive material, forcing the sound to strike the sound absorptive material which causes the sound to be reduced
significantly by the time it exits.
Q. What type of noise problem are you facing?
Airborne Noise (engine, pump, compressor)
Structure Borne Noise (vibrating panel, hard mounted
objects)
30

Reducing airborne noise transmission from mechanical rooms.
When trying to control
sound reflections and
reverberation, in
Industrial / commercial
spaces like lobbies, board
rooms, offices, music
rooms, community halls
and today’s residential
spaces and home
theatres, sound
absorbing products, such
as acoustic wall and
ceiling panels and baffles,
are used to reduce noise
level and control sound
within the space. The
appropriate quantity and
location of acoustic
treatment will reduce the
reverberation time and
echoes in spaces with
hard reflective surfaces.
Acoustiblok k is a high performance soundproofing and noise deadening solution, now available in India where strict laws have been put in place to reduce noise
pollution. Ideal for new industrial, commercial, and residential projects, new builds and renovations.
Acoustiblok 3mm and 6mm materials can be applied to floors, walls or ceilings to provide a noise barrier between you and any sound source.
Acoustiblok 3mm material offers a sound reduction index (SRI) of 26 decibels – six decibels more than lead. Acoustiblok is a proprietary viscoelastic polymer material
with a high density mineral content, heavy and yet extremely flexible. As sound waves cause the Acoustiblok material to flex, internal friction occurs and the acoustical
energy is dissipated into undetectable trace amounts of heat. Our 6mm product is 9.76kg per square meter, and offers an SRI of 32 decibels
31

Acoustic container for Noise Control & Ventilation system
The cooling air requirements for each generator set must be
determined before accurate sizing can take place. This mostly
involves obtaining information from the engine's radiator
manufacturer, where they will be able to advise airflow
volume and additional fan pressure (ie pressure available over
and above the requirements of the radiator alone)
In general, a 500kW engine will produce a noise level of
110dBA at one metre distance, without any acoustic
treatment. This could drop to 95dBA for small engines or up
to 115dBA for large engines. (Petrol or gas powered engines
tend to be quieter.) This is generally too noisy for comfortable
use and acoustic treatment is normally necessary.
Noise levels reduce with increasing distance and the chart
shows the reductions that can be expected. The end customer
will either expect you to suggest a suitable noise level,
dependant on the actual site conditions, how close the
neighbors are (who have every legal right to sue the operator
for nuisance if they choose) and the best orientation to direct
hot air and exhaust fumes away from sensitive areas - or will
set his own limits required.
As a guide, where business or residential neighbors are likely
to be affected by noise from the generator set, a local
authority would suggest a limit of 45dBA at the boundary of
the property for daytime - or 35dBA for nighttime operation.
This can be extremely difficult to achieve, especially when you
have no distance to assist with the attenuation.
32

33
The calculator below can be used to calculate the NR - Noise Rating:
Sound Pressure Level (dB) - Octave band mid-frequency (Hz)
40db 31.5 Hz
40 62.5 Hz
50 125 Hz
55db 250 Hz
60 500 Hz
50 1000 Hz
55 2000 Hz
45 4000 Hz
45db 8000 Hz
For this the calculated NR is @ 58db
Why architects need to use their ears
Because of poor acoustics, students in classrooms miss
50 percent of what their teachers say and patients in
hospitals have trouble sleeping because they continually
feel stressed. Ar. Julian Treasure sounds a call to action
for designers to pay attention to the "invisible
architecture" of sound
The calculation model in Acousticfacts.com
uses Sabine’s relationship between the total amount of
sound absorption in a room and the room’s
reverberation time. In essence, the relationship states
that the reverberation time is lowered if you introduce
more absorption.
Calculations are made in
the octave bands 125, 250, 500, 1000, 2000 and 4000 Hz.
This means that the total sound absorption from the
room itself (including an absorbing ceiling) and from all
furniture and wall absorbers is summed in each octave
band. Sabine’s relationship is then used to calculate the
corresponding reverberation time in each octave band.
These values can be seen for your individual rooms by
pressing the graph button.
https://www.youtube.com/watch?feature=player_embedded&v=y5nbWUOc9tY
Sound Pressure - Sound Pressure is the force of sound on a surface area perpendicular to the direction of the sound

Conclusions of study :
Design strategies can optimize acoustic performance, and acoustical materials can provide further enhancements. Acoustic
performance encompasses several issues, including:
Speech privacy —Good acoustic design manages speech transmission both through solids (like roofs and walls) and through air
(like in ducts and conduits).
Speech intelligibility —Key to schools and other learning and performance spaces, intelligibility relies on controlling reverberation.
Noise transmittance —Noise is unpleasant, chaotic sound that may come from building equipment or from sources outside the
structure.
Typical design strategies for addressing these issues require starting early in design:• Right-sizing of rooms—Smaller rooms can
optimize speech intelligibility by preventing reverberation common in larger spaces. High-performance building envelope—In addition
to improving energy performance, high insulating value and airtightness can help reduce the amount of sound that enters the building
from the outside.
Studies show that acoustics are an essential consideration when designing an office for optimum performance. The work place should
provide occupants freedom from distracting noise, and enable them to work without distracting others. Undesirable noises affects
concentration, the ability to think clearly, the ability to communicate effectively, and increases error rates. It has been estimated that
productivity can increase by as much as 26% if noise is controlled.
Considerations
• Effective design of partitions, correct size, placement and absorption material
• Layout of workspaces
• Correct ceiling materials
• Eliminate impact noise
• Background noise levels
Acoustic considerations in work spaces
A church building is built for good communication, verbal, musical and emotional.
If the acoustics are poor and the sound system is badly designed for speech communication ,
then it becomes a bad design
34

Acoustical design considerations
Acoustics in schools;
Class room Satisfactory size for 40 students is 8.5x7
m Acceptable noise level 40 db.
RT in class room is dependent on purpose for which
it is used, capacity & age group.
In kindergarten schools more sound absorptive
materials is necessary comparatively.
In Lecture halls of higher schools the volume should
be kept as small as possible ie,. About 12 cum / seat.
Racking may be suggested for seating & ceiling
should be designed to give beneficial reflections of
sound. A ratio of length to width of 1.2 : 1.0 is ideal.
Rooms for music should have higher ceilings than
ordinary class rooms & some sound diffusers are to
be used. Sound lock rooms may also act effectively to
rehearsal rooms.
The school library areas ceilings may be made
absorptive and walls are usually lined with book
racks which provide absorptive surfaces. The flooring
can be with carpets to avoid movement noises.
Auditoriums
Important feature is the geometry of the room ie,. Space
(volume) , Shape &Sound Absorption
A Concert hall should have more volume for RT of 1.5 to 2
sec Theaters must have comparatively lesser volume & a
lesser RT of 1.3 sec.
The shape of auditorium is a very important feature as we
usually use sound reinforcing system which makes it a much
more difficult factor for acoustics.
•Concave walls –have concentration of sound
•To control reflections from walls, solid convex segments
may be added in the wall areas.
•Sounding boards on platforms of speech helps in reducing
ECHO effects as they produce parallel waves
•The audience during program shall absorb nearly 70 to 80
percent of the sound produced & cushioned seats have a
greater advantage.
•Fan shaped Auditorium plan with converging side walls
are considered best for acoustical reasons with proportions
of1;2;3 for height, width & length is advisable.
•The rear wall should be made sound absorbent to prevent
Echos.
•Some important design considerations for Multy purpose halls are a) No deep under
Balcony spaces b) No… Domed ceilings c) No Curved Walls d) acoustical Doors to
Lobbies e) well isolated Mechanical equipments & Stage Scenery Stores. F) Fabric
upholstered seats for stable RT characteristics' 35

Now days, many children seem to have learning disabilities, language
learning problems, behaviour problems, auditory processing disorders,
reduced cognition skills etc. It is therefore imperative that the
classroom they are learning in does not hinder their learning ability
but allows clear listening and communication.
Considerations:
• Volume per seat ratio
• Placement and choice of sound reinforcement materials
• Distance between speaker and rear of audience
• Path difference between reflected and direct sound
• Good sight lines and reduce audience attenuation
• Teacher/lecturers voice needs to be 15dB louder than background
noise
• Background noise levels should be below 34dBA
• Overall sound levels should not exceed 79dBA
• Reverberation time less than 0.6 seconds
• Design and placement of sound reinforcement system
Classrooms and educational facilities
36
Good projection of
sound to the rear of
the enclosure.
Desirable acoustic
properties of the
auditorium can be
contributed by its
architecture. The
desired properties at
left are correlated
with the measurable
parameters at right.
Long enough reverberation
time.
Good clarity and
articulation
RReverberation time not
too long.
Good balance of low
and high frequencies.
Reverb time for low freq.
longer than for highs.
Even dispersion of
sound. Absence of
annoying echoes.
No large reflective surfaces
or focusing of sound.
A feeling of "intimacy"
or "presence".
Short delay between direct
and first reflected sound.

CONSIDERATIONS
• Background noise levels
• Correct use of space
• Placement of buffer areas to reduce
background noise levels
• Treat noisy areas with the absorption to
control noise build up
• Ensure volume per seating ratio is
correct
• Good sight lines
• Visual and seating considerations should
not be the only consideration when
determining the shape.
• Determine desired reverberation times
at various frequencies
• Ceilings and side walls should provide
useful sound reflections.
• Choice of carpet and placement.
• The HVAC system should not exceed
preferred noise criteria
• Stage enclosure should provide good
distribution of strong early reflections
• Correct positioning and set up of sound
reinforcement system if required
• Correct placement of control console
Design criteria for multipurpose auditoriums
A good auditorium will accomplish effective projection of the sound to the rear of the
auditorium so that those distant listeners will not experience the extreme loss of sound
level caused by the inverse square law. That projection is normally achieved by having a
sufficiently long reverberation time. Another significant contributor will be a high,
reflective ceiling to reflect sound to the back of the auditorium. People seem to prefer
diff Reverberation times for diff types of musical performance. 1.3 sec for general music
1.5 to 2.1 Sec . for Symphony Orchestra etc,.
in order to have the best sound quality, all noise has been controlled. To avoid any
possible interference with the noise of the city, rubber joints have been interposed
between the floating part of the ceiling and the fixed one. The stage is also built
separately, such that it is completely unplugged from the real floor.
End of the Program
Thank you By Prof Mukund
37

Sound-scape in the environment
Sound-scape and the Environment
A soundscape is a sound or combination of sounds that forms or arises from a Place. The study of soundscape is the subject of Ecology. The
idea of sound-scape refers to both the natural environment, consisting of natural sounds, including animal vocalizations and, for instance, the
sounds of Weather and other natural elements; and environmental sounds created by humans, through music, and other ordinary human
activities including conversation, work, and sounds of mechanical origin resulting from use of industrial technology.
The term "sound-scape" can also refer to an performance of sounds that create the sensation of experiencing a particular acoustic
environment, or compositions created using the instruments of an acoustic environment, either exclusively or in conjunction with musical
performances
There are two distinct sound-scape, either high fidelity or Low fidelity hi-fi or lo-fi, created by the environment. A hi-fi system possesses a
positive SNR. These settings make it possible for discrete sounds to be heard clearly since there is no Background Noise to obstruct even the
smallest disturbance. A rural landscape offers more hi-fi frequencies than a city because the natural landscape creates an opportunity to hear
incidences from nearby and afar. In a lo-fi sound-scape, signals are obscured by too many sounds, and perspective is lost within the broad-
band of noises. In lo-fi soundscapes everything is very close and compact. A person can only listen to immediate encounters; in most cases
even ordinary sounds have to be exuberantly amplified in order to be heard.
All sounds are unique in nature. They occur at one time in one place and can't be replicated. In fact, it is physically impossible for nature to
reproduce any phoneme twice in exactly the same manner. Today, there is a split between original sounds and unnatural acoustics brought
on by the transmission and storage of sound. In other words, recordings have made it possible to simulate any sound environment anywhere.
The portability of acoustics has transformed the idea of soundscape because it made hi-fi gadgetry mainstream in a lo-fi setting. Producers
have displaced sounds found in the countryside, wildlife, and water and injected them into the homes of people everywhere, further
enhancing the lo-fi problem found in urban spaces today.
Why it is said that "plantation of trees decreases the noise pollution " ?
Trees, not only absorb carbon dioxide, provide shade, prevent soil erosion, but they can also help muffle noise. Think
of trees as big, leafy, air-purifying, oxygen-producing, Acting as shields, trees reduce the intensity of the sound waves
considerably and it is the sound produced by the wind passing through the leaves that really helps muffle noise. A
properly-designed buffer of trees and shrubs can reduce noise by about five to ten decibels-or about 50 percent as
perceived by the human ear, planting a variety of both hedges or shrubs and taller trees to create a wall of foliage
from the ground up. Such examples as cottonwoods, poplar and aspen trees are especially good at noise reduction
because their leaf-shapes produce a good, strong rustling sound
38

Indian Code IS 2526 is a code of practice for acoustical design for
The designing of Auditoriums & Conference halls
Click this twice …..to get internet
connection
39

Architectural Acoustics
Question Bank VTU 6thSem B. Arch 2015
Draw sketches wherever necessary
1. (a) What is Sound ? Explain its difference with human Voice 04
(b) Explain the difference between Pitch & Frequency of sound 06
2. (a) How sound is produced in the human body 04
(b) Explain the concept of Sound Wave & Amplitude 06
3. (a) What do you understand by “DECIBEL” of sound, 04
(b) Differentiate between Audible /infrasonic & Ultrasonic sounds 06
4. (a) On what factors the speed of sound is determined 04
(b) What is “Attenuation of sound & Quality of sound 06
5. (a) Differentiate between Sound Pressure & Sound power 04
(b) Explain the fundamental characteristics of sound in detail 06
6. (a) Explain Threshold of Hearing & Threshold of Pain 04
(b) What is- Inverse Square Law 06
7. (a) Explain with sketches the features of the Ancient Greek Theater 04
(b) Explain the Operative concepts of Roman Theater Acoustics 06
8 (a) Explain with sketches the sound & surface interactions in buildings 04
(b) Explain the acoustic consideration in the work spaces 06
9. (a) What do you know about Sabine formula ? 04
(b) Explain the Ambient Noise & Reverberation issues 06
10(a) What causes Echo in Buildings 04
(b) Explain the importance of Reverberation time in the halls for music 06
11. Discuss the behavior of sound in enclosed spaces & its solutions 10
12. Explain the sound Reinforcing & Insulations methods 10
13. Discuss about i) Speech intelligibility ii) Room modes 10
14. Explain Good Acoustic design, What factors affect the basic goal 10
15. Describe the methods adopted for noise control in buildings 10
16. Briefly explain the following (answer any two) 10
a.) Standing waves b)Bass Traps c.) Sound Absorbing Materials
d.) sound masking purpose e.) Time delay mechanism
e.) Audio spot lighting
ksm.
END- Thank You
Prof. K S Mukunda
40

Accoustics tech notes 2014

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