Module 1 Part 2.pptx building servicesssss

Module 1, Part 2
BUILDING SERVICES - IV
WADIYAR CENTRE FOR ARCHITECTURE | MYSURU

Room Acoustics: Reflection - Nature of reflection from plane, convex and concave
surfaces, diffraction, Absorption, Echoes, focusing of sound, dead spots, flutter echo.
Room Acoustics: Room resonances, Reverberation - reverberation time (RT) calculation
using Sabine’s and Eyring’s formulae. Effect of RT on speech and music
Module 1, Part 1

HOW SOUND BEHAVES IN AN ENCLOSURE
• Room acoustics is concerned
with the physical properties of
Sound, with respect to its
effect on the inside of a
building and how this effect
can be altered to suit the
room/building’s intended use.
• It is concerned with
• providing the best
conditions for the
production and the reception
of desirable sounds.
• Setting a rate of decay
(reverberation time)
suitable for the type of room
• Reducing Background
noise and external noise to
acceptable levels
• Reduction of echoes and
similar acoustic defects

• To understand the behaviour of sound in an
enclosure, one must understand what effects it may
produce in a space
• The basic mechanisms of sound wave propagation
are
• TRANSMISSION – The passage of sound from one
point to another, e.g., from one room in a building to
another, or from the street into a room in the building.

produce in a space
are
• TRANSMISSION
• REFLECTION – When sound travels in a given
medium, it strikes the surface of another medium and
bounces back in some other direction, this
phenomenon is called the reflection of sound. The
waves are called the incident and reflected sound
waves

produce in a space
are
• TRANSMISSION
• REFLECTION
• REFRACTION – When sound changes mediums
(enters a different material) at an angle other that 90
degrees, it is bent from its original direction. This
change in angle of direction is called refraction.
Because of the angle, part of the wave enters the new
medium first and changes speed. Example - Sound
waves travel slower in cooler air than they do in
warmer air.

produce in a space
are
• TRANSMISSION
• REFLECTION
• REFRACTION
• DIFFRACTION – The bending of waves around small
obstacles and the spreading out of waves beyond
small openings

produce in a space
are
• TRANSMISSION
• REFLECTION
• REFRACTION
• DIFFRACTION
• ABSORPTION – When a sound wave strikes one of
the surfaces of a room, some of the sound energy is
reflected back into the room and some penetrates the
surface. Parts of the sound wave energy are absorbed
by conversion to heat energy in the material, while
the rest is transmitted through.

produce in a space
are
• TRANSMISSION
• REFLECTION
• REFRACTION
• DIFFRACTION
• ABSORPTION
• DIFFUSION - This is the scattering of waves from a
surface. It occurs as a result of the texture and
hardness of the obstacle.

• These basic mechanisms of sound wave propagation
create sound effects like
• ECHOES
• An echo is produced when the reflected sound wave
reaches the ear just when the original sound from the
same source has been already heard.
• Thus, there is repetition of sound.
• The sensation of sound persists for 1/10th of a second
after the source has ceased.
• Multiple echoes may be heard when a sound is
reflected from a number of reflecting surfaces placed
suitably.

Module 1 Part 2.pptx building servicesssss

• FOCUSSING OF SOUND &
DEAD SPOTS
• Focussing occurs when a
dispersed sound wave comes
to focus on one spot in the
enclosure
• This increases the Intensity
of sound at that one spot
• At the same time, this causes
the occurrence of spots with
diminished or deficient
sound intensity.
• These are called dead spots

• FLUTTER ECHO
• Flutter echoes occur when a
sound wave bounces
between 2 parallel surfaces,
causing a series of echoes
• The more closely the surfaces
are spaced, the faster the
flutter.
• These can cause audio
distortion that makes speech
and music sound muddy and
unclear. This problem is
more acutely noticed in
recording studios

Room Acoustics: Reflection - Nature of reflection from plane, convex and concave
surfaces, diffraction, Absorption, Echoes, focusing of sound, dead spots, flutter echo.
Room Acoustics: Room resonances, Reverberation - reverberation time (RT) calculation
using Sabine’s and Eyring’s formulae. Effect of RT on speech and music
Module 1, Part 2

REVERBERATION:
• This is the persistence of sound in an enclosed
space as a result of continuous reflection or
scattering of sound after the source has stopped.
• It is one the most prominent behaviors of sound in an
enclosure.
• It occurs when sound waves hit a surface and are
reflected toward another surface which also
reflects it.
• Some of the sound is absorbed with this
continuous reflection which gradually reduces the
energy of the sound to zero.

REVERBERATION TIME:
• The time taken by the sound in a room from its
average intensity to inaudibility level is called
reverberation time.
• In technical terms, Reverberation time is the time
taken for the sound pressure level to diminish to
60 dB below its initial value.
• Reverberation Time can affect the audibility of
sound in an enclosure, especially if the reverberation
time is considerably long.
• The optimal reverberation time depends on the use of
the enclosure
• For example, factory halls should have short
reverberation time. In such spaces the criteria is to
prevent the accumulation of sound energy. This will
help reduce the overall sound pressure level and noise
exposure for all workers in the hall.
• In churches and large concert halls longer
reverberation times are often desired since this
supports the music.

MEASURING REVERBERATION TIME
SABINE’S FORMULA:
• Reverberation time depends on 3 factors
• Volume of the Room
• Surfaces of the Room
• Furniture and objects in the room
• Increasing height makes rooms more reverberant
• Absorbent surfaces – Carpets, curtains, ceiling
panelling, furniture, and even people present in the
room – reduce the reverberation time
• Each material bears a constant, known as the
absorption coefficient, which will affect the
calculation of Reverberation time
• Sabine’s formula postulates that the reverberation
time is directly proportional to the volume of the
room, and inversely proportional to the surface area
and absorption coefficients of all the materials in the
room

SHORTCOMINGS in SABINE’S FORMULA:
• Sabine's formula, while a foundational tool in acoustics, has several shortcomings
• Assumption of diffuse sound field:
• Sabine's formula assumes a perfectly diffuse sound field where sound waves are equally distributed throughout the
room, which is rarely achieved in practice, especially in rooms with irregular shapes or localized absorption areas.
• Low absorption requirement:
• The formula works best when the overall absorption in a room is low and becomes less accurate as absorption levels
increase.
• Neglecting room geometry:
• Sabine's formula does not directly account for the specific geometry of a room, which can significantly impact sound
propagation and reverberation.
• Frequency dependence limitations:
• The formula does not adequately consider how sound absorption coefficients vary across different frequencies,
which can affect the accuracy of reverberation time calculations in different frequency ranges.
• Edge effects ignored:
• Sabine's formula does not consider the effects of sound diffraction at room edges, which can influence sound
distribution.

EYRING’S FORMULA:
• Eyring’s formula doesn’t vary from Sabine's formula in
the base assumptions made for calculations,
regarding diffuse sound field, room geometry and
frequency limitations
• Unlike the Sabine formula, though, the Eyring
equation is designed to provide more accurate
reverberation time estimations in rooms with high
sound absorption, often called "dead" rooms.
• The key difference is that the Eyring formula uses a
logarithmic function of the absorption coefficient,
which better accounts for the effects of high
absorption levels.
• This formula is commonly used in acoustics design
for spaces like recording studios or critical
listening environments where precise control over
reverberation is important.

SAMPLE PROBLEM
A classroom, 18m long by 10 m wide by 5 m high, has sound absorption coefficients ( ) of 0.30 for walls, 0.04 for
ɑ
ceilings and 0.10 for the floor, for a frequency of 500 Hz.
• Find the reverberation time T, using Sabine’s and Eyring’s formula, respectively
• Find the reverberation time T, if 50% of the ceiling was panelled with an acoustic material with absorption
coefficient ( ) of 0.85
ɑ

EFFECTS of REVERBERATION TIME
ON SPEECH:
• Clarity – A long RT (above 1.5 seconds) causes
overlapping of speech sounds, making words less
distinct and reducing intelligibility. Short RT (around
0.5 seconds) improves clarity.
• Echo and Smearing – Excessive reverberation leads
to prolonged reflections, causing syllables and words
to blend, making it difficult to understand fast speech.
• Loudness and Warmth – Moderate RT can enhance
perceived loudness and add a sense of warmth to
speech, which may be beneficial in certain settings
like auditoriums.
• Listening Effort – High RT forces listeners to exert
more cognitive effort to separate speech from
reverberant noise, increasing listening fatigue.
• Optimal RT for Speech – Ideal RT for speech is
typically between 0.4 to 0.8 seconds, depending on
room size and purpose (e.g., classrooms require
lower RT for better learning conditions).

EFFECTS of REVERBERATION TIME
ON MUSIC:
• Tonal Richness and Warmth – Longer RT (above
1.5 seconds) enhances musical fullness, adding
depth and warmth, which is desirable for orchestral
and choral music.
• Clarity vs. Blend – Short RT (below 1 second)
improves clarity but may make music sound dry and
lifeless. Longer RT allows for better blending of notes,
beneficial for sustained and harmonic music.
• Rhythmic Definition – Excessive RT can blur fast or
percussive music (e.g., jazz, rock), making rhythms
less distinct. Shorter RT helps maintain rhythmic
precision.
• Spatial Perception – Longer RT enhances the sense
of space and envelopment, making the music feel
more immersive. Short RT provides a more intimate,
focused sound.
• Optimal RT for Music – Ideal RT varies by genre:
around 1.2–2.0 seconds for classical and choral
music, 1.0–1.5 seconds for jazz, and 0.5–1.0
seconds for amplified or speech-heavy
performances.

Module 1 Part 2.pptx building servicesssss

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