Acoust.ppt

 Acoustics
 The science of sound, including its
production, propagation and effects
 The objective study of the physical behavior
of sound in an enclosed space
 Sound
 A wave motion consisting of a series of
condensations and rarefactions in an elastic
medium produced by a vibrating body

Requirements to Produce
Sound
1. Presence of vibrating body
2. Presence of transmitting medium
3. Presence of receiving medium

AUDIBLE FREQUENCY RANGE
 Infrasonic/Subsonic
 frequencies below the audible range
 Ultrasonic/Supersonic
 frequencies above the audible range
Audible Range: 20 Hz– 20kHz

General Interpretations of
Sound
1. Physical phenomenon consisting of
wave motion in a transmitting
medium (objective)
2. Sensation due to outside simulation
(subjective)

Physical Properties of Sound
1. Amplitude – magnitude of the
vibration (pressure, current, voltage)
2. Period – time it takes to complete a
vibration/cycle
3. Frequency – number of vibrations /
cycle per unit time

Physical Properties of Sound
4. Wavelength – physical length of a
vibration
5. Velocity of Propagation
Vsound << VRF
(344 m/sec << 3 x 108 m/sec)

Velocity of Sound
Solids
Where:
E = Young’s Modulus of elasticity, dynes/cm3
d = density of the medium, g/cm3

Velocity of Sound
Liquids
Where:
E = Bulk’s Modulus of elasticity, dynes/cm3
d = density of the medium, g/cm3

Velocity of Sound
Gases
Where:
k = specific heat ratio = hsp/hsv
hsp = specific heat at constant pressure
hsv = specific heat at constant volume
p = gas pressure, dynes/cm2
d = density, g/cm3

Velocity of Sounds
Dry Air/Air (for TC ≤ 20 0C)

Velocity of Sounds
Dry Air/Air (for TC > 20 0C)
where:
TK = temperature in Kelvin

Velocity of Sounds
 Notes
 Sounds travel more slowly in gases than in
liquids, and more slowly in liquids than in
solids.
 Sounds travels slower with an increased
altitude (elevation if you are on solid earth),
primarily as a result and humidity changes.

QUESTIONS
1. Which best describes the sound wave?
a. It may be longitudinal
b. It is always transverse
c. It is always longitudinal
d. All of the above

2. Which of the following cannot travel
through a vacuum?
a. Electromagnetic wave
b. Radio wave
c. Sound wave
d. Light wave

3. Through which medium does sound
travel fastest?
a. Air
b. Water
c. Steel
d. Mercury

4. Speed that is faster than that of
sound.
a. Ultrasonic
b. Supersonic
c. Subsonic
d. Transonic

5. What is the speed of sound in air at
20°C?
a. 1087 ft/s
b. 1100 ft/s
c. 1126 ft/s
d. 200 ft/s

6. Calculate a half wavelength sound
for sound of 16000 Hz
a. 35 ft
b. 10 ft
c. 0.035 ft
d. 100 ft

7. The lowest frequency that a human
ear can hear is
a. 5 Hz
b. 20 Hz
c. 30 Hz
d. 20 kHz

8. Sound that vibrates at frequency too
high for the human ear to hear (over 20
kHz)
a. Subsonic
b. Ultrasonic
c. Transonic
d. Stereo

9. What is the speed of sound in a material
having a density of 1000 kg/cu.m. and
Young’s modulus of elasticity of 2.3 x 10exp
9 N/sq.m.?
a. 1517 m/sec
b. 1571 m/sec
c. 1715 m/sec
d. 1751 m/sec

10. In acoustics, the volume velocity
component is a function of the _____ of the
material.
a. density
b. volume
c. diameter
d. Young’s modulus

11. A sound intensity that could cause
painful sensation to the human ear.
a. threshold of sense
b. threshold of pain
c. hearing threshold
d. sensation intensity

Possibilities when a
Propagated Sound is
Obstructed (3)

Propagated Sound is
Obstructed (3)
 Sound is Reflected
 Echo
 Becomes apparent to the listener only when the
distance from the source and the reflecting medium is
great and the difference between the original and
reflected sound is greater or equal to 1/17 of a second.
 Flutter
 Brought about by a series of reflections between two
parallel surfaces resulting to prolongation of sound
 Creates listening fatigue
 Interference
 Reflection caused by two parallel surfaces, producing
standing waves

Propagated Sound is
Obstructed
 Sound is absorbed
 Conversion of sound energy to heat energy
 Onward transmission through
obstruction

Physiological Characteristics
of Wave Motion (3)
 Pitch

of Wave Motion (3)
 Pitch
 Number of cycles a wave goes through in a
definite interval
 The higher the frequency, the higher the
pitch
 Mel – unit of pitch
 1000 mels – pitch of 1000Hz tone at 40dB
 Octave – pitch interval 2:1; frequency is twice
the given tone

of Wave Motion (3)
 Tone
 Timbre quality of sound
 Pure Tone – a sound composed of only one
frequency in which the sound pressure varies
sinusoidally with time.
 Musical Sound – composed of the
fundamental frequency and its harmonics

of Wave Motion (3)
 Loudness
 Fluctuation of air pressure created by sound waves
 Observer’s auditory impression of the strength of a
sound and is associated with the rate at which
energy is transmitted to the ear.
 Depends on the amplitude of the sound
Loudness Level – measured by the sound level of a
standard pure tone or specified frequency which is
assessed by normal observers as being equally loud

PHON
Phon is the unit of loudness level
when:
 The standard pure tone is produced by a
sensibly plane sinusoidal progressive
sound wave coming from directly in front of
the observer and having the frequency of
1kHz
 The sound pressure level in the free
progressive wave is expressed in dB above
2 x 10-5 N/m2

SONE
Sone is the unit of loudness of an
individual listener.
Phon = 40 + 10 log2 sone

Sound Levels
Sound Pressure (P) and
Sound Pressure Level (SPL)
Sound Pressure
 The alternating component of the pressure at
a particular point in a sound field
 Expressed in N/m2 or Pa

Sound Levels
Sound Pressure Level
 Equal to 20 times the logarithm to the base 10
of the ratio of the RMS sound pressure to the
reference sound pressure
SPL = 20 log (P/Po)
Where:
P = rms sound pressure
Po = reference sound pressure
Po = 2 x 10-5 N/m2 or Pa or 2 x 10-4
dynes/cm2
Po = 0.0002 μbar or 2.089 lb/ft2

Sound Pressure Levels
 Sound Pressure Level (SPL) at any unit of
pressure in dB
SPL = 20log(P+N)
Where:
PN = rms sound pressure expressed in any of
pressure in dB
N = SPL constant corresponding to the unit
at which sound pressure is expressed

SPL Constants
Unit of Sound Pressure Designation
SPL Constant
(N)
Microbar μbar 74
Pascal N/m2 94
lb/ft2 psf 127.6
mmHg mmHg 136.5
torr torr 136.5
lb/in2 psi 170.8
atm (technical) atm 193.8
atm (standard) atm 194.1

Sound Levels
Sound Intensity (I) and
Sound Intensity Level (SIL)
Sound Intensity
 Defined as the acoustic power per unit area
 The basic units are W/m2 or W/cm2
 The average rate of transmission of sound
energy through a cross-sectional area of 1
m2 at right angles to a particular direction.

Sound Levels
For sound produced at ground level

Sound Levels
Sound Intensity
I = ρ2 / d v
Where: d – density of the medium (kg/m3)
v – velocity of sound in medium (m/sec)
ρ – rms pressure in Pa (N/m2)

Sound Levels
Sound Intensity in Air
I = ρ2 / 410
Where: dv – 410 ray/sec
ρ – rms pressure in Pa (N/m2)

Sound Levels
 Sound Intensity Level
Where:
I = sound intensity,
Io = threshold intensity,
Io = 10-12 W/m2 or 10-16 W/cm2

Sound Levels
Sound Power (W) and
Sound Power Level (PWL)
Sound Power (W)
 The total energy radiated per unit time.

Sound Levels
 Sound Power Level (PWL)
Where:
W = sound power , W
Wo = reference sound power
Wo = 10-12 w

12. The frequency interval between two
sounds whose frequency ratio is 2.
a. Octave
b. Half octave
c. Third-octave
d. Decade

13. A 16 KHz sound is how many octaves
higher than a 500 Hz sound
a. 2
b. 5
c. 4
d. 8

14. Sound waves composed of but one
frequency is a/an
a. Infra sound
b. Pure tone
c. Structure borne
d. Residual sound

15. Sound wave has two main
characteristics which are
a. Highness and loudness
b. Tone and loudness
c. Pitch and loudness
d. Rarefactions and compressions

16. _____ is the sound power measured over
the area upon which is received.
a. Sound pressure
b. Sound energy
c. Sound intensity
d. Sound pressure level

17. A measure of the intensity of sound in
comparison to another sound intensity
a. Phon
b. Decibel
c. Pascal
d. Watts

18. Calculate the sound intensity level in
dB of a sound whose intensity is 0.007
W/m2.
a. 95 dB
b. 91 dB
c. 98 dB
d. 101 dB

19. What is the sound pressure level for a
given sound whose RMS pressure is
200 N/m2?
a. 200 dB
b. 20 dB
c. 140 dB
d. 14 dB

20. The amplitude of sound waves, the
maximum displacement of each air particle,
is the property which perceive as _____ of a
sound
a. Pitch
b. Intensity
c. Loudness
d. Harmonics

21. If the sound source radiates 1 watt, what
is its sound power level?
a. 0 dB
b. 60 dB
c. 120 dB
d. 240 dB

22. If a note has a fundamental frequency of
100Hz, what is its 5th octave?
a. 6400 Hz
b. 3200 Hz
c. 500 Hz
d. 1600 Hz

23. What is the sound intensity for an RMS
pressure of 200 Pascal?
a. 90 W/m2
b. 98 W/m2
c. 108 W/m2
d. 88 W/m2

24. The sound pressure level is increased by
_____ dB if the pressure is doubled.
a. 3
b. 4
c. 5
d. 6

25. The sound pressure level is
increased by _____ dB if the intensity is
doubled.
a. 3
b. 4
c. 5
d. 6

26. If four identical sounds are added
what is the increase in level in dB?
a. 3
b. 4
c. 5
d. 6

27. A unit of noisiness related to the
perceived noise level
a. Noy
b. dB
c. Sone
d. Phon

28. What is the loudness level of a 1KHz
tone if its intensity is 1 x 10-
5W/cm2?
a. 100 phons
b. 105 phons
c. 110 phons
d. 100 phons

29. What is the unit of loudness of an
individual listener?
a. Sone
b. Phon
c. Decibel
d. Mel

30. It is the weakest sound that average
human hearing can detect.
a. SPL = 0 dB
b. Threshold of hearing
c. Reference pressure = 2 x 10-5N/m2
d. A, b, c

31. When waves bend away from straight
lines of travel, it is called
a. Reflection
b. Diffraction
c. Rarefaction
d. Refraction

32. The amplitude of sound waves, the
maximum displacement of each air particle,
is the property which perceive as _____ of a
sound
a. Pitch
b. Intensity
c. Loudness
d. Harmonics

Room Acoustics
 Room Acoustics
 Concerned with the behavior of sound
within an enclosed space with a view to
obtaining the optimum acoustic effect on
the occupants

Room Acoustics
 Requirements
 Adequate amount of sound must reach all
parts of the room.
 Even distribution of sound
 Noise must be reduced to an acceptable
level.
 Optimum Reverberation time, RT60

Reverberation
 Reverberation
 Tendency for the sound to persist over a
definite period of time after it has been
produced originally and stopped at the
source.

Reverberation
 Reverberation

Reverberation
 Reverberation Time, RT60
 Time taken for the density of sound energy
in the room to drop to 1 millionth (60dB)
below of its initial value

Optimum Periods of
Reverberation

Factors Affecting
Reverberation Time
 Volume of the room
 Type of materials
 Surface area of
material

TYPES OF ROOM
LIVE ROOM
- Little absorption (RT60 > 1 sec)
DEAD ROOM
- Large absorption (RT60 < 1 sec)
ANECHOIC ROOM
- 100% absorption (free field conditions)

Room Acoustics
 Coefficient of absorption, α
 Ratio of incident sound and absorbed sound
 Efficiency of sound absorption

Room Acoustics
Coefficient of Absorption

Reverberation Time
Equations
a. Sabine’s Equation
 For actual reverberation time with average
absorption less than or equal to 0.2; (absorption
coefficient, α ≤ 0.2)
Where;
V = room volume,
m3
A = total absorption
units

Reverberation Time
Equations
Where;
V = room volume, ft3
A = total absorption units

Reverberation Time
Equations
33.Calculate the reverberation time of a
broadcast studio 8 ft. high by 13 ft
wide by 20 ft. long. The material used
has a total absorption of 180.75
sabines.

Reverberation Time
Equations
b. Norris – Eyring Equation
 For actual reverberation time with average
absorption greater than 0.2; ( α ≥ 0.2 )
Where;
V = room volume, m3
α = average coefficient
of reflecting surfaces

Reverberation Time
Equations
34. A lecture room, 16 m. long, 12.5 m.
wide and 5 m. high has a reverberation
time of 0.75 sec. Calculate the average
absorption coefficient of the surfaces
using the Eyring formula.

Reverberation Time
Equations
c. Stephens and Bate Equation
 For ideal reverberation time computation
Where:
r = 4 for speech
r = 5 for orchestra
r = 6 for choir

Optimum Volume / person
Concert Halls 7.1
Italian type opera houses 4.2 – 5.1
Churches 7.1 – 9.9
Cinemas 3.1
Rooms for Speeches 2.8

Reverberation Time
Equations
35. Suggest the optimum volume and
reverberation time for a concert hall to
be used mainly for orchestral music
and to hold 450 people.

36. A church has an internal volume of
2550 cu.m. When it contains
absorption of 186 metric sabines, what
will be its reverberation time in sec.?
a. 2
b. 2.2
c. 2.5
d. 3.0

37. The transmission of sound from one room
to an adjacent room, via common walls,
floors or ceilings.
a. Flanking transmission
b. Reflection
c. Refraction
d. Reverberation

38. _____ is the continuing presence of an
audible sound after the sound source has
stop.
a. Flutter echo
b. Sound concentration
c. Sound shadow
d. Reverberation

39. Required time for any sound to decay
to 60 dB
a. Echo time
b. Reverberation time
c. Delay time
d. Transient time

40. A room containing relatively little
sound absorption
a. Dead room
b. Anechoic room
c. Live room
d. Free-field

41. A room in which the walls offer essentially
100% absorption, therefore simulating free
field conditions.
a. Dead room
b. Anechoic room
c. Live room
d. Closed room

42. Calculate the reverberation time of the
room, which has a volume of 8700 ft3 and
total sound absorption 140 sabines.
a. 0.3 sec
b. 3.5 sec
c. 3 sec
d. 0.53 sec

43. _____ is early reflection of sound.
a. Echo
b. Pure sound
c. Reverberation
d. Intelligible sound

 Microphone
 An acoustic device classified as a transducer
which converts sound waves into their
corresponding electrical impulses
 Transducer
 A device which when actuated by energy in
one transmission system, supplies energy in
the same form or in another form, to a
second transmission system

Classification of
Microphones
A. General Categories
1. Passive (Generator Type) Microphone
 Does not require external power source
2. Active (Amplifier Type) Microphone
 Needs an external power source for its
operation

Classification of
Microphones
B. According to Impedance
1. High Impedance
 Greater than 1000 ohms
2. Low Impedance
 1000 ohms and below

Classification of
Microphones
C. According to Method of Coupling
Pressure Type
- Actuated by the
pressure of sound
waves against
the diaphragm.

Classification of
Microphones
Velocity Type
- actuated by
velocity of
sound waves

Classification of
Microphones
Contact Type

Classification of
Microphones
D. According to Elements Used
1. Dynamic
 Uses the principle of electromagnetic
induction
 Electromagnetic moving coil microphone
 A medium-priced instrument of high
sensitivity

Classification of
Microphones
2. Ribbon
 Velocity microphone
 Ribbon moves as if it is a part of the air
that experiences rarefactions and
condensations

Classification of
Microphones
3. Capacitor
 Condenser type or electrostatic
microphone

Classification of
Microphones
4. Carbon
 Uses principle of variable resistance

Classification of
Microphones
5. Crystal
 Uses principle of piezoelectric effect

Classification of
Microphones
6. Magnetic
 Operated on the magnetic reluctance due to
the movable core

Classification of
Microphones
E. According to directional
Characteristics
 Unidirectional

Classification of
Microphones
Characteristics
 Bidirectional

Classification of
Microphones
Characteristics
 Omnidirectional

Classification of
Microphones
Characteristics
 Cardioid

Characteristics of
Microphone
1. Frequency Response
 Frequency over which the microphone will
operate normally
Magnetic : 60 – 10 000Hz
Crystal : 50 – 10 000Hz
Condenser : 50 – 15 000Hz
Carbon : 200 – 3 000Hz

Characteristics of
Microphone
2. Sensitivity
 Ability of the microphone to detect very
slight changes of sound.
3. Dynamic Range
 Range of sound intensity that would be
covered by the microphone

Special Types of
Microphones
 Line Microphone
 Capable of picking up sound from a great
distance at an angle of 45 degrees and is
highly sensitive

Special Types of
Microphones
Differential Microphone
 Used in noisy places; good up to 3-in
distance

53. A transducer that converts acoustic
signals into electrical signals.
a. microphone
b. loudspeaker
c. both a and b
d. none of these

54. A characteristic of a microphone which
indicates the frequency range over which
the microphone the frequency range over
which the microphone will operate
normally.
a. sensitivity
b. frequency response
c. dynamic range
d. directional characteristic

55. An ability of the microphone to detect very
slight changes of sound.
a. sensitivity
c. dynamic range

56. The range of sound intensity that would
be covered by the microphone.
a. sensitivity
c. dynamic range

57. It is an audio transducer that converts
acoustic pressure in air into its equivalent
electrical impulses
a. Loudspeaker
b. Amplifier
c. Baffle
d. Microphone

58. _____ is a pressure type microphone with
permanent coil as a transducing element.
a. Dynamic
b. Condenser
c. Magnetic
d. Carbon

59. A microphone which has an internal
impedance of 25 kΩ is _____ type.
a. High impedance
b. Low impedance
c. Dynamic
d. Magnetic

60. A microphone that uses the
piezoelectric effect
a. Dynamic
b. Condenser
c. Crystal
d. Carbon

61. It describes the output of a
microphone over a range of frequencies.
a. Directivity
b. Sensitivity
c. Frequency response
d. All of the above

62. A special microphone characterized by a
long perforated tube and high sensitivity,
suitable for TV applications.
a. line microphone
b. dynamic microphone
c. differential microphone
d. ribbon microphone

63. Using a microphone at less than the
recommended working distance will
create a _____ which greatly increases
the low frequency signals.
a. Roll-off
b. Proximity effect
c. Drop out
d. None of the choices

Loudspeakers
Are transducers that convert
electrical signals to sound
waves.

Types of Loudspeakers
Direct Radiator Type
 Those in which the vibrating surface
(diaphragm) radiates sound directly into the
air
1. Dynamic or Moving Coil Loudspeaker
 Makes use of a moving coil in a magnetic
field and a permanent magnet

Dynamic or Moving Coil Loudspeaker

Electrostatic Loudspeaker
 Operates on the same principle as a
condenser microphone

Horn Type
 Those in which a horn is interposed between the
diaphragm and the air
 Used for efficient coupling of sound into the air
 Types:
 Conical Horn
 Parabolic Horn
 Exponential Horn
 Hyperbolic Horn

 To cover the entire range of audible
frequencies, the following speakers
are used:

 Woofer – for low frequencies

Tweeter – for high frequencies

Midrange – for normal range

 Subwoofer – for very low frequencies

Loudspeaker Phasing
 When more than one speaker is used:
 Phasing must be uniform
 Polarities and voice coils are in phase such
that the cone of all the speakers move
inwards at the same instant.

Loudspeaker Enclosure
(Baffle)
 Loudspeaker mounting that is used to
prevent the sound waves from the rear
from interfering with the sound waves
in the front of speaker

DOLBY DIGITAL
 Dolby Digital is the name for audio
compression technologies developed by
Dolby Laboratories. It was originally named
Dolby Stereo Digital until 1994. Except for
Dolby TrueHD, the audio compression is
lossy.

DOLBY DIGITAL
 The first use of Dolby Digital was to provide
digital sound in cinemas from 35mm film
prints. It is now also used for other
applications such as HDTV broadcast,
DVDs, Blu-ray Discs and game consoles.

DIGITAL THEATRE SOUND
 DTS is a series of multichannel audio
technologies owned by DTS, Inc. (formerly
known as Digital Theater Systems, Inc.), an
American company specializing in digital
surround sound formats used for both
commercial/theatrical and consumer grade
applications. It was known as The Digital
Experience until 1995.

DIGITAL AUDIO
BROADCASTING
 Digital Audio Broadcasting (DAB) is a
digital radio technology for broadcasting
radio stations, used in several countries,
particularly in Europe. As of 2006,
approximately 1,002 stations worldwide
broadcast in the DAB format.[1]

64. An amplifier can deliver 100 W to a
loudspeaker. If the rated efficiency of the
loudspeaker is -60 dB. What is the
maximum intensity 300 ft from it?
a. 10 dB
b. 20 dB
c. 30 dB
d. 40 dB

65. Speaker is a device that
a. Converts sound waves into current
and voltage
b. Converts current variations into
sound waves
c. Converts electrical energy to
mechanical energy
d. Converts electrical energy to
electromagnetic energy

66. The impedance of most drivers is about
_____ ohms at their resonant frequency.
a. 4
b. 6
c. 8
d. 10

67. It is a transducer used to convert
electrical energy to mechanical energy.
a. Microphone
b. Baffle
c. Magnetic assemble
d. Driver

68. It is an enclosure used to prevent front
and back wave cancellation.
a. Loudspeaker
b. Driver
c. Baffle
d. Frame

69. A circuit that divides the frequency
components into separate bands in order to
have individual feeds to the different
drivers.
a. Suspension system
b. Dividing network
c. Magnet assembly
d. Panel board

70. What is a device that is used to measure
the hearing sensitivity of a person?
a. Audiometer
b. OTDR
c. SLM
d. Spectrum analyzer

71. _____ is a type of loudspeaker driver with
an effective diameter of 5 inches used at
midrange audio frequency.
a. Tweeter
b. Woofer
c. Mid-range
d. A or C

72. _____ is measure of how much sound is
produced from the electrical signal.
a. Sensitivity
b. Distortion
c. Efficiency
d. Frequency response

73. A loudspeaker radiates an acoustic
power of 1 mW if the electrical input is
10 W. What is its rated efficiency?
a. -10 dB
b. -20 dB
c. -30 dB
d. -40 dB

74. What is the device used in measuring
sound pressure levels incorporating a
microphone, amplification, filtering and a
display.
a. Audiometer
b. OTDR
c. SLM
d. Spectrum analyzer

75. It is the device used to calibrate an
SLM?
a. Microphone
b. Pistonphone
c. Telephone
d. Filter

76. Noise reduction system used for film
sound in movie.
a. Dolby
b. DBx
c. dBa
d. dBk

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