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1. BUILDING SCIENCES
- ACOUSTICS
Module 1

2. Content
• Introduction to Acoustics
• Physics of sound- Amplitude, Frequency, Wavelength, Period, Pitch and
Velocity.
• Behavior of sound in an enclosed space – Reflection, Absorption, Refraction,
Diffusion, Diffraction, Transmission and factors that affect the behavior of
sound.
• Criteria for acoustic environment
• Basic understanding of echo and Reverberation time
• Sound Absorption, Sound absorption coefficients and measurement of
absorption.
• Noise rating curves

3. Introduction to Acoustics
Source - The science and applications of acoustics, Daniel R.
Raichel
• Of the five senses that we possess, hearing probably ranks second only to sight.
• Animal skins, horns were used as musical instruments which were later made of
metals.
• With sound as a major factor affecting human lives, it was only natural for
interest in the science of sound, or acoustics, to emerge.
• The earliest acousticians probably falls to the Greek philosopher Chrysippus, the
Roman architect-engineer Vitruvius and the Roman philosopher Severinus
Boethius.
• Joseph Sauveur suggested the term acoustics (from the Greek word for sound) for
the science of sound.
• Acoustics is no longer limited to the telephone and broadcasting industries, the
military, and university research centers.
• Architects are increasingly using acoustical engineers and acoustical designs to
ensure environmental harmony with the esthetic aspects of their designs.
• Architectural Acoustics can be defined as the science, study and application of
acoustic principles as they are implemented inside a building or structure.

4. Introduction to Acoustics
• All acoustics situations can be described by three parts:
a) Source – speech, HVAC equipment
b) Path – air, earth, building material
c) Receiver – human/animal ear
• Acoustical requirements to be considered at earliest
stages of building as later corrections may be difficult
and extremely expensive.
• Essential elements of Architectural Acoustics :
a) Room Acoustics
Volume
Shape and Proportion
Layout – Floor space, slope, distances from source
Finishes – Selection & Placement of furnishes
Special treatments
b) Sound Isolation
Site Noise characteristics
Outdoor Barriers – building,
vegetation
Location of activities within
buildings
Wall, floor and ceiling construction

5. Introduction to Acoustics
c) Electronic sound system
System compatibility with room acoustics
Loudspeaker selection, placement &
orientation
System components and controls
d) Sound Isolation
Site Noise characteristics
Outdoor Barriers – building,
vegetation
Location of activities within
buildings
Wall, floor and ceiling construction
e) Mechanical system
Equipment characteristics
Location of mechanical equipment
Vibration isolation – springs, pads
Air duct & pipe treatment- lining

6. Introduction to Acoustics
• Sound (or noise) is the result of pressure variations, or oscillations, in an elastic
medium (e.g., air, water, solids), generated by a vibrating surface, or turbulent
fluid flow.
• Sound propagates in the form of longitudinal (as opposed to transverse) waves,
involving a succession of compressions and rarefactions in the elastic medium,
as illustrated.
• When a sound wave propagates in air the oscillations in pressure are above and
below the ambient atmospheric pressure.
Figure - Representation of a sound
wave
(a) compressions and rarefactions
caused in air by the sound wave.
(b) graphic representation of pressure
variations above and below
atmospheric pressure.

7. Introduction to Acoustics
• The decibel is commonly used in acoustics to quantify sound levels relative to a
0 dB reference which has been defined as a sound pressure level of
.0002 microbar, or 20 micro Pascal.
• The ratio of the sound intensity that causes permanent damage during short
exposure to the quietest sound that the ear can hear is greater than or equal to 1
trillion. Such large measurement ranges are conveniently expressed in logarithmic
units: the base-10 logarithm of one trillion (1012) is 12, which is expressed as an
audio level of 120 dB.
• Here are some common sounds and their decibel ratings:
a) Near total silence - 0 dB
b) A whisper - 15 dB
c) Normal conversation - 60 dB
d) A lawnmower - 90 dB
e) A car horn - 110 dB
f) A rock concert or a jet engine - 120 dB
g) A gunshot or firecracker - 140 dB

8. Physics of sound -
Wavelength of sound-
• The distance a sound wave travels during one cycle of vibration is termed as
wavelength.
• Wavelength is commonly designated by the Greek letter lambda (λ).
• The S.I unit of wavelength is meter.
• For sound waves in air, the speed of sound is 343 m/s.
• The wavelengths of sound frequencies audible to the human ear (20 Hz–20 kHz) are
thus between approximately 17 m and 17 mm, respectively.
• Wavelength is inversely proportional to frequency waves with higher frequencies
have shorter wavelengths, and lower frequencies have longer wavelengths.
wavelength

9. Physics of sound -
Frequency of sound-
• It is defined as the no. of vibrations completed by a particles in a second.
• It is also defined as no. of cycles over a period of time is known as frequency.
• Unit = Hertz(Hz) named after the 19th C. physicist 1 cycle/sec = 1 Hz
• The range of normal human hearing is approx. 20 Hz - 20 kHz; the higher number
tends to decrease with age.
• The wavelength of a sound is the inverse of its frequency.
• The formula is: wavelength (λ) = speed of sound/frequency.

10. Physics of sound -
Amplitude of sound-
• The average minimum distance of a particle from its mean position is called as
amplitude or
• This is the distance between a crest (the highest point) and a valley (the lowest
point)
• Amplitude on the positive and the negative side will be same in distances.
• Amplitude represents the loudness of sound.

11. Physics of sound -
Period of sound-
• This is the time taken for on complete oscillation. This is measured in seconds(s)
and represented with the letter ‘T’.
Velocity of sound
• This is the rate at which a sound wave travels from a source through a medium to
the receiver.
• The unit is m/s.
Pitch
• It is the highness or lowness of a tone determined by the rapidity of the
oscillations producing it. or
• It is the subjective response of human being to frequency. Low frequencies are
considered bloomy and high frequencies are screechy and hissy.

12. Behavior of sound in an enclosed space -
• An enclosed space is a room or area bounded on every of its sides. The materials
for enclosure may be classified into two:
a) Those that allow sound rays to pass through and
b) Those that do not allow sound rays to pass through.
• Areas bounded with materials that allow sound rays to pass through tend to enjoy
good acoustic as the effect of indirect sound from reflection is reduced within the
space.
• Sound from external source can pass into the enclosure as background noise.
• The vibration of some of the materials can be a source of noise within the space
which affects clarity and audibility of speech/music.

13. Behavior of sound in an enclosed space -
• On encountering barriers posed by the enclosure, sound waves are likely to behave
in the following ways:
a) Reflection
b) Absorption
c) Refraction
d) Diffusion
e) Diffraction
f) Transmission

14. Behavior of sound in an enclosed space -
a) Reflection
• Occurs when the wavelength of a sound wave is smaller than the surface of an
obstacle.
• In the case of an enclosed space, the sound waves hit every side of the enclosure
continuously until the sound energy reduces to zero.
• The amount of waves reflected depends on the smoothness, size, and softness of
the materials of enclosure.
• The angle of incidence of sound rays is equal to that of the reflected rays only if
the surface of the reflector is flat. But when it is curved, the angles are different.

15. Behavior of sound in an enclosed space -
b) Absorption
• When sound waves hit the surface of an obstacle, some of its energy is reflected
while some are lost through its transfer to the molecules of the barrier.
• The lost sound energy is said to have been absorbed by the barrier.
• The thickness and nature of the material as regards its softness and hardness
influences the amount of sound energy absorbed.

16. Behavior of sound in an enclosed space -
c) Refraction
• This is the bending of sound when it travels from one medium into another
medium.
• The difference in the composition of the two different media bends the sound i.e.
the angle of incidence changes into an angle of refraction as it travels into the
new medium.

17. Behavior of sound in an enclosed space -
d) Diffusion
• This is the scattering of waves from a surface.
• It occurs as a result of the texture and hardness of the obstacle is comparable to
the wavelength of the sound.
• The direction of the incident ray changes when it strikes the surface of the
obstacle.
• Satisfaction is achieved when sound is heard in all direction at equal level.

18. Behavior of sound in an enclosed space -
d) Diffraction
• When the wavelength of a sound wave is smaller or equal to the size of the
obstacle, the sound rays tend to bend round the edge of the obstacle thereby
turning the edge to a sound source.

19. Behavior of sound in an enclosed space -
e) Transmission
• In this phenomenon, sound wave is carried by molecules of the obstacle through
vibration and re-emitted at the other side irrespective of the medium.
• It can be structure borne, air borne or impact sound.

20. Behavior of sound in an enclosed space -
The behavior of sound in an enclosed space depends on many factors which include:
a) Reduction in its intensity of sound – due to the distance between source &
receiver.
b) Absorption of direct sound by the audience – listeners absorb sound while
hearing.
c) Absorption of direct and reflected sound by surfaces – walls, ceiling and floor
of the enclosure absorbs & reflects sound waves thereby controlling the behavior
of sound.
d) Reflection of sounds from right-angled corners - if surfaces are acoustically
reflective. This can in turn produce echoes especially in large spaces.
e) Dispersion of the sides of an enclosure - Reflections can be controlled by
making one surface dispersive i.e. not at right angle to each.
f) Edge diffraction of sound - causes the obstacles to scatter the sound waves
making it behave like a source of sound.
g) Primary reflection – This depends on the angle of incidence which is equal to the
angle of reflection. Also, the nature of sound reflector is important.
h) Panel resonance - Sound waves can propagate "through" a solid material by panel

21. Criteria for acoustic environment -
• Architectural acousticians seek to understand
and optimize the sound environment in rooms
and buildings of all types.
• Some of the earliest attempts to optimize
sound transmission were practiced in the design
of ancient amphitheaters.
• In a room, most of the sound waves that reach
the ear have been reflected by surfaces or by
objects in the room.
• Therefore the acoustical properties of rooms
play an important role in determining the
nature of the sound heard by a listener.
• The acoustical concerns in buildings are
providing the occupants with privacy and with a
quiet environment, which means dealing with
noise sources within the building as well as
noise transmitted from outside.

22. Criteria for acoustic environment -
Requirements for rooms
a) Reverberation time
The optimum value for reverberation time is
dependent on the particular use and room
volume.
b) Speech intelligibility
This is used to judge the degree of audibility of
the spoken word.
c) Impression of space
This is determined by the reception of
reflections with respect to time and direction.
For music, diffuse reflections are favorable for
sound volume.

23. Criteria for acoustic environment -
a) Reverberation time
• Several repetitive sound reflections from
different surfaces in a room reduce the
sound energy.
• Even when the sound source is stopped
some sound energy is retained in the
room for sometime.
• This process of sound energy retention is
called ‘reverberation’.
Reverberation Time (RT) :
• It is the time required for a sound of 60
dB level to become inaudible i.e. to
come down to 0 dB- level. Or
• It is the time taken by the sound to
diminish to one-millionth of its original
intensity is called reverberation time.
Effects of Reverberation:-
• Leads to overlapping of sound over
the previous sound.
• Clarity of sound decreases which
reduces the acoustical condition.
• Deadness of sound takes place and
there will be no brilliance in the
hearing.
• Early decay results in longer
prolongation of sound it becomes
loud and becomes noise
Sabine’s Formula:-
T=0.16V/S
Where, t=reverberation time, v=volume of space
in meter cube, s=total sound absorption of that
space.
t=0.16V/λa1+λa2+λa3
=0.16V/∑S

24. Criteria for acoustic environment -
b) Speech Intelligibility
• Speech level versus ambient noise level - Every effort should be made to
minimize noise due to air handling systems and outside interferences.
• In general, the noise level should be 25 dB or greater below the lowest speech
levels which are expected.
• However, for quite high levels of reinforced speech, as may be encountered
outdoors, a noise level 10 to 15 dB below speech levels may be tolerated.
• Speech clarity and intelligibility are so important that real, objective tests have
been devised and established as industry standards.
• Objective clarity is a measure of the ratio of direct sound to indirect sound in an
enclosed space; high levels of clarity indicate high levels of direct sound relative
to indirect sound.
• One such test is based on a long list of unpredictable, rhyming words. The lists
are devised to gage the percentage of lost consonants (%ALcons) that might exist
in an acoustical environment.
• Others are: The two talker test, The hand clap test and The room mode test.

25. ECHO
• They occur when sound is reflected off a boundary with sufficient magnitude and
delay to be perceived as another sound, distinct from the direct sound. Or
• This occurs when the reverberation time is long enough to cause a distinct
repetition of the direct sound.
• A true echo is a single reflection of the sound source. The time delay is the extra
distance divided by the speed of sound.
• This condition is an advanced form of reverberation where the sound is heard
clearly and repeatedly after some time until it fades.
• As a rule, if the delay is greater than 1/25 sec (14m) for speech and 1/12 sec
(34m) for music then that reflection will be a problem.
• Solution: Either alter the geometry of the offending surface or apply absorber or
diffusion.