2. Acoustics – What we will cover
1. Acoustics I
• Properties and defects of sound
• Parameters for good acoustical condition of a room
2. Acoustics II
• Noise control methods for air-borne and structure-borne noises
• Acoustical materials and construction
• Sound amplification system
3. Acoustics III
• Reverberation time calculation and recommendations for acoustical
treatment
• Acoustical treatment Layout design
3. Acoustics 1
What is Acoustics?
The study of the generation, propagation and transmission of sound.
Architectural Acoustics can be defined as the science, study and application of
acoustic principles as they are implemented inside a dwelling or building or structure.
Sound
Desirable Sound Undesirable Sound (Noise)
•Live Concert
•Lecture
• Seminar
•Speeches
•Theatres
•Traffic, Heavy Vehicles
•Loud music in late-night
party
•Factory/Workshop
•Train/Flight
4. Acoustics 1
Fundamentals of Sound
• Sound is generated by oscillations/vibration
(Wave format).
• Sound requires a medium to propagate.
• Sound waves are elastic in nature. Sound
travels in medium that has the properties of
mass and elasticity.
• If a particle of such a medium is displaced
then the elastic forces present will tend to
pull the particle back to its original position.
• The displaced particle possesses inertia and
can therefore transfer momentum to a
neighboring particle. The initial disturbance
can therefore be propagated throughout the
entire medium.
• Sound waves are longitudinal waves.
5. Acoustics 1
Frequency of Sound
When the oscillation repeats itself, the
motion is said to have completed one cycle.
The number of cycles per second is called
the frequency, f. The unit of frequency is the
Hertz. 1 Hertz = 1 cycle/sec. The time taken
for the oscillation to repeat itself is known
as the period, T.
Sound emitted at a single frequency is called
a pure tone
The wavelength, A, is the distance
between two successive pressure
maxima or between successive
pressure minima in a plane wave.
Wavelength of Sound
6. Acoustics 1
Frequency & Wavelength Relationship
Frequency is inversely proportional to Wavelength. Shorter the Wavelength, more will
be the no. of cycles per unit time.
c=λf, Where, f is frequency
λ is wavelength
and c is a constant denoting speed of sound~340m/s
7. Acoustics 1
Displacement & Amplitude
The distance between the instantaneous position of a vibrating particle and its mean
position is the displacement of the particle.
The maximum displacement experienced by a vibrating particle is known as the
amplitude of vibration.
The amplitudes range from 13 about 1 0 - 7 mm up to a few mm. The smaller
amplitude corresponds to the sound which is just perceptible by the ear while the
greater amplitude is the limit beyond which the ear would suffer damage.
8. Acoustics 1
Directivity of Sound
Directivity is a measure of the directional characteristic of a sound source. It is often
expressed as a Directivity Index in decibels, or as a dimensionless value of Q.
Directivity is important because it helps indicate how much sound will be directed
towards a specific area compared to all the sound energy being generated by a source.
9. Acoustics 1
Intensity of Sound – Decibel
The decibel (abbreviated dB) is the unit used to measure the intensity of a sound.
The human ear is incredibly sensitive. Your ears can hear everything from your
fingertip brushing lightly over your skin to a loud jet engine. In terms of power, the
sound of the jet engine is about 1012
times more powerful than the smallest audible
sound. Decibel scale is designed to address this huge variation.
On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound
10 times more powerful is 10 dB. A sound 100 times more powerful than near total
silence is 20 dB. A sound 1,000 times more powerful than near total silence is 30 dB.
1. Near total silence - 0 dB
2. A whisper - 15 dB
3. Normal conversation - 60 dB
4. A lawnmower - 90 dB
5. A car horn - 110 dB
6. A rock concert or a jet engine - 120 dB
7. A gunshot or firecracker - 140 dB
11. Acoustics 1
Acoustics of a room
Consider a sound source which is situated in a room.
Sound waves will propagate away from the source until they encounter one of the
room's envelope/enclosing wall where, in general, some of the sound energy will be
reflected back into the room, some will be absorbed and some will be transmitted
through the boundary.
The complex sound field produced by the multitude of reflections and the behavior of
this sound field as the sound energy in the room is allowed to build up and decay
constitutes the acoustics of the room.
12. Acoustics 1
Acoustics of a room
The sound waves behave similar to light waves. Like
the light waves, Sound waves also follow the laws of
reflection when sound waves are incident on a rigid
surface/wall.
If the wall surface is curved, sound waves will either be focused or dispersed
depending on whether the surface is concave or convex .
13. Acoustics 1
Acoustics of a room
Considering the angle of incidence of sound waves, large spaces can be design to
enhance sound quality experience of users.
14. Acoustics 1
Growth and decay of Sound
When a source begins generating sound within a room, the sound intensity measured
at a particular point will increase suddenly with the arrival of the direct sound and will
continue to increase in a series of small increments as indirect reflections arrive and
act to contribute to the total sound level. Eventually an equilibrium is reached where
the sound energy absorbed by the room surfaces is equal to the energy being radiated
by the source. This equilibrium is usually found quite quickly because the absorption of
most building materials is proportional to sound intensity. Thus, as sound levels
increase, so too does their absorption.
If a constant sound source is abruptly switched off, the sound intensity at any point will
not suddenly disappear, but will fade away gradually as the indirect sound field begins
to die off. This occurs because of the path difference between the direct sound and all
the different reflections. Even after the source has been turned off, some of its energy
will still be bouncing around on complex reflection paths. As more surfaces are hit,
more energy is lost and the reflections get weaker and weaker. The rate of this decay is
a function of room shape and the amount/position of absorbent material.
15. Acoustics 1
This gradual decay of sound energy is known as reverberation.
Reverberation is the phenomenon of persistence of sound after it has been stopped as
a result of multiple reflections from surfaces such as furniture, people, air etc. within a
closed surface. These reflections build up with each reflection and decay gradually as
they are absorbed by the surfaces of objects in the space enclosed.
16. Acoustics 1
The reverberation time defined as the time taken for a sound to decay by 60 dB after
the sound source is abruptly switched off.
Reverberation Time
The empirical relationship between the volume of the space, the amount of absorptive
material within the space and the reverberation time is termed as Sabine’s Formula.
RT=0.161V
A
Where, RT = Reverberation time
V = volume of space in m3
A=Total Absorption of space in m2-sabins (Absorption Coefficient)
The absorption unit of 1m2 -sabin represents a surface capable of absorbing sound at
the same rate as 1m2 of a perfectly absorbing surface e.g. an open window.
The absorption coefficient of a material, as originally defined by Sabine, is the ratio of
the sound absorbed by the material to that absorbed by an equivalent area of open
window hence the absorption coefficient of a perfectly absorbing surface would be 1
17. Acoustics 1
Reverberation and echo are often perceived as two separate acoustical phenomena,
but in reality they are very much the same thing; just perceived differently because of
the size and geometric characteristics of a room.
Reverberation & Echo
Reverberation (reverb) is sound energy that has reflected off of several surfaces or
structural boundaries.
Echo is reflected sound energy, which exceeds our ear/brain "integration time".
The human ear/brain system perceives direct sound, and reflected sound that arrives
within a range of about 30-60 milliseconds (ms), as being one in the same signal. The
two discrete time arrivals are integrated or merged into one. The time arrival of the
two is so close that we can't tell them apart.
• 60ms represents the upper limits of this integration time, after which the late
arriving sound is perceived as a discrete echo.
• Based on many factors, the integration time for music can be slightly longer than 60
ms, but speech integration, and therefore intelligibility, are almost always the
dominant concern.
18. Acoustics 1
Flutter echoes occur between hard parallel walls where the sound repeatedly bounces
back and forth.
Flutter Echo
19. Acoustics 1
1. An appropriate reverberation time
2. Uniform sound distribution
3. An appropriate sound level
4. An appropriately low background noise (undesirable sound)
5. No echo or flutter echo
Good Acoustical Condition/Acoustical Comfort
