ACOUSTICS

Module - 2
Sound in Rooms
Room acoustics describes how sound behaves in an enclosed space.
The way that sound behaves in a room can be broken up into roughly
four different frequency zones:

• The first zone is below the frequency that has a wavelength of twice
the longest length of the room. In this zone, sound behaves very
much like changes in static air pressure.
• Above that zone, until the frequency is approximately
11,250(RT60/V)1/2, wavelengths are comparable to the dimensions
of the room, and so room resonances dominate.
• The third region which extends approximately 2 octaves is a
transition to the fourth zone.
• In the fourth zone, sounds behave like rays of light bouncing around
the room.
1) REFLECTION (x > 4 λ)

Reflection is the return of a sound wave

from a surface. If the surface dimension x
is larger than about 2 to 4 times the
wavelength (λ) of the impinging sound
wave, the angle of incidence angle i will
equal the angle of reflection angle r.

2) DIFFUSION (x = λ)

Diffusion is the scattering or random

redistribution of a sound wave from a surface. It
occurs when the surface depths of hard-surfaced
materials are com parable to the wavelengths of
the sound. Diffusion does not “break up” or
absorb sound—sound is not fragile or brittle.
However, the direction of the incident sound
wave is changed as it strikes a sound-diffusing
material. Diffusion is an extremely important
characteristic of rooms used for musical
performances. When satisfactory diffusion has
been achieved, listeners will have the sensation
of sound coming from all directions at equal
levels.
CONCAVE REFLECTOR
Concave sound-reflecting surfaces (such as barrel-
vaulted ceilings in churches and curved rear walls in
auditoriums) can focus sound, causing hot spots and
echoes in the audience seating area. Because concave
surfaces focus sound, they also are poor distributors of
sound energy and therefore should be avoided where
sound-reflecting surfaces are desired (e.g., near stage,
lectern, or other source locations in rooms).

FLAT REFLECTOR
Flat, hard-surfaced building elements, if large enough
and oriented properly, can effectively distribute
reflected sound. The reflector shown is tilted slightly
to project sound energy toward the rear of an
CONVEX REFLECTOR
Convex, hard-surfaced building elements, if
large enough, can be most effective as sound-
distributing forms. The reflected sound energy
from convex surfaces diverges, enhancing
diffusion, which is highly desirable for music
listening. In addition, reflected sound from
convex surfaces is more evenly distributed
across a wide range of frequencies.
3) DIFFRACTION (x < λ)

Diffraction is the bending or “flowing” of a

sound wave around an object or through an

opening. A single frequency can be

emphasized (called diffraction grating effect)

when an array of small overhead panels are of

equal length and width or vertical projecting

slats on walls are of equal depth and spacing. -

SOUND ABSORPTION AND TRANSMISSION

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.
1. Transmitted energy
A material's sound absorbing properties are 2. Converted energy
expressed by the sound absorption coefficient, 3. Incident energy
4. Reflected energy
α, (alpha), as a function of the frequency. α
ranges from 0 (total reflection) to 1.00 (total
absorption).  
Reverberatio
• Reverberation Definition : sound that persists in an enclosed space, as a result of repeated
reflection or scattering, after the sound source has stopped.
• Reverberation Time : RT : RT60 or T60 – It is the time interval with in which the sound level
in a room has faded away by 60 dB.
• Measuring reverberation times also enables the calculation of the total Sound Absorption
of a room. The reverberation time varies with frequency.
• Reverberation Time may range from 0.1 seconds (or less) in Anechoic Chambers, to 10 or
more seconds in large public spaces.

• Reverberation Times vary between positions in a room, so it is usually measured at several

positions and the average is taken.
• Optimum Reverberation Time : The reverberation time which is most desirable for a given
room size and a given use.
The Sabine equation The Sabine equation is named for its inventor, and tells
us how many sabins of absorption we would need in order to achieve a specified
sound decay time for a given room. A sabin is defined as one sq. ft. of surface
area that is 100% absorbent at all frequencies. Here’s the equation:

Reverberation Time ie. RT60 = .049 V

                Sa
where  RT60 = reverb time, seconds
            V = room volume, cu. ft.
             S = room surface area
             a = average absorption coefficient of room surfaces = α · S
α = absorption coefficient or attenuation coefficient
S = absorbing surface area

Or
Reverberation Time ie. RT60 = .161 V
               Sa
Where V = room volume in cu. metre
For example, heavy carpet may absorb 70% of the sound at 4 kHz
(.70), 50% at 500 Hz (.50), and only 10% at 150 Hz (.10). A more
ideal material like 8” compressed fiberglass spaced 8” from the
surface would be closer to 100% at all frequencies. Although it is
wise to run the numbers for different frequency bands to account
for different materials’ absorption coefficients, we’ll run a simple
solution of the equation presuming a 1.00 absorbing material for
a simple rectangular room. If we have a 15 ft. x 20 ft. x 30 ft.
room and we desire a 1 sec. reverb time, then substituting 1 for
RT60, 9000 cu. ft. for V, 2700 sq. ft. for S, we get:
1 = .049 x 9000
      2700 x a    
Solving for a, we get a =  441   = .16333 
                                          2700
So if .16333 is the average absorption coefficient of all surfaces,
roughly 16% of the surface area of the room would need to be
treated with our theoretically perfect absorber material to
achieve a 1 sec. reverb time.
CHECKLIST FOR ROOMS
When auditoriums are used primarily for speech (e.g., theaters, conference rooms, classrooms),
the design goal should be high intelligibility of spoken words throughout the room. To achieve
high signal-to-noise ratios (> 15 dB), rooms should be shaped to direct sound from the speaker’s
location toward the audience, designed to avoid echoes and “hot” (or “bright”) spots, and
planned to have low background noise levels. Important acoustical parameters affecting the
perception of speech in lecture rooms are summarized by the list below.
1. To help achieve satisfactory loudness, provide compact room shape with relatively low room
volume.
2. 2. Reverberation times should be less than 1.2 s from 250 to 4000 Hz for theaters and less
than 0.8 s for classrooms. Long reverberation times reduce the intelligibility of speech the
same way noise masks speech signals . Select sound-absorbing finishes so absorption will be
constant within the frequency range for speech. It is preferable to place absorption on side
walls rather than on ceilings. In small rooms, use sound-absorbing panels with air space
behind to prevent “boominess” at low frequencies.
3. Distance between speaker and the rear of the audience area should be short so that loudness will be
sufficient throughout the room and the audience will have ability to see the person talking. For drama, it's
difficult to see expressions of performers beyond 40 ft gestures beyond 65 ft. and large body movements
beyond 100 ft. For fan-shaped rooms, seating should be within 1400 angle measured at the location of the
speaker.

4. Ceiling or overhead sound-reflecting surfaces should provide short-delayed sound reflections directly to
the audience.

5. Seating should be sloped greater than 70 to provide good sight lines and re duce audience attenuation.

6. Background noise levels from the mechanical system should not exceed 34 dBA. Enclosing constructions
should reduce intruding noise to below this preferred criterion to avoid interference with desired sounds and
prevent distractions. Even lower limits should be considered where rooms are to be used by young children,
older adults, or hearing-impaired persons.
7. When seating capacity exceeds about 500, provide a sound-reinforcing system to augment the natural
sound from source to listener. Smaller lecture rooms, courtrooms, conference rooms, and the like may also
require a sound-reinforcing system to assist weak-voiced speakers and to project recorded material evenly.
Noise Source
Noise can come from many places. Let us see a few good sources:

Household sources:
Gadgets like food mixer, grinder, vacuum cleaner, washing machine and dryer, cooler, air
conditioners, can be very noisy and injurious to health. Others include loud speakers of  sound
systems and TVs, ipods and ear phones. Another example may be your neighbor’s dog barking all
night everyday at every shadow it sees, disturbing everyone else in the apartment.

Social events:
Places of worship, discos and gigs, parties and other social events also create a lot of noise for the
people living in that area. In many market areas, people sell with loud speakers, others shout out
offers and try to get customers to buy their goods. Whether it is marriage, parties, pub, disc or
place of worship, people normally flout rules set by the local administration and create nuisance in
the area.

Commercial and industrial activities:

Printing presses, manufacturing industries, construction sites, contribute to noise pollutions in
large cities. In many industries, it is a requirement that people always wear earplugs to minimize
their exposure to heavy noise. Apart from that, various equipments like compressors, generators,
exhaust fans, grinding mills also participate in producing big noise.

Transportation:
Think of aero planes flying over houses close to busy airports like Heathrow (London) or Ohare
(Chicago), over ground and underground trains, vehicles on road—these are constantly making a
lot of noise and people always struggle to cope with them.
Effect Of Noise Polluti
Noise pollution effects both health and behavior. Unwanted sound (noise) can damage
psychological health. Noise pollution can cause hypertension, high stress levels, tinnitus, hearing
loss, sleep disturbances, and other harmful effects.

1. Hearing Problems: Our ears can take in a certain range of sounds without getting damaged.
Man made noises such as jackhammers, horns, machinery, airplanes and even vehicles can be
too loud for our hearing range. Constant exposure to loud levels of noise can easily result in the
damage of our ear drums and loss of hearing. It also reduces our sensitivity to sounds that our
ears pick up unconsciously to regulate our body’s rhythm.

2. Health Issues: Excessive noise can influence psychological health. Studies show that the
occurrence of aggressive behavior, disturbance of sleep, constant stress, fatigue and
hypertension can be linked to excessive noise levels. These in turn can cause more severe and
chronic health issues later in life.

3. Sleeping Disorders: Loud noise can certainly hamper your sleeping pattern and may lead to
irritation and uncomfortable situations.

4. Cardiovascular Issues: Blood pressure levels, cardio-vascular disease and stress related heart
problems are on the rise. Studies suggest that high intensity noise causes high blood pressure
and increases heart beat rate as it disrupts the normal blood flow.

5. Trouble Communicating: High decibel noise can put trouble and may not allow two people to
communicate freely.
Noise Leve
In atmospheric sounding and noise
pollution, ambient noise level
(sometimes called background noise
level, reference sound level, or room
noise level) is the background sound
pressure level at a given location,
normally specified as a reference
level to study a new intrusive sound
source.

Ambient noise level is measured

with a sound level meter. It is
usually measured in dB above a
reference pressure level of 0.00002
Pa, i.e., 20 μPa (micropascals) in SI
units. Sounds that are louder than
85 dB can cause permanent hearing
loss.
Noise Level Chart
A noise level chart showing
examples of sounds with dB levels
ranging from 0 to 180 decibels.
What is noise criteria curv
It is basically a curve on a graph, with

octave bands on the x-axis, and Sound

Pressure Level on the y-axis. they are

used by designers and engineers as well

as acoustics engineers to determine the

noise floor of a given space.

Noise Criterion - NC - were established

in U.S. for rating indoor noise, noise

from air-conditioning equipment etc. In

Europe it is common to use Noise

Rating Curves - NR.

Recommended Noise Criterion - NC
The noise in different types of rooms should not exceed the Noise Criterion limits listed below:
Noise Contro
NOISE CONTROL – GENERAL
• Noise control is NOT sound proofing.
• Noise control is a reduction of sound expressed in decibels (dB).
• Over a range of frequencies (Hz) from low (bass) to high frequency
ir Borne and Structure Bor
Sound
Noise control or noise mitigation is a set of strategies to reduce noise pollution or to reduce the
impact of that noise, whether outdoors or indoors.

Noise control techniques include:

• Sound insulation: prevent the transmission of noise by the introduction of a mass barrier.
Common materials have high-density properties such as brick, thick glass, concrete, metal
etc. A sound baffle is a construction or device which reduces the strength (level) of airborne
sound.

• Sound absorption: a porous material which acts as a ‘noise sponge’ by converting the sound
energy into heat within the material. Common sound absorption materials include decoupled
lead-based tiles, open cell foams and fiberglass

• Sound isolation: Noise isolation is isolating noise to prevent it from transferring out of one
area, using barriers like deadening materials to trap sound and vibrational energy. Vibration
isolation: prevents transmission of vibration energy from a source to a receiver by
introducing a flexible element or a physical break. Common vibration isolators are springs,
rubber mounts, cork etc.

• Sound masking is the active addition of noise to reduce the annoyance of certain sounds; the
opposite of soundproofing.

• Electronic quieting: Electronics, sensors, and computers are now employed to reduce
vibration
• Sound refraction & Sound Redirection: the principle of sound refraction can be used to
prevent certain observers from hearing the noise. To reduce the received sound level of an
observer , the observer is placed out of the path of the highest amplitude sounds

• Vibration damping: applicable for large vibrating surfaces. The damping mechanism works
by extracting the vibration energy from the thin sheet and dissipating it as heat. A common
material is sound deadened steel. Acoustic quieting is the process of making machinery
quieter by damping vibrations to prevent them from reaching the observer.

• Material selection: By choosing nonmetallic components, the transmission of sound and

vibrations can be minimized.

• Acoustic decoupling: certain parts of a machine can be built to keep the frame, chassis, or
external shafts from receiving unwanted vibrations from a moving part. Example: Volkswagen
has registered a patent for an "acoustically decoupled underbody for a motor vehicle.“

• Hearing protection: An observer may be forced to wear ear plugs in areas of high ambient
noise levels.
The definition of transmission loss
(TL) is, " The accumulated decrease
in acoustic intensity as an acoustic
pressure wave propagates
outwards from a source." As the
acoustic wave propagates outwards
from the source the intensity of the
the signal is reduced with
increasing range due to:
1) Spreading
2) Attenuation
Sound Transmission Loss (STL)
represents the amount of sound, in
decibels (dB), that is isolated by a
material or partition in a particular
octave or 1/3 octave frequency
band.