PSY/MC/P-III

Acoustics of Buildings
Introduction:

Acoustics of building is the part of science of physics which deals with the control of sound in
buildings, cinema hall, auditoriums etc. The purpose of this control is to create conditions by which
people can hear with comfort. It is necessary to consider the principles of sound and the factors
affecting the architectural acoustics.

Sometimes buildings are constructed such that there is no consideration about the acoustical
properties of rooms and halls. Such a construction leads that people cannot hear the sound with
comfort. So, proper design and planning is needed for construction of hall or room so as to hear the
best sound to every audience in the hall.

The branch of Physics which deals with the planning, design and construction of a hall,
auditorium or a room to provide the best audible sound to every audience is called Acoustics of
Building.

Essential Features about Good Acoustics:

According to Prof. W.C. Sabine an auditorium must be acoustically good. These are the
essential features about good acoustics.
1. The sound heard in an auditorium or a hall must be sufficiently loud to every part of
the hall and there should not be echo.
2. The quality of speech of music must remain same always.
3. There should not be overlapping of successive syllables, for the sake of clarity.
4. The reverberation should be quite proper (~ 1 to 2 seconds)
5. There should be no concentration of sound in any part of the hall.
6. The boundaries should be sound proof to exclude extraneous noise.
7. There should be no echelon effect.
8. There should be no resonance within the building.

Reverberation and Time of Reverberation:

When a sound is produced in a hall or building, it lasts for long time. The listener receives
the sound directly from the source and then after subsequently reflection from walls, window,
ceiling and floor of the hall. The small part of the sound energy is lost at every reflection. Thus,
listener receives series of sounds of diminished intensity, which creates confusion to listener.
Reverberation: Reverberation is defined as the persistence or prolongation of sound in a
hall even though the sound source is stopped. The prolonged reflection of sound from the
walls, ceiling, and floor of the room or hall is called reverberation.
Time of reverberation: The time taken for the sound to fall below the minimum
audibility measured from the instant when the source stopped emitting.
Reverberation time or time of reverberation is also defined as the time required for sound
to decay 60 dB from its initial level.
Sabine found that the time of reverberation depends upon the size of the hall, loudness of

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the sound and the kind of music or sound for which hall is to be used.
Acoustically good auditorium or a hall is that where time of reverberation is negligibly
small. In case of a speech, series of notes are produced in a hall, each one has its own intensity.
The rate of decreasing intensity of an impulse should be such as to allow the other without
confusion. Hence there should not be any confusion.
It was found that, for a sound of frequency 512 Hz, the time of reverberation is 1 to 1.5
seconds for a capacity of 4500 m2 hall. To avoid longer time of reverberation, anti-
reverberation materials must be used. Porous tiles, asbestos for plastering can be used to
reduce reverberation time.

Sabine Formula:

Prof. Wallace C. Sabine (1868-1919) of Harvard University investigated architectural

acoustics scientifically, particularly with reference to reverberation time. According to Sabine
that the reverberation time is:
 directly proportional to the volume of the hall
 inversely proportional to the effective absorbing surface area of the walls and the
materials inside the hall
V
T 
aS
where, V is the volume of the hall, ‘a’ is the absorption coefficient of an area S. If the volume
is measured in cubic meters and area in square meters, then the experimentally obtained value
of the constant of proportionality, according to Sabine is 0.161. Then,
0.161V
T=
∑ aS
If there are different absorbing surfaces of area S1, S2, S3, S4, etc., having absorption
coefficients a1, a2, a3, a4 etc., then,
0.161 V
T=
a1S1  a 2S2  ... ... ... ...

Coefficient of Absorption:

The coefficient of absorption of a material is defined as the ratio of sound energy absorbed by the
surface to that of total incident sound energy on the surface. Its unit is Sabine.

1 sound energy absorbed by surface

Thus, Absorption coefficient (a)¿ =
T sound energy incident by surface

Let a source of audible frequency (512 Hz) is excited in the hall and the time of reverberation is
determined without absorbing material.

0.161 V
T 1= then
∑ aS
1
=
∑ aS … …(1)
T 1 0.161V

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Where, a = coefficient of absorption, S = surface area, V = volume of the room or a hall.
If a1 is the absorption coefficient of sound absorbing material in the hall and S1 be the area of
0 .161V
absorbing part then the reverberation time (T2) is T 2=
∑ aS+ a1 S1 then
1 ∑ aS+ a1 S 1
= … …(2)
T2 0.161V

1 1 a 1 S1
− = … …(3)
T 2 T 1 0.161V

∴ a1 =
S1 (
0.161 V 1
−
1
T2 T1 )
… …(4 )

Acoustics of Buildings:

Some of the common defects in music halls and auditorium are the presence of
excessive Reverberation time, Focusing and interference effect, Echoes and Echelon effect,
Resonance and Extraneous noise. To avoid these defects Sabine prescribed the following
requirements for a good auditorium:
1. The sound should be sufficiently loud in every part of the hall.
2. The quality of sound must remain unaltered.
3. The successive sound of speech must remain distinct & must be free from one another
and from extraneous noises.
4. There should not be undesirable focusing of sound in any part of the hall and depletion
in any other.
5. There should not be unpleasant reinforcement of any articulation by objects inside the
hall.
6. There should not be any vibration due to resonance.
7. There should not be any other noise from other source in the hall.

Factors Affecting the Acoustics of Buildings:

The reverberation is one of the important factors affecting the acoustics of the auditorium. There are
additional factors like focusing of sound, echoes, unwanted resonance, interference, echelon effect and
extraneous noise.

1. Focusing of sound:
The sound waves falling on concave surfaces of buildings after reflection get focused at a point. Due to
this rise intensity of sound to be maximum at such points & zero at other places. This is called focusing
effect. Concentration of sound into one part of the hall causes dead spaces in other parts. Hence, sound
cannot be heard uniformly with equal intensity throughout the hall. When defect like this is detected, to
cover this surface with poorly reflecting materials, having walls rough and using number of cloth
curtains. Curved surface can be avoided.

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2. Echoes:

Echoes produced from sound getting reflected at walls and surfaces at large distances returning long
after the original impulse have died down. Echoes are particularly troublesome in big halls. They can
be removed almost entirely by making the surfaces of the walls rough and by inclining the walls
outward. A full hall is less troubled by echoes than an empty one. Further echoes can be avoided if the
hall is provided with big windows and door.

3. Resonance:

Window panels, concave surfaces, walls, hideouts of ceilings, decorative articles, loose hanging
pendants and latches sometime pick up particular frequencies of sound impulses and will start
resonating with the sound produced in the hall due to forced vibrations. These resonant vibrations
very often stand out of tune with homogeneity of the sound impulses. Such resonance vibrations
should be damped. While it is not always possible to avoid use of such resonating objects, special
care must be taken that they are not in a position to deliver resonant sounds. This defect can be
rectified by hanging a large number of curtains with many folds in the hall.
4. Interference:

When the sound waves produce in the hall superpose with each other, interference patterns are
produced in the halls giving rise to maxima and minima of sound at various places. The vibration of
intensity in the room. At different points in the room, different responses can be heard even in the
absence of focusing surfaces. During speech, this interference patterns shift from place to place with
the change of frequency due to this effect is unnoticed. Interference can be minimized by avoiding
smooth or polished corners in the halls.

5. Echelon effect:

If there is a regular structure similar to the flight of stairs or a set of railings in the hall, the sound
produced in front of such a structure may produce a musical note due to regular successive echoes of
sound reaching the listener. Such an effect is called echelon effect. If frequency of such note is within
the audible limit, the listener will hear only this note prominently. Echelon effect affects quality of
original sound. To avoid this effect, the stair cases are covered with the absorbing material like carpets.

6. Extraneous noise:

It is due to: (i ) external sources (ii) due to internal sources. The discontinuous vibrations produced by
one objects known as noise. The external noise which reaches the hall from outside through open
windows, door and ventilators is air borne noise. Noise can be due to machinery used inside the
building. It can be minimized using double walls and sound absorbing materials like carpets. It can also
be minimized by making hall air- conditioned. The air- conditioning pipes must be covered with cork
and insulated acoustically from the main building. The noise which is conveyed through the structure
of the building is called structure borne noise. The structural vibration may be due to street traffic,
operation of heavy machines etc. it can be minimized by using double walls with air space between
them.

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Sound Distribution in an Auditorium:

From an acoustical perspective, auditorium presents a unique design challenge. The two primary uses of
the auditorium; dramatic and musical, each have inherent acoustical requirements.
To function well, the auditorium must be conducive to its intended use. An auditorium with proper
acoustics encourages the audience to contemplate the acoustical content of the sound source. An
auditorium with poor acoustics is immediately apparent and detracts from the presentation.

Auditorium Design Criteria:

Successful acoustics in the auditorium are built on four pillars:
9. The background noise level must be low enough so as not to interfere with the perception of
the desired sound,
10. The desired sound(s) must be sufficiently loud,
11. The sound within the auditorium should be distributed with considerable uniformity (this
statement implies the avoidance of focusing, echoes, and areas of deficient sound level when
compared with other positions in the room), and
12. The reverberation time should be well-suited to the intended use of the space.
1. Background Noise:
A common metric used to characterize background noise is known as the noise criterion rating.
This is a single number rating given to the measured background sound levels within a space. The
background sound, which contributes to the noise criterion rating, can originate from any number of
sources, including the air handling equipment, noise in adjacent rooms, nearby traffic, lighting dimmers,
etc.
When designing to achieve the acceptable noise criterion rating for an auditorium, knowledge of
another acoustical metric is required. The sound transmission class rating for a wall or roof indicates the
ability of that composite construction to resist the transmission of sound from one side of the
construction to the other. A high Sound Transmission Class (STC) rating indicates the composite
construction in question (wall or roof) functions well in preventing undesired sounds from entering the
auditorium. An auditorium with high STC walls limits sound generated in adjacent spaces from entering
the auditorium, and conversely, high STC walls limit sound generated in the auditorium from entering
adjacent spaces.
2. Reinforcement of the Desired Sound:
Although sound reinforcement is typically a phrase used to describe the electronic equipment that
accomplishes this task, it is important to understand that the successful auditorium itself functions to
reinforce the desired sound. This reinforcement is accomplished through properly placed hard reflecting
surfaces. These surfaces provide early reflections, which reinforce the direct sound arriving at the
listeners’ ears.
3. Distribution of Sound:
The distribution of sound within the auditorium is not surprisingly related to how the space is
articulated, that is both the room’s shape and the room’s interior finish materials. The distribution of
sound over the audience area is dependent on the efficiency by which sound travels from the stage house
to the main space of the auditorium. Fig 1.1 shows that the reflected sound distributed evenly in the
whole auditorium. In the case of a musical performing ensemble, a stage enclosure is essential for
assisting in the projection of sound into the auditorium.
4. Reverberation Time:
The design of an auditorium with the correct reverberation time is perhaps the most delicate of the

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four acoustic design concepts presented here. The reverberation time of a given spaces is, roughly
defined, the time it takes for sound to decay from a given level to the threshold of audibility. The
space must be reverberant enough to meet expectations for musical performances, but not too
reverberant so as to compromise speech intelligibility. Experience with relatively reverberant spaces
reveals that good speech acoustics may be achieved if the ratio between the direct sound energy and the
reverberant energy is high. Also, if the space is sufficiently large enough so as to require an integrated
speech reinforcement system, highly directional loudspeakers may be employed to distribute sound to
the audience with a minimum excitation of the reverberant room. Typical reverberation times for
auditoria range from 1.2 to 1.5 seconds. Because the reverberation time measured in an auditorium is
related to the amount of sound-absorbing material present, it is influenced by the number of people
sitting in the audience. To maintain a certain degree of reverberation time invariability with respect to
audience size, it is important to choose upholstered seats; upholstered seats most closely resemble the
absorption characteristics of seated audience members.

Assignment questions:
1. Define reverberation time. State Sabine’s formula for reverberation time. State its
significance.
2. What is meant by reverberation and reverberation time?
3. Define absorption coefficient of a material and hence determine the relation between
reverberation time of a hall and absorption coefficient.
4. Explain the factors affecting the acoustics quality.
5. State the features that the acoustically auditorium should have.
6. Describe in detail sound distribution in an auditorium.