ROHINI COLLEGE OF ENGINEERING AND TECHNOLOGY
2.6 SOUND ABSORBING MATERIALS
The special materials used to increase the absorption of sound waves or to
reduce the reflection of sound waves in a room or hall are known as sound
absorbing materials.
The important facts in connection with sound absorbing materials are
as follows:
An ideal absorbing material should be economical in construction and
maintenance,water-proof,fire-proof,sufficiently strong and good in
appearance.
In the hall treated with absorbing materials, the speech can be heard clearly
and music can be fully enjoyed.
All the absorbing materials are found to be soft and porous. They work on the
principle that the sound waves penetrate into the pores and in this process, the
sound waves are converted into other form of energy by friction.
The absorbing capacity of the absorbing materials depends on the thickness
of the material, its density and frequency of sound.
The acoustic properties of the absorbing materials are considerably changed
by their modes of fixing.
Great care should be exercised while prescribing the covering for an absorbing
material so as to improve its appearance. The improper covering destroys the
absorbing properties of the material.
It should be remembered that in a big hall, the audience is a major absorbing
factor.
The requirements of a good acoustical material are as follows:
It should be durable and should not be liable to the attacked by insects,
termites, etc.
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ROHINI COLLEGE OF ENGINEERING AND TECHNOLOGY
It should be easily available at a reasonable cost.
It should be efficient over a wide range of frequencies.
It should be fire resistant.
It should give pleasing appearance after fixing.
It should have high coefficient of absorption.
It should have sufficient structural strength.
Classification of sound absorbing materials
The sound absorbing materials are broadly classified into the following
four categories:
(a) Porous absorbents
(b) Cavity resonators
(c) Resonant absorbing or panel absorbers
(d) Composite types of absorbents.
Have we shall discuss these materials one by one
(a) Porous absorbents. When sound waves strike the porous material, a part of
waves is reflected while the other enters the porous material.
The part that enters the porous material is converted into heat energy while
the reflected part is reduced in energy.
The examples are: fibre boards, soft plasters, rock wool, wood wool, mineral
wools, glass silk, asbestos fibre spray, etc.
(b) Cavity resonators. A cavity resonator is a chamber or container having a
small opening. When sound waves enter the resonator, the waves are
absorbed due to multiple reflections.
(c) Resonant absorbents or Panel absorbers. In this system, the absorbent
materials is fixed on a framing (usually timber) with an air space between
the framing and the wall. It acts as a panel absorber.
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ROHINI COLLEGE OF ENGINEERING AND TECHNOLOGY
The common examples are: gypsum boards, wood and
hard-board panels, suspended plaster ceilings, rigid
plastic boards, windows, doors, etc.
(d) Composite absorbers. When the functions of all the
three types described above is combined in a single
unit, then it is known as composite absorber.
The composite absorbers consist of a perforated panel
fixed over an air space containing porous absorbent.
When sound waves strike the panel, they pass through it
and damped by resonance of the air in the cavity.
Following are some of the common types of sound
absorbing materials:
1. Hairfelt
The average value of coefficient of absorption of 25 mm
thick hairfelt is 0.60.
2. Acoustic plaster
This is also known as the fibrous plaster and it includes
granulated insulation material mixed with cement. The acoustic
plaster boards are also available. They can be fixed on the wall
and their coefficient of absorption varies from 0.15 to 0.30.
3. Acoustical tiles
These are made in factory and sold under different trade
names. The absorption of sound is uniform from tile to tile and
they can be fixed easily.
4. Strawboard
This material can be also used as absorbent material.
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5. Pulp boards
These are the soft boards which are prepared from the
compressed pulp. They are cheap and can be fixed by ordinary
panelling. The average value of coefficient of absorption is
0.17.Compressed fiberboard
This material may be perforated or unperforated. The
average coefficient of absorption for the perforated is 0.30.
6. Compressed wood particle board
This material is provided with perforations and it can be
painted also. With a thickness of about 13 mm, the average
coefficient of absorption is 0.40.
7. Perforated plywood
This material can be used by forming composite panels
with mineral wool and cement asbestos or with mineral wool
and hardboard. It is generally suspended from the trusses.
8. Wood wool board
This material is generally used with a thickness of 25 mm.
The average value of coefficient of absorption is 0.20.
9. Quilts and mats
These are prepared from mineral wool or glass wool and
are fixed in the form of acoustic blankets.
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