Sound Absorbing Materials

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

• 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 odeli penetrate into the pores and they are converted into other form of energy.

• The absorbing capacity of the absorbing materials ni bosu depends on the thickness of the material, its density and frequency of sound.

• Great care should be exercised while prescribing the sound absorbing material SO as to improve its

• 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 be attacked by insects, termites, etc.

• 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. 02

• 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.

 

(a) Porous absorbents. When sound waves strike the ojai bo 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.

Examples: 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.

Examples: 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.

 

Common types of sound absorbing materials:

1. Hairfelt

The average value of coefficient of absorption of 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 also be used as absorbent material.

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.

6. Compressed fiberboard

This material may be perforated or unperforated. The average coefficient of absorption for the perforated is 0.30.

7. Compressed wood particle board

This material is provided with perforations and it can also be painted. With a thickness of about 13 mm, the average coefficient of absorption is 0.40. bluoria

8. 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.

9. Wood wool board

This material is generally used with a thickness of 25 mm. The average value of coefficient of absorption is 0.20.

10. Quilts and mats

These are prepared from mineral wool or glass wool and are fixed in the form of acoustic blankets.

 

Factors Affecting Acoustics of Buildings and their Remedies

The factors affecting the acoustics of a building are:

1. Optimum reverberation time

2. Loudness

3. Focussing

4. Echoes

5. Echelon effect

6. Resonance

7. Noises

 

Noise

Unwanted sound is called noise. There are three types of noises.

They are

(i) Inside noise

(ii) Airborne noise

(iii) Structure-borne noise

Noises produce a disturbing and displeasing effect on the ear. Hence, noise should be avoided and controlled.

(i) Inside noise

Noise produced inside the room is known as inside noise

Example of inside

• The sound created by moving of people, crying babies, movement of furniture.

• The sound produced from machines, typewriters, etc.

Remedy

• Machines and typewriters can be placed over sound absorbing materials.

• The walls, floors and ceilings can be covered with suitable sound absorbing materials

• The engines that create noise may be fitted on the floor with a layer of wood or felt between them.

 

(ii) Air-borne noises

Noises coming through open windows, doors and ventilators are known as air-borne noises.

These types of noises are common in a densely populated area.

Remedy

• The use of double doors and windows with separate frames HOOT and by placing sound absorbing material in between them.

 • By allotting proper places for doors and windows.

• Using heavy glasses in doors, windows and ventilators.

• By making the hall air-conditioned. gols aszlet Ji

 

 (iii) Structure borne noise to

Noises conveyed through the structures of a building are called structure-borne noises.

Example

The machinery operation, movement of furniture, foot steps etc. produce structural vibration giving rise to structure-borne noise.

Remedy

• Noises from water pipe can be controlled by using rubber couplings at junctions.

• Using double walls with an air space between them.

• Covering the floors and ceilings  with suitable sound absorbing materials and anti-vibration mounts.