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Sound absorbent materials, classifications, materials are used for Sound absorbent

Er. Parbhakar Dwivedi
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Sound absorbent materials

The sound absorbent materials can be incorporated in building structures either in compressed state or in suspended state or in Free State. In compressed state, they are provided between the load bearing panels of ceiling and floor. In suspended state, they are provided in the form of slabs fastened to ceiling so as to provide an air space. In Free State, they are provided in non-compressed or loose manner.

According to the nature of absorbing sound, the sound absorbent materials can be classified as follows:

(1) Porous materials: This type includes light weight concrete with porous aggregate, foam glass, etc.

(2) Porous-cum-elastic material: This type includes porous materials with an elastic backing.

(3) Baffle materials: This type includes thin panels from veneer, rigid wood fibre slabs, solid cardboard, etc.

(4) Perforated materials: This type includes perforated panels and slabs. The holes may be of equal diameter or different diameters and they may be symmetrically arranged or located at random on the surface of panels or slabs.

Most of the common building materials absorb sound to a small extent and hence, for better acoustical requirement, some other materials are to be incorporated on the surfaces of the room. Such materials are known as the sound absorbent materials and they help a great deal in making the room acoustically good.

These materials are used for:

(i) Damping sound in ventilation installations;

(ii) Developing special acoustic effects in TV, radio and film shooting studios, etc.

(iii) Facing interiors of premises which require a low noise level such as offices, restaurants, commercial centres, banks, etc.

(iv) Providing adequate acoustic in theatre halls, auditoriums, etc.

The various types of absorbent materials are available in the market under different trade names. The value of coefficient of absorption is supplied by the manufacturer.

Following are some of the common types of absorbent materials:

(1) Hairfelt: This material was used by Prof. Sabin in his experimental work. 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. If quantity of cement is more than required, the plaster will not have sufficient pores to become effective for acoustics. If quantity of cement is less, the plaster will not have enough strength. Thus the quantity of cement should be carefully decided. For thickness of 20 mm and density of I kN/m3, the acoustic plaster possesses an absorbent coefficient of 0.30 at 500 cycles per second. 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. However the acoustical tiles are relatively costly than other absorbent Materials. They are most suitable for rooms in which small area is available for the acoustical treatment.

(4) Strawboard: This material can also be used as absorbent material. With a thickness of 15 mm and density of 24 IN/m, it possesses a coefficient of absorption of 0.30 at 500 cycles per second.

(5) Pulp boards: These are soft boards which are prepared from the compressed pulp. They are cheaper and can be fixed by ordinary panelling. The average value of coefficient of absorption is 0.17.

(6) Compressed fibreboard: This material may be perforated or unperforated. The average coefficient of absorption for the former is 0.30 and for the latter is 0.52. It has a density of 3 kN/m3.

(7) Compressed wood particle board: This material is provided with perforations and it can be painted also. With a thickness of about 15 mm, the average coefficient of absorption is 0.40.

(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 trusses. The average value of coefficient of absorption for the former composite panel is as high as 0.95 and for the latter composite panel, it is about 0.20.

(9) Wood wool board: This material is generally used with a thickness of 25 mm and it has a density of 4 kN/m. 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. The absorption coefficients of such quilts and mats depend on the thickness, density, perforations, mode of fixing nature of backing and frequency of sound.

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