**2.2 Noise and its effects**

*Noise and Environment*

9.It should be remembered that in a big hall, audience is a major absorbing factor. This is especially true in the high frequency zone. Hence, low frequency absorbent materials should be provided to achieve optimum reverberation time over

**Material Absorption coefficient per m2**

Open window 1.00 Ventilators 0.10 to 0.50 Brick wall 40 cm thick 0.03 Plaster on wall surface 0.02 Glass against solid surface 0.03 Marble 0.01 Stage curtain 0.20 Linoleum or concrete floor 0.03 Solid wooden floor 0.09 Framed wooden floor 0.13 Plywood on battens 0.17 to 0.26 Window glazed 0.18 Curtains in heavy folds 0.40 to 0.75 Metal 0.01

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. The choice of the absorbent material should be made after carefully considering various factors such as appearance, cost, workability, flame resistance, durability, and light reflection. Following are some of the common types of absorbent

1.Hairfelt: the average value of coefficient of absorption for Hairfelt for 25 mm

2.Acoustic plaster: this is also known as 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

coefficient of 0.30 at 500 cycles per second. Acoustic plaster boards are also available. They can be fixed on the wall and their coefficient of absorption

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, acoustical tiles are relatively costly than other absorbent

, the acoustic plaster possesses an absorbent

a wide range of frequency of sound.

mm and density of 0.10 g per cm3

varies from 0.15 to 0.30.

*2.1.1 Types of absorbent materials*

thickness is 0.60.

**104**

materials:

**Table 1.**

*Absorption coefficients.*

When the sound waves are non-periodic, irregular and of short duration, they produce a displeasing effect and such a sound is known as noise. Thus a noise is an unwanted abrupt sound of complex character with an irregular period and amplitude originating from a source of non-periodic motion.

Following are the important effects of noise: (i) noise creates uncomfortable living conditions; (ii) prolonged exposure to noise may result into temporary deafness or nervous breakdowns; (iii) it is observed that noise has an influence on blood pressure, on muscular strain and even on sleep; (iv) noise leads to fatigue and consequently, the efficiency of persons exposed to noise decreases considerably; (v) it is an established fact that reduction in noise increases to a great extent the output of labor; (vi) presence of noise takes away the essence of music and speech.
