Acoustics And Sound Insulation | Effects Of Audible Sound | Principles | Defects | Acoustical Materials

ACOUSTICS AND SOUND INSULATION 

The average sound travels at a speed of 340 m/s at ordinary temperature. It depends on the medium through which it travels.

 

EFFECTS OF AUDIBLE SOUND

Sound can travel through some medium like air. It can not travel in a vacuum. Thus for the sound to be audible to the ears, the sound source and ear must be connected by an elastic medium like air. The characteristics of audible sound are discussed below.

1. Frequency of Sound

The frequency of pitch of sound is defined as the number of cycles or vibrations per second. The highest audible sound (e.g., whisle) has a frequency of 20000 cps (cycles or vibrations per second). The lowest audible sound has a frequency of 20 cps (e.g., whispering). The frequency is a measure of the quality of sound.

 

2. Intensity of Sound

The intensity of sound is defined as the flow of sound energy per second through the unit area. The intensity of sound is the strength of the sensation received by the human ear. The intensity of sound is a purely physical quantity. But loudness of sound depends on the characteristics of the ear.

 

3. Measurement of Sound

The range of intensity of sound is very large. The loudest sound is about 1013 times the sound which is just audible by the human ear. There is a wide range of sound levels, it is realised that a scale has to be adopted as guidance. The intensity of sound is measured on a logarithmic scale due to wide range of variations of the intensity of sound. Bel is the measure of the intensity of sound named after Graham Bell, the inverter of the telephone.

As the unit of bel is comparatively large, hence a shorter practical unit decibel (db) is equal to 1/10th of a bel unit. The range of audible sound to painful noise varies from 1 to 1013 which is covered on a logarithmic scale between 1 and 130 db units. Thus one db unit is approximately the smallest change of sound intensity which the human ear can hear. Table below (Sharma, 1988) shows the rating to sound generally experienced.

 

PRINCIPLES OF ACOUSTICS

The behaviour of sound plays an effective role in the acoustical design of different types of buildings and in the sound insulation process.

A sound originating from a source, such as music or the operation of a machine, is transmit- ted through the medium in all directions. The transmitted sound strikes on some surface, like a wall, ceiling, floor or any other barrier. Depending on the type of surface part of it is

 

The absorption coefficient for different surfaces

Sl. No.	Surface	Absorption coefficient per m2
1.	Marble, metal	0.01
2.	Plaster on wall surface, stage curtain	0.02
3.	Unplastered brick wall (40 cm thick), Glass against solid surface, Linoleum or concrete floor	0.03
4.	Solid wooden floor	0.09
5.	Framed wooden floor	0.13
6.	Window glazed	0.18
7.	Plywood on battens	0.17–0.26
8.	Ventilators	0.10–0.50
9.	Sound absorbent or acoustical materials	0.50
10.	Curtains in heavy folds	0.40–0.75
11.	Open window	1.00

 

The reduction of the intensity of the sound of a transmitted sound through a barrier is called as transmission loss. This transmission loss is a measure of the effectiveness of a surface as an insulating material.

Thus transmitted and absorbed sounds have an important bearing on the acoustical condition of a building. However, both transmitted and absorbed sounds are interrelated and influence acoustic and sound insulation.

Highly porous materials have the quality to dissipate considerable energy and the absorption will be relatively high. Ideal material for sound absorption should possess the following qualities:

(i)     It should have a high coefficient of sound absorption of the economy.

(ii)    It should be fire-resistant.

(iii)    It should have high structural strength.

(iv)    It should be unaffected by decay, attack by insects and vermin.

(v)    It should be easily workable to fit in any location.

 

ACOUSTICAL DEFECTS

The acoustical design of an enclosed space basically depends on the behaviour of the reflected sound. Due to the reflection of sound two main defects are developed, viz., echoes and reverberation.

1. Echoes

Echo is said to be produced when a reflected sound wave reaches the ear just when the original sound from the same source has already been reached. The sensation of sound persists for one-tenth of a second after the source has ceased. Thus an echo forms when the time lag between the two sounds is about 1/17th of the second.

Further considering the velocity of sound in the atmosphere air as 34.3 m/s, it is shown that when the distance of the reflecting surface is between 8 and 17 m, echoes are formed.

The defect of echoes also occurs when the shape of the reflected surface is curved with a smooth character. Echoes are unpleasant to hear and cause disturbance to hear.

 

2. Reverberation

When the surfaces of an indoor place are hard and smooth, very small energy is lost at each impact of sound and many reflections take place before the sound dies down. This repeated reflection of sound is called prolongation or reverberation. Then if sound exists too long, then successive words of a speech will overlap and confuse.

The remedy for this defect is to select a correct time of reverberation known as an optimum time of reverberation. This is achieved by suitably selecting proper absorbent or acoustical materials for different reflecting surfaces.

The acoustical level of a room and reverberation times are shown in Tables Below shows the recommended values of the optimum reverberation time in India for halls after considering the indicated audience strengths.

 Qualities of good acoustical materials are as follows:

ACOUSTICAL MATERIALS

Common building materials are absorbents of different levels. Such materials are called as absorbent materials.

Thermal Insulation Of Buildings | Purpose | Principles | Materials | Methods

Acoustics level and reverberation time

Acoustics Level	Reverberation Time
Excellent	0.50-1.50
Good	1.50-2.00
Fairly Good	2.00-3.00
Bad	3.00-5.00
Very Bad	Above 5.00

 

 

Optimum reverberation time for closed area

Sl. No.	Type Of Hall	Audience Factor	Optimum Reverberation Time (Sec)
(A)	Big halls	Full	2.00–3.00
(B)	Music concert halls	Full	1.60–2.00
(C)	Churches	Two-thirds	1.8–3.00
(D)	Theatre	Two-thirds	1.30
(E)	Public-lecture halls	One-thirds	1.50–2.00
(F)	Law courts, Committee rooms, Conference hall	One-thirds	1.00–1.50
(G)	Parliament house, Assembly halls, Council chambers	Quorum	1.00–1.50

Must read: Thermal Insulation Of Buildings | Purpose | Principles | Materials | Methods

Qualities of good acoustical materials are as follows:

(i) An absorbent material should be economical for construction and should be water-proof, fireproof, adequate strength and heat resistant.

(ii) It should be durable and easy to maintenance.

(iii) It should have a high absorption capacity.

(iv) It should be easy to fix and should be self-supporting. Some of the acoustical materials are discussed below.

 

1. Acoustic Plaster

It is a granulated insulation material mixed with cement. The quantity of cement used in such materials is to provide enough strength and have sufficient pores. The coefficient of absorption varies from 0.15 to 0.30.

 

2. Acoustic Tiles

These are commercially available materials with adequate absorbent qualities. The absorption of sound is uniform and most suitable for small rooms. But it is costly compared to other acoustical materials.

 

3. Porous Boards

Straw boards, pulp boards and compressed fibre boards have varied coefficients of absorption. These boards can be fixed by ordinary panelling. The thickness is about 17 cm and the coefficient of absorption varies from 0.17 to 0.50.

 

4. Perforated Boards

The material can be formed by the combination of mineral wool and cement asbestos or with mineral wool and hardboard. The coefficient of absorption for the first combination is about 0.95 and for the latter is 0.20. Such boards are generally suspended from trusses.

 

5. Quilts and Mats

These materials are made out of mineral wool or glass wool. The coefficient of absorption varies which depend on the thickness of the board or mat, density, perforations, mode of fixing and frequency of sound.

SOURCE: BUILDING CONSTRUCTION MATERIALS AND TECHNIQUES | P. PURUSHOTHAMA RAJ

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