23 ACOUSTICS AND SOUND ABSORPTION

P.G. Padma Gowri

Introduction:

The word Acoustics is originally derived from the Greek word meaning ‘to hear’. Hence, Acoustics is defined as the science of sound and as such, it discusses the origin, propagation and auditory sensation of sound. When a sound wave sticks, part of its energy is absorbed by friction, part of energy is transmitted and their main part of its energy is reflected.

Objectives:

• To know about the details of audible range of sound and its properties.
• To analyse various materials used in building construction and its acoustic properties.
• To evaluate sound qualities of various sound absorption materials used in building construction.
• Methods to enhance the acoustics in building construction and eliminate noise.

Acoustics:

Sound is produced when part of the atmosphere is compressed suddenly. This compression would have retained stationary at that place only in case the air was not elastic. But due to the elasticity of air, the particles originally disturbed in turn disturb the neighboring particles.

Ultimately the compression is propagated or spared away from the source. Sound thus travels in the form of waves and when these waves come near our ear drums, we feel a sensation of hearing.

Important facts in connection with sound are :

1. The sounding body throws sound waves in the state of vibration. If vibrations of the sounding body die out, the sound emitted by the sounding body will also die out.
2.  It is absolutely necessary that, for a sound to be heard by our ears, the sound body and ear must be connected by an uninterrupted series of portions of elastic matter. The physical state of matter namely Solid, Liquid or Gas.
3.  Presence of some matter is required for the transmission of sound. Sound cannot travel in vacuum.
4. The sound waves are longitudinal waves and hence each particle of the medium through which sound wave is proceeding moves backwards along a line in the direction in which the sound wave is travelling.

Velocity of sound:

The speed at which the sound waves travel or pass through any medium is termed as “sound velocity” and it depends on the nature and temperature of medium through which sound travels.

Table: Sound velocities in different medium

Important facts to be remembered in connection with Velocity of sound are:

1)The time required by sound waves to travel from one place to another is quite appreciable.

2)The Velocity of sound in air is affected by the amount of moisture in the air, temperature of the air and the intensity of the wind.

3)Velocity of sound is independent of the frequency or pitch of sound.

4) It can be seen from the table that sounds travels much faster in solids and liquids than in air.

Frequency and intensity of sound:

Frequency or pitch is defined as the number of cycles which a sounding body makes in each unit of time. The greater the number of cycles, higher will be the pitch.

The intensity or loudness of the sound is defined as the flow of sound energy per unit of time through unit area. Thus frequency or pitch is a measure of the quality of sound. The intensity or loudness is a measure of the quantity of sound.

The audio frequency scale for human covers a wide range varying from 20 cycles per second to 15000 cycles per second. The upper limit of frequency depends on the age of the person and his physical fitness to receive the sound. It can be noted that intensity of sound in db and frequency of sound in cycles per second are physical quantities which are defined arbitrarily.

Table: Some typical Sound levels

Reflection of sound:

Convex surface tends to spread the reflected sound waves. Hence, convex surfaces may be used with its advantage to spread the sound throughout the room. The study of law of reflection of sound helps in selecting proper shape of the room with regard to the distribution of sound in that room.

The assumption that sounds are reflected in the same manner as light is true only for limited circumstances and hence great caution should be exercised while applying the law of reflection of sound.

Fig 1. Reflection of Sound wave in hard plane

When a free sound wave travelling through air sticks a uniform large solid plane surface, it is reflected in the same manner as that of light ray. It is shown in figure 1. The angle which the incident wave makes with the plane surface is equal to the angle which the reflected wave makes with the plane surface.

Reverberation:

It is quite clear that some interval of time will be required for the sound energy to convert into other form of energy.

This transformation is usually brought about by friction between the sound wave and air particles as well as sound and surfaces with which it comes into contact. Naturally the more the friction, quicker will be the transformation. The gradual process of transformation operates over a certain interval of time and this is known as Reverberation.

Now, m=10log 10 I1/I2  ,  m=decibels

It is found that a normal person speaks, the ratio I1/I2=106. That is m=60 db. The time of Reverberation is thus defined as the time required by the Reverberant sound to decay to one millionth of its initial value. This naturally corresponds to a drop of 60 db.

Table: Reverberation time and acoustics

The optimum reverberation time will depend on the use of the room. The table gives the relation between the reverberation time and acoustics of the room. If the reverberation time is too long, it results in overlapping of speech and on the other hand, if it is too short it produces the effect of deadness.

The table shows the recommended values of optimum reverberation time in India for halls designed for various purposes.

Table: Optimum reverberation time for halls

Noise and its effects:

Noise is defined as an unwanted sound and it may be due to frequency of sound or intensity of sound or both. Noise due to high frequency sounds is more unpleasant and harmful than that of low frequency sounds.

Types of noise:

1.  Indoor noise – created in adjacent room or same room in a house.

2.  Outdoor noise – created from nearby streets and other source from outside.

Indoor noise elimination:

Improvement in working methods:

·  A working method creating less noise may be adopted.

·  Acoustical treatment: The walls, floors and ceilings should be provided with sound absorbing materials.

·  Personal protective devices: It is possible to reduce the noise to the extent of 20db to 30db by using Personal protective devices like head phones, ear plugs etc.,

Absorption of sound:

When a sound wave strikes a surface, part of its energy is absorbed by friction, part of its energy is transmitted and the remaining part of its energy is reflected. But as reverberation directly depends on the loss of energy of sound wave due to friction, it is of greater importance. This property of a surface by which sound energy is converted into other form of energy is known as absorption and absorption coefficient of a surface indicates the degree to which these surfaces affect the absorption of sound. It is thus the ratio of energy absorbed by the area to the energy striking the area.

The value of coefficient of absorption will depend on the frequency of sound. The table gives the value of coefficients of absorption for some of the common surfaces.

Table: Absorption coefficients

These values correspond to the normal frequency of 500 cycles per second. It may be noted that the coefficient of absorption for an open window is taken as unity. This is very easy to understand as sound wave approaching an open window must completely pass through it.

Sabin’s equation:

Prof. W.C Sabin of Hardward University, U.S.A, carried out number of experiments in rooms of different sizes and he was able to establish the following formula which is known as Sabin’s equation,

Where t = reverberation time in seconds

V = volume in m3

A= total absorbing power in m3

(a1s1+a2s2+a3s3+……) absorption of individual objects

s1,s2,s3 etc., being coefficients of absorption of corresponding surfaces a1,a2,a3 etc.,

Since V and A can be worked out from the plans and specifications of the structure, it is possible to design a structure for any desired value of time reverberation.

Absorbent materials

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 absorbent materials and they help a great deal in making the room acoustically good.

The important facts in connection with Absorbent materials are:

An ideal Absorbent material,

1)  Should be economical in construction and maintenance, water proof, fire proof, sufficiently strong and good in appearance.

2) Noise level of the room provided with absorbent materials is considerably reduced.

3)  In the hall treated with Absorbent materials, speech can be heard clearly and music can be fully enjoyed.

4)  All the Absorbent materials are found to be soft and porous. They work on the principle that sound waves penetrate into the porous and in this process sound waves are converted into other form of energy by friction.

5)  The absorbing capacity of the absorbent materials depends on the thickness of the material, density of sound and frequency of sound.

6)  The acoustic properties of the absorbent materials are considerably changed by their modes of fixing. Suspended absorbers in the form of inverted cones may be provided in the ceiling to make the hall acoustically good.

7)  Great care should be exercised while prescribing the covering for an absorbent material so as to improve its appearance.

8)  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 a wide range of frequency of sound.

Types of Absorbent materials:

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, light reflection etc.,

Following are some of the common types of absorbent materials:

Hair felt:

Hair felt was used by Prof. Sabin in his experimental works. If the average value of coefficient of absorption is 25 mm thick, hair felt is 0.60.

Acoustic plaster:

Acoustic plaster is also known as fibrous plaster. 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. It can be fixed on the wall and their coefficient of abortion varies from 0.15 to 0.30.

Acoustical tiles:

Acoustical tiles 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. Because of high cost they are most suitable for rooms in which small area is available for acoustical treatment.

Strawboards:

With a thickness of 13 mm and density of 0.24 g/cm3, it possesses a coefficient of absorption of 0.30 at 500 cycles per second.

Pulp boards:

Pulp boards are soft boards which are prepared from compressed pulp. They are very cheap and can be fixed by ordinary paneling. The average value of coefficient of absorption is 0.17.

Compressed fire board:

Compressed fire board may be perforated or imperforated. The average coefficient of absorption for the former is 0.30 and for the latter is 0.52. It has a density of 0.30 g/cm3.

Compressed wood particle board:

Compressed wood particle board 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.

Perforated plywood:

Perforated plywood can be used by forming composite panels with mineral wool and hard board. The average coefficient of absorption is 0.75.

Quits and mats:

Quits and mats are prepared from mineral wool or glass wool and are fixed in the form of acoustic blankets. The coefficient of absorption of depends on thickness and size of Quits and mats.

Conditions for good acoustics of an auditorium:

Following conditions should prevail in an auditorium possessing good acoustical properties:

1) The initial sound should be of adequate intensity or loudness. The quite obvious as it is impossible for a speaker to be heard over a long distance without the help of sound –amplification system.

2) The sound which is produced should be evenly spread over the whole area covered by the audience. If this condition is not satisfied, conditions tending the formation of echoes will be established.

3) The initial sound should be clear and distinct so that there is no possibility of distorted speech.

4)  For hall to be used for music, the initial sound should reach the audience with the same frequency.

5)  All undesired sound should be reduced to such an extent that it will not interfere with the normal hearing of music or speech. The undesired sound may be created either from inside or outside.

Factors to be considered in the acoustic design of an auditorium

Volume:

The hall should be of enough volume and it should be decided by considering intensities of sounds likely to be developed in the hall. Hall of small volumes is useful for weak sounds.

Shape:

Shape is an important consideration in the acoustic design of an auditorium and it involves geometrical aspects of the hall. The paths followed by reflected sound waves are traced and concentration of sound waves. Concave walls are not good for acoustic purposes as they tend to concentrate sound waves. Plain walls are better. But convex walls are excellent and are used to reduce the possibilities of echoes to the minimum extent.

Sound absorption:

Adequate absorbing surfaces should be provided in the hall to control the reverberation. As a matter of fact, the acoustics design of an auditorium will be incomplete without the provision of such a surface. Careful study of the hall should be made before recommending the type of absorbent material and its location in the hall.

Defects of auditorium and their remedies:

There are three objects involved in the design of an auditorium:

1)   There should be good direct path for the sound .This will enable the sound to be heard loudly and clearly and will avoid confusion with subsequent reflections.

2)   It should be seen that none of the subsequent reflections has the same strength as the original sound.

3)  Desired degree of reverberation sound is provided in the auditorium.

The following defects are however found in most of the auditoriums:

Reverberation:

The defect of excessive Reverberation is very common in many auditoriums. The effect is that sound once created prolongs for a longer duration resulting in confusion with the sound created next. The remedy of this defect is to correct the time of reverberation by suitably installing the absorbent materials.

Formation of Echoes:

When the reflecting surfaces are situated at a distance greater than 15 meters or when the shape of the auditorium is unsuitable, formation of echoes takes place. This defect can be removed by selecting proper shape and porous interior surfaces to disperse the energy of echoes.

Sound foci:

In case of concave reflecting interior surfaces, certain spots are formed where reflected sound waves meet and create a sound of large intensity. These spots are known as sound foci and can be eliminated by avoiding curvilinear interiors or by providing absorbent materials on focusing areas.

Dead spots:

Dead spots defect is the reverse of sound foci. Because of high concentration of reflected sound at sound foci, there is deficiency of reflected sound at some other points. These points are known as dead spots and the sound level at dead spots is generally inadequate for satisfactory hearing. This defect can be remedied by installation of suitable diffusers so as to have even distribution of sound in the hall.

Loudness:

This defect is due to lack of reflecting surfaces near the source of sound and excessive absorption of sound in the hall. This defect can be rectified by arranging hard reflecting surfaces near the source of sound and by adjusting the absorption of the hall so as to give an optimum time reverberation.

  Exterior noise:

Exterior noise defect is due to poor sound insulation and hence, exterior noise enters the auditorium through loose doors and windows. The remedy to correct this defect is the various components of the auditorium.

Conclusion:

The concepts of audio sounds and its properties were discussed. The materials with different acoustic properties were elaborated. To enhance sound qualities in different types of building was discussed. There is no royal road for making a particular room acoustically good. It mainly depends on the ideas of the Engineer or the Architect. Each case is to be studied separately and after proper thinking, suitable Absorbent materials may be selected

Web links:

• https://www.acousticalsurfaces.com/acoustic
• https://en.wikipedia.org/wiki/Absorption_(acoustics)
• www.ecophon.com/en/acoustic-solutions/acoustic-knowledge…/Basic-Acoustics/
• www.acoustic-glossary.co.uk/sound-absorption.htm
• https://en.wikibooks.org/wiki/…Acoustics/Sound_Absorbing_Structures_and_Material..
• www.acoustic-supplies.com/sound-absorption/