Absorption of Sound | Noise Pollution

After reading this article you will learn about:- 1. Introduction to Absorption of Sound 2. Sound Intensity in a Room 3. Noise Reduction 4. Coefficient of Absorption of Sound 5. Classification of Absorbents.

Contents:

1. Introduction to Absorption of Sound
2. Sound Intensity in a Room
3. Noise Reduction
4. Coefficient of Absorption of Sound
5. Classification of Absorbents

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1. Introduction to Absorption of Sound:

Absorption of sound is important in lessening the general level of noise within a room, and also minimizing excessive background noise. Absorption is especially beneficial in large general offices and workshops, where the noise sources are confined to certain parts of the room.

Reflection of noise back into other areas of quiter activities in an office can be prevented by sound absorption treatment of the office. We should note, however, that additional sound insulating barriers may be necessary to reduce the noise radiated in a direct path from noise sources.

There is a clear distinction between absorption and insulation of sound. A sound absorbent surface absorbs a proportion of the sound energy incident on it so that the level of sound reflected from such a surface is substantially reduced. It follows, therefore, that the technique of sound absorption is basically intended to reduce the loudness of reflected sound in a room enclosure, and decrease the reverberation of sound.

At the same time, however, a high proportion of sound energy may be transmitted through the absorbent surface since most sound absorbing materials are, in general, poor insulators of sound. Basically, therefore, sound insulation is intended to reduce the intensity of transmitted sounds.

In many cases (for example, in auditoria, churches, concert halls, theatres, etc.), sound absorption treatment can have a dual purpose. In the first place, sound absorption treatment can be effective in reducing the intensity of background noise to a suitable level to ensure speech intelligibility. Secondly, such a treatment can also have a marked effect on the quality of sound generated in the room by controlling reverberation.

In such cases, only a limited amount of sound absorption (producing a small reduction in the loudness) is then normally required. Excessive deadening of sound (by sound absorption treatment) within the auditoria or theatres, for example, would be undesirable, especially for music. The primary aim of sound absorption treatment in such cases is to achieve an optimum reverberation time.

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2. Sound Intensity in a Room:

Noise emitted directly from a simple noise source is radiated uniformly in all directions. The intensity of such noise decreases with the square of the distance between the listener and the source. This situation is represented by the solid straight line in Fig. 1. This figure shows graphically the relationship between the intensity of sound and distance of the listener from a simple source of sound (inside a room).

Within a room, sounds are reflected from and between the boundary walls, travelling at random in all directions. If the noise source inside a room is continuous and uniform, the intensity of the reflected sound will be substan­tially constant throughout the whole room.

However, the actual intensity of reverberant sound will depend on the degree of sound absorption provided by the boundary walls of the room, as shown by the two dashed lines in Fig. 1.

The three solid curves in this figure show the intensity of actual sound in a room corresponding to various degrees of absorption. The actual sound results from the combined effect of the sound coming directly from a simple source (inside a room) and the reverberant sound resulting from reflections from the boundary walls (and other surfaces inside the room).

The listener, however, has no means of distinguishing between the direct and reflected sounds and he usually hears only the resultant sound. It is evident from Fig. 1 that the (resultant) sound intensity inside a room decreases rapidly at first as the listener moves away from the noise source; but the rate of change gradually falls, until the level of sound becomes substantially constant over the remainder of the room.

The discussion given above may be summarised as follows. The noise level at a point close to the noise source (inside a room) is almost entirely due to the direct radiation from the source, whereas at greater distances it is all due to multiple reflection between the walls.

It is hardly necessary to discuss in detail the direct component of sound inside a room. The reason is that the dissipation of sound energy due to its absorption in air is negligible in normal circumstances. Consequently, the decrease in the intensity of direct sound with increase in distance can be easily obtained from the well-known inverse square law.

At larger distances in the room, however, the intensity of sound is almost constant. At such distances, practically the whole of sound energy consists of components that have been reflected many times between the boundary walls of the room (and other surfaces).

All the surface materials (in general use) absorb some sound energy from a sound wave striking the surface. Such absorption varies from about 5% (for a hard painted surface) to as much as 90% (for some of the specially prepared sound- absorbent materials).

Obvious­ly, surfaces having high values of sound absorption coefficients will produce a large reduction in sound intensity at each reflection. Consequently, the final steady uniform level of reflected sound in this case will be lower than would result if the surfaces had low absorption coefficients.

It can be shown, in fact, that the steady uniform sound level will be inversely proportional to the total amount of sound absorption present in the room. Each time the total amount of sound absorption in the room is doubled, there is a reduction of 3 dB in the average sound level.

In principle, therefore, the level of reflected sound can be reduced to any desired extent by installing sufficient sound-absorbing material inside the room. In practice, however, this solution is not so simple (or so effective) as it appears. We know that a doubling of the total absorption reduces the average noise level by 3 dB (i.e., halves the intensity of the reflected sound).

When there is only a small amount of sound absorption present in the room, it is fairly easy to double this amount On the other hand, when the ceiling of the room has been acoustically tiled and its floor covered with carpet, it generally becomes difficult to find more surfaces of sufficient area to allow a further doubling of the total sound absorption (even using acoustic treatment of high efficiency).

It is this practical difficulty which limits the amount of noise reduction in a room that can be achieved by the use of sound- absorbing materials.

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3. Noise Reduction:

On the basis of the discussion given above, several important general deductions can be made.

Some of these are as follows:

(a) The addition of sound absorbent materials to the walls or ceiling of a room is particularly effective in reducing noise if the amount of such sound absorbents initially present in the room is small.

(b) The converse of the preceding conclusion is equally true. If the room is well furnished, therefore, a further addition of sound absorbents will be of little benefit in reducing the noise level. In rooms (in residential buildings) containing soft furnishings, for example, it is very unlikely that a noise reduction of more than 5 dB will be achieved with sound absorption treatment.

On the other hand, an improvement (i.e., noise reduction) of upto 10 dB may well be obtained in acoustically “loud” rooms (e.g., workshops and school classrooms).

(c) Since the application of sound-absorbent materials is not effective in reducing the direct sound, the noise level to which the operator of a noisy machine (in a workshop) is exposed cannot be significantly reduced by the sound absorbent treatment of the walls of the room in which the noisy machine is operated. This is especially true in large rooms (for example, the average machine shop).

(d) On the other hand, since the sound-absorbent treatment is effective in reducing the indirect (or reflected) sound, the noise criticised by persons remote from a noisy machine may be appreciably reduced by the addition of acoustic treatment to the boundary walls and other surfaces of the room.

(e) In a workshop, therefore, the operator of one machine disturbed by the noise of another machine may benefit by the addition of sound absorbents to the walls; but there is advantage only during those intervals when his own machine is at rest (i.e., not operating).

We note here that a sound-absorbent material (or composite structure) provides the absorption of sound energy by one or more of the following processes:

(i) Friction between the fibres of a porous, fibrous material;

(ii) Absorption in the voids of a porous, non-fibrous material; and

(iii) Absorption within narrow entries to an air space.

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4. Coefficient of Absorption of Sound:

The performance of an absorbent (or, alternately, its effectiveness as a sound absorbent) is usually expressed by its absorption coefficient, which is given by the ratio of the sound energy absorbed by such a material to the total sound energy incident upon it. Thus the absorption coefficient of a material indicates the fraction of incident sound energy absorbed by the material.

It follows, therefore, that the absorption coefficient can only vary between zero and unity. An absorption coefficient of 0.0 represents total reflection (i.e., no absorption) of the incident sound energy, while a coefficient of 1.0 repre­sents total absorption (i.e., no reflection) of sound. The absorption coefficient multiplied by 100 thus represents the actual percentage of sound energy absorbed.

It can be easily inferred from the preceding discussion that the absorption coefficient is not an absolute constant quantity for any material (or a composite structure).

In fact, the absorption coefficient varies with frequency, and is also affected by the size, position and method of mounting of the absorber. It follows, therefore, that the absorption coefficients, determined by different test methods, for the same absorbent material or composite are not directly comparable.

Typical absorption coefficients for some commonly used sound absorb­ents are shown in Fig. 2. This diagram clearly shows the frequency dependent nature of the absorption coefficient in the case of fibrous or perforated materials.

An increase in the thickness in fibrous or porous absorbents will generally improve the absorption of sound mainly over the low and middle ranges of frequency. As a general rule, porous absorbents should preferably be a minimum of 2.5 cm thick unless absorption is only required at high frequencies.

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5. Classification of Absorbents:

A general classification for sound absorbent materials is based on the values of their absorption coefficients over the middle and high frequency range, as shown in Table 1.

Alternatively, sound absorbents can also be broadly classified according to their effectiveness (or performance) over specific ranges of frequency, as shown in Table 2.

General characteristics of some sound-absorbent materials commonly used in buildings are given in Table 3.

Empirical determination of sound absorption coefficients (or acoustic absorptiveness) is commonly confined to six frequencies in the range 125 Hz to 4,000 Hz, although measurements may be extended to higher frequencies.

It has been found, in general, that the values of absorption coefficients for frequencies above 4,000 Hz are very similar to those for 4,000 Hz. The sound absorption (or noise reduction) coefficient is usually quoted as a single number average value, based on the sound absorption coefficients at 125, 250, 500, 1000, 2,000 and 4,000 Hz. In all cases, the values of absorption coefficients are rounded off to the nearest 0.05.

In the case of porous materials with inter-connecting pores, friction is the predominant factor in absorbing the energy of sound waves by progressive damping. Such materials offer a “direct resistance” (in the terminology of electrical engineering) and, therefore, the damping effect is largely inde­pendent of frequency. There is, however, an optimum value for the resistance.

If, for example, the resistance is too high, sound waves will be rejected (or reflected) instead of penetrating the absorbent material in depth and being absorbed. If the resistance is too low, however, there will not be sufficient friction to provide enough damping to make such a material effective as a sound absorber.

On the other hand, in the case of perforated materials opening into a body of porous materials, solid materials or membranes, damping is provided by “reactance” rather than pure resistance. As a result, the performance in this case can be markedly dependent on the frequency.

Even this frequency dependence is further dependent on the proportion of open area in the case of a perforated surface, or the mass in the case of an impervious membrane.

In addition, the total depth of the air volume between the face of the material and the rigid backing can also modify the frequency-dependent characteristics. This air volume includes open-pore volumes in the case of porous materials.

The basic requirements of a sound-absorbent material are as follows:

(a) It should be sufficiently porous to allow the sound waves to enter into the material; and

(b) The nature of the material should be such that the maximum proportion of sound energy is transformed into heat energy by friction, thus providing dissipation of the sound energy.

Some Relations:

The “turnover frequency” is the frequency at which the low frequency absorption characteristics deteriorate rapidly. This turnover frequency f_t is given by

f_t = C/2d, …(1)

where C = velocity of sound in air and d = total depth of air volume.

The actual porosity of a porous material is defined as the ratio of the volume of voids present in the material to the total volume. In the case of solid, fibrous materials the porosity can be estimated directly from the density of the fibres and total mass:

Porosity = 1

where mass, volume and density are all expressed in consistent units. If binders are present, an allowance for this must be made to estimate the true mass. For materials of mixed or composite structure, on the other hand, porosity can be determined accurately by direct measurement.

The equivalent absorption of a surface is given by the product of its surface area and the absorption coefficient. In the case of a room, the total absorption is given by the sum of equivalent absorption of each surface (also including the absorption given by furnishings, seats, occupants, etc., where applicable). Thus we have Total absorption A (in Sabines)

= ∑α_iS_i…(3)

where α_i and S_i are the absorption coefficient and surface area, respectively, of the i^th surface. Mean values are usually utilized for the absorption coefficients

The average sound pressure level (SPL_av) of reflected sound in a room is given by the relation

SPLav (in dB) = 10 log₁₀W-10 log₁₀ A + 1364, … (4)

= W-10 log10A + 6.1, -(5)

where W = power level of the source (ref. 10^-12 watt), and surface areas S_i are expressed in m² while calculating the total absorption A using Eq. (3). Strictly speaking, Eq. (4) and (5) apply only for diffuse distribution of sound within the room, and for a single source of sound.

The total sound level in a room is the logarithmic sum of the direct and reflected sounds, viz.,

Total sound level (in dB) =

10 log₁₀ (antilog (L_d/10) + antilog (L_r / 10), – (6)

where L_d = level of direct sound and L_r = level of reflected sound, both

being expressed in dB.

The total sound level in a room, as given by Eq. (6), will be only slightly higher than the level of the larger sound, and never more than 3 dB higher up to a distance of about 0.5 A from the source A = total absorption in the room), the direct sound will be the louder one.

At greater distances, the reflected sound level will be greater than the direct sound and, therefore, the total sound level will be substantially the same as that of the reflected level of sound (and constant at that level).

At very high sound levels (about 150 dB and above), sound absorbing media may disintegrate (or “burn out’). This occurs due to the reason that high-intensity sound waves are non-linear.

This non-linearity occurs when the rarefication of the wave approaches about half the atmospheric pressure, causing the wave to become increasingly asymmetrical. At substantially higher pressure levels, the sound wave degrades into a saw-tooth form, with the impact on materials in its path becoming similar to that of hammer blows.

The general properties (i.e., absorption coefficients at various frequencies) of a range of acoustically absorbent materials, and also some other materials for the sake of comparison) are given in Table 4. A broad classification of acoustic materials for buildings and other similar applications is shown in Table 5.

We note here that both tiles and boards (see Table 5) may be further classified according to their geometry, perforations and surface characteristics.

Surface Finishes:

The glass tissue interleaves are often placed immediately over perforated ceiling panels (and a fibrous absorbent directly above). Mineral fibre, on the other hand, is often applied in the form of a quilt with a covering of fine muslin (or scrim). The glass tissue or scrim cloth used for surface finishing prevents any loose fibres (from the un-faced absorbent) from falling through large perforations.

Thin plastic films may, alternatively, be placed over the perforated ceiling trays (to act also as a vapour check). It is generally impracticable to obtain a complete vapour barrier with suspended ceilings. This is because of the requirement for access, and the extent of joining.

The effect of applying a non- porous plastic film to the surface of fibrous insulator is to reduce the absorption of sound at high frequencies. A similar effect is obtained by the application of perforated panels. When these two are employed together, the absorption at high frequencies is increased further.

In situations (e.g., hospitals) where facility for cleaning is desired, porous absorbent tiles in ceiling may be covered with flexible plastic film, without appreciably affecting their sound absorption characteristics. The plastic film used for this purpose should not, however, be thicker than 0.05 mm, and it should be secured only around the edges of tiles, thus allowing the film to vibrate independently.

The painting of porous ceiling tiles is not desirable unless it can be assured that the decoration would not clog up the pores and thus reduce the absorption of sound by porous tiles. If the surface of ceiling tiles is sealed by painting, reflection of sound will be increased, and this will have a deleterious effect on absorption performance of the ceiling.

Effect of Density:

Most porous absorbents have an optimum density and flow resistance at which maximum absorption is achieved. Too small a pore structure, for example, will restrict the passage of sound waves. On the other hand, when the pore structure is too large, it offers a low frictional resistance. In both of these extreme cases, low absorption is the result. The more porous materials generally have a low density.

The larger-density absorbents can be gainfully employed in raising the level of sound absorption, when it is not practicable or desirable to include a cavity of appreciable depth within a wall or ceiling structure. Table 6 shows the absorptive properties of rock-wool products of different densities.

Cavity Depth:

An airspace formed between the porous absorbent and solid backing will generally improve absorption of sound at lower frequencies. Absorption is increased with increasing cavity. Airspaces up to about 40 cm have cor­respondingly higher absorptive power at frequencies below 250 Hz.

Since airspaces increase absorption of sound at low frequencies, their effect is similar to that obtained by increasing the thickness of mineral fibre. The use of a cavity alone, however, is not generally sufficient for normal requirements, and some absorbent material is usually included. When an absorbent material is also used, it will often increase and extend the absorption coefficient over a broader range of frequencies.

Perforated Panels:

Porous absorbents (such as mineral fibres are commonly applied behind perforated or slotted hardboard, plywood or plasterboard, mounted upon battens of sufficient thickness to accommodate a minimum 2.5 cm mineral fibre and an airspace.

Absorbent materials are also used in conjunction with hardwood straps with narrow continuous gaps between them. The porous absorbent material should be placed close to the vertical or horizontal panel to obtain maximum absorption efficiency. If a cavity is also constructed, absorp­tion at low frequencies will be improved.

Acoustic treatment with perforated panels, however, is not suitable for applications in areas of high humidity (such as laundries, swimming baths and some industrial situations). The reason is that large amount of moisture is usually present in such situations, and it could gain access to the fibrous absorbent material and affect its performance.

When it comes to tiles, sticking them tightly to the walls or ceiling is the least effective way of treating a given area with sound-absorbent tiles. In this position, the airflow through a tile is at a minimum, since the velocity of air particles is zero at the wall surface. The result is that the dissipation of energy is minimized in the areas of absorbent material adjacent to the wall.

The best method is to mount the tiles off the wall, leaving a cavity behind them. The method of mounting tiles in this way is almost as effective as the use of a sound absorbent with a thickness equal to the total thickness of cavity plus tile. However, if the energy of sound waves is concentrated in the high-frequency range, a cavity behind the tile does not offer much advantage.

An absorbent tile mounted in the middle of one of the long walls of a room has an absorption of sound which is only about one quarter that of the same area of tile mounted in the same way, but in a corner at the junction of three surfaces. Similarly, the same tile stuck to the wall at the junction of two surfaces is about twice as efficient as the tile mounted in the middle of the long wall.

On the other hand, if the tile is used to cover a column well away from the walls, or to face a free standing screen, it may be 3-4 times as effective as when mounted on the wall. The same considerations apply when the absorbent is suspended from the ceiling or from the roof trusses.

Perforated Facings:

Such panels are intended to form a decorative or protective facing for sound-absorbent materials. One should see to it, however, that these panels exert least influence upon the absorption characteristics of the absorbent materials. It is well known that the absorptive behaviour of perforated panels is controlled by the extent of perforation.

The amount of sound absorption that can be achieved by the application of perforated panels depends on:

(a) The spacing between perforations;

(b) The diameter of perforations;

(c) The percentage of “open” area; and

(d) The thickness of panels.

Other things being equal, the effect of the facing is minimized with “open” area of the perforated panel in the region of 10-20%. When this is the case, the performance of the sound-absorbent structure is influenced mainly by the backing material and cavity.

In the case of perforated panels a reduction in open area alters the overall performance, in general, by increasing the low- frequency absorption efficien­cy and decreasing it at higher frequencies.

In situations where low-frequency noises are the most disturbing ones, and a lowering of absorption efficiency at higher frequencies is acceptable, perforated panels with as little as 3% open area may be preferred. In fact, perforated panels function essentially as low-frequency absorbers. The absorption performance of perforated ceiling tiles of different designs and open area is given in Table 7.

The results of the study of absorptive properties of perforated panels indicate that effective control over the pattern of perforations, often for the purpose of decoration, will allow a wide variety of designs to be produced. These, in turn, can provide equally favourable degrees of absorption. In such cases, direct relationship between the open area of different panels is not the critical factor.

Resonant Absorbers:

A simple resonant absorber comprises a cavity enclosing a mass of air, with a narrow opening to the outside, as shown in Fig. 3. The air mass inside such a cavity can effectively act as a spring at the resonant frequency of the cavity and, under those conditions, can absorb appreciable amounts of sound energy, thus exciting the resonance.

The resonant frequency f_r of such a cavity (known as “Helmholtz resonator”) is given by the relation

f_r (in Hz) = 55 LV, … (7)

where S = area of cross section of the cavity opening (in m²), L = length of the opening (in m) and V = volume of the cavity (in m³). At the resonant frequency, such a resonator is capable of absorbing up to nearly 90% of the sound impinging on the cavity (via the opening).

The performance of such a resonant cavity is modified considerably by lining the cavity with an absorbent material. Although the peak absorption is substan­tially reduced when the resonant cavity is lined, the absorption is spread over a wider range of frequencies, as shown in Fig .4.

Perforated panels backed by a sub-divided air space have the property of acting as multiple resonant absorbers. The resonant frequency F_r of such a panel is given by the relation

f_r (in Hz) = 5,000 = P/L(t+ 0.8 d), . ………. (8)

where P = percentage of open area, L = depth of air space (in mm), t = thick­ness of the panel (in mm), and d = diameter of the perforations (in mm).

We note here that Eq. (8) is only an approximate relation; but it will indicate optimum values of P, L and t to provide maximum absorption of sound at a specific frequency.

However, the actual performance of such a perforated panel may be considerably modified by the introduction of absorbent material behind the panel, when the percentage of open area will largely govern the absorption of sound achieved at higher frequencies.

If the perforated panel is thin, and employed primarily is a protective cover for an absorbent material, its principal effect is again to reduce the absorption at higher frequencies. This reduction in absorption is inversely proportional to the percentage of open area. The frequency f at which this reduction is likely to become apparent can be estimated from the approximate relation

f = 1,000 P/d, -(9)

where P = percentage of open area, and d = diameter of the perforations (in mm), as before. It follows from the last relation that a large number of small-diameter holes giving a specific open area are more beneficial in delaying the loss of absorption with increasing frequency.

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