Everything you need to learn about reducing and controlling noise.
Introduction:
It may not be possible to have a total elimination of annoying sounds but attempts should be directed to reduction at source, reduction of the duration of exposure etc.
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The sounds generated and transmitted in air directly to human ears are known as air borne sounds. The air-borne noise possesses less power, continues for a long duration and is confined to places near its origin.
It is transmitted to the receiving room in two ways:
(i) By air path between two rooms such as doors, windows, ventilators, key holes, ducts, pipes etc.; and
(ii) By forced vibration set up by the transmitting room to the walls, floors and ceiling of the receiving room.
The sounds which originate and progress on the building structure are known as structure borne sounds or impact sounds. The structure borne noise is powerful, propagates over long distances and persists for a very short duration. It is developed in solid structures and is then transmitted as air-borne noise. The closing of doors, vibration of machines etc. set up vibrations in solid materials of the structure which result in transmission of noise to the receiving room.
Noise Reduction at Source:
A great deal can be done to cut down the amount of noise generated by plant, machinery or equipment at source. The correct use of attenuators, as has been seen, will greatly reduce the level of noise transmitted through a building by means of ducting, trunking or other noise transmission media.
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Where, however, the initial source of noise is a particularly troublesome one, so that even the most careful selection of equipment and mounting techniques fails to produce the amount of noise reduction required, other measures must be considered.
There may also be instances where the degree of noise reduction obtained by these methods, though sufficient for normal purposes, is not enough for an area where exceptionally quiet conditions are required for special reasons. In such cases, the additional steps which can be taken include the use of absorbent materials or the erection of special sound barriers and enclosures to control the transmission of noise from source.
Reducing noise at the source itself is the most promising method, which is accomplished as follows:
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Noise should be reduced as near the source as possible so that acoustical treatment is less expensive and a large number of people are protected from the noise. While the operational processes in a factory may be fixed and may have no quieter alternative, careful selection of the machine tools and equipment to be used may considerably help attaining lower noise levels in the machine shop. One make of machine tool may have noisy mechanical system compared with another of similar performance.
2. Reducing Noise from Potential Sources:
Impact that is not essential to a process should be quietened. Noise from handling and dropping of materials on hard surfaces may be reduced by using soft resilient materials on containers, fixing rubber tyres on trucks, trolleys etc. Machine noise may be kept to a minimum by proper maintenance. Proper lubrication reduces noise by friction from conveyors, rollers etc.
3. Noise from Radiating Surfaces:
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This can be reduced by reducing the radiating area. For example – if the area is halved, the noise intensity will be reduced by three dB and at low frequencies the reduction will be much greater. Supporting structures for vibrating machines and other equipment should be frames rather than cabinets or sheeted enclosures. If an enclosure is used, precaution should be taken to isolate it and line it on the inside with sound-absorbent material. The noise radiated by machinery guards can be minimised by making them of perforated sheet or of wire mesh.
4. Reducing Transmission of Mechanical Vibrations:
A vibrating source does not usually contain a large radiating surface but the vibration is conducted along mechanically rigid paths to surfaces that can act as effective radiator. If the rigid connecting paths are interrupted by resilient material, the transmission of vibration and consequently the noise radiated may be greatly reduced. The reduction depends on the ratio of the driving (forcing) frequency of the source to the natural frequency of the resilient system.
Vibration isolators are usually made of resilient materials like steel in the form of springs, rubber, cork and felt. Because of the large range of deflections obtainable in coil springs, they may isolate vibrations over a large spectrum of low frequencies. Metal springs transmit high frequencies (from about two hundred to several thousand c/s) very readily. Transmission of these frequencies can be reduced by eliminating direct contact between the spring and the supporting structure. Rubber or felt pads may be inserted between the ends of the spring and the surfaces to which it is fastened.
Rubber pads may be used to isolate very effectively, relatively small machinery, engines, motors etc. It may be used in compression or in shear. Some rubber mountings use rubber in shear as the primary elastic elements and rubber in compression as a secondary element which furnishes snubbing action if the mounting is subjected to an overload. Felt or cork or both may be used as resilient mats or pads under machine bases.
Large press droppers which create serious impact vibration in heavy machine shops may be mounted rigidly on massive blocks of concrete having weights many times greater than the weights of the supported machines. The inertia blocks may, in turn, be isolated from the building structure by large wooden block and with thick pads of cork.
In critical installations, attempt should be made to locate the resilient mounts in a plane which contains the centre of gravity of the mounted assembly. It is also preferable to locate the mounts laterally as far away as possible from the centre of the machine.
Rigid mechanical ties between vibrating machine and building structure short circuit or reduce the effectiveness of isolators. Loose and flexible connection should be inserted in all pipes and conduits leading from the vibrating machine. Where flexible connections are impracticable, bends should be inserted into the pipes or the pipes themselves should be supported on vibration mounts for a considerable distance from the source.
6. Noise Reduction by Enclosures and Barriers:
Air borne noise generated by a machine may be reduced by placing the machine in an enclosure or behind a barrier. Much larger noise reduction can be achieved with complete enclosures. The enclosure may be in the form of close-fitting acoustic box around the machine such that the operator performs his normal work outside the box and thus is not subjected to the high noise levels of the machine. The enclosure may also be made of sheet metal lined inside with an acoustical material.
Where size of the machine, working area and the operation do not permit close-fitting enclosures, the machine may be housed in a room of its own. The inside of the enclosure should be lined with sound-absorbing materials to reduce the noise level of the contained sound. The walls of the enclosures shall also have adequate transmission loss to provide proper sound insulation.
A partial elimination of noise in certain directions may be obtained by barriers or partial enclosures. Two sided or three sided barrier, with acoustic absorption material may affect appreciable noise reduction. Where it is possible, the opening should face a wall covered with sound absorbing material. If the top of the enclosure is open, the reduction may be increased by placing the sound-absorbing material on the ceiling overhead.
7. Acoustical Absorption Devices:
In order to reduce the general reverberant noise level in machine shops, acoustical material may be placed on the ceiling and side walls. With this treatment three to eight dB reductions of middle and high frequency noise may be achieved. This would bring down the general reverberation noise level and as a consequence, the noise conditions may become less confusing.
For efficient noise reduction functional sound absorbers may be clustered as near the machine as possible. These units may be suspended and distributed in any pattern to obtain lower noise levels within the machine shop. Compared on the basis of equal total exposed surface areas, functional sound absorbers have slightly higher noise reduction coefficients.
For noisy industries, the workers should be provided with ear protection aids like earplugs, headphones or noise helmets. A unique form of operational modification is employing deaf persons to handle noisy equipment, wherever possible, subject to suitable safe guards. The noise levels must be reduced to a point where the noise hazard will be reduced to a condition of acceptability.
ii. Design of Doors and Windows:
For reducing noise, it is necessary to design carefully the doors and windows of the room. The sound travels through very thin cracks between the door and wall. Excellent sound insulation is obtained by constructing glazed windows with double or triple panes of glass. The air space at the edges of such panes is filled with sound absorbing material.
iii. Treatment of Floors, Ceilings and Floorings:
It is known that floating floors and suspended ceilings help considerably in reducing noise. Suitable sound absorbing materials like hair, felt, acoustical tiles and perforated plywood and specially made porous materials are available and can be fixed on walls, floors and ceilings to have reduction in noise. It is suggested to use a few suspended absorbers from the ceiling for reducing deflections from the ceiling and for absorbing the machine noise. These absorbers may be suspended on the top of the machine and as close as possible to it.
A new concept gaining acceptance is the planting of trees like Neem, Tamarind, Coconut etc. near schools, hospitals, public offices and such sites. The presence of trees is likely to reduce the noise to the extent of about 8-10 dB. Similarly indoor plants are helpful to reduce noise in a house by 8 to 10 dB.
Belts of trees or shrubs may also be used as barriers of sound. It has been found that in dense evergreen woods, the attenuation of sound is about 18 dB per 300 m at 500 c/s. This is due to absorption by the foliage on one hand and multiple scattering on the other. Obviously, denser the barrier, greater is the attenuation.
v. Use of Silencers or Filters:
This method is applicable to the control of noise from ducts, exhausts or conveys systems, the ends of which must be open to the atmosphere. For this purpose glass wool or mineral wool covered with a sheet of perforations may be used.
This arrangement is attempted to reduce vibration. A layer of damping material in the form of resilient pads made of rubber, neoprene, cork and plastic may be adopted for high frequency vibrations. It is desirable to make massive base for a vibrating machine.
vii. Noise Control by Locations:
Another practical method of reducing noise is by increasing the distance between the source and work vicinity. Machines, processes and work areas which are approximately equally noisy should be located together. Areas that are particularly noisy should be segregated from quiet areas.
The office space in a factory should be as far as possible segregated from the production area and preferably be located in a separate building. If a common wall is unavoidable it should be heavy with minimum connecting doors and no permanent openings.
8. Noise Control by Absorption of Reflected Sound:
The absorptivity of the ceiling can be increased by introducing large absorbing area without interference with other operations.
Two methods are commonly employed:
(i) By suspended grid system, using lay- in or concealed grid type panels or tiles; and
(ii) By vertically suspending panels of absorbent material.
If ceiling treatment does not give required attenuation, reflections from the walls should be reduced by applying absorptive treatment to them.
After the above treatments if the noise is to be reduced further, it is advisable to use screens around the noisy machines. Sound reduction of the order of 10 dBA to 15 dBA may be achieved by using screens in an already treated area. The various other absorbing materials are acoustic plaster (a plaster of granulated insulting material and cement), compressed cane or wooden boards, perforated asbestos, teak plywood packed by wire etc.
It is important at the outset to recognise the clear distinctions between sound absorption (which is the purpose of absorbent materials) and sound insulation or prevention of the transmission of sound (which is the function of the partition). Thus, if a room (or for that matter, a partition) is treated with absorbent materials, whilst these will reduce the noise level within the room they will not necessarily substantially reduce the amount of noise which is transmitted through the wall or partition to the next room.
Sound absorption is defined as the prevention of noise reflection from surfaces and thus, the reduction of reverberation times. It is a term which relates to the loss of sound energy which can be reflected from a hard surface and so is mainly a property of the surface construction or material.
Sound insulation, on the other hand, is the term applied to the noise reduction which is obtained when sound passes from one room to another or from one side of a partition to another. It can, therefore, apply to a wall, roof, floor or any other barrier through which sound can travel to the detriment of people on one side or the other of it. Sound insulation is a function of the whole construction of the wall or partition, influenced by the surrounding structure and other relevant factors.
Two extreme examples will perhaps help to make this distinction clear. An open window, which reflects no sound at all, can be reckoned to be almost 100 per cent absorbing, whereas, at the same time, it is also almost 100 per cent transmitting. Another example of the fact that sound absorption is not a measure of sound insulation is seen in the case of a sponge which, whilst it will absorb a great deal of water, will also transmit nearly as much as it absorbs.
But, although there is no direct relationship between the sound insulation value of a wall or partition and the sound absorption of its surface, this is not to say that both factors must be considered entirely in isolation from one another. There are, for example, a number of ways in which sound absorption affects sound insulation. The amount of absorption units in a room affects the level of the reverberant noise, which, in turn, affects the level of the transmitted noise.
Changes in noise levels due to absorbent treatment of the room surfaces are usually small relative to sound insulation, but may have to be taken into account where the problem is being considered within very precise limits. It is important to remember, however, that the absorbent materials are not acting as a sound barrier but are simply having this effect through reduction of reverberation times and a small noise level reduction due to a minimum of reflection.
The primary purpose of absorbent materials is, therefore, to control noise levels inside a room and only incidentally to reduce the amount of noise transmitted which, in any case, will only be a relatively small amount.
Sound is absorbed by means of a phenomenon under which it is converted into other forms of energy and ultimately into heat. There are two main ways in which this process works, depending on whether the transformation of energy is taking place through absorbent materials or panels. In the case of absorbent materials, the main factor is the porosity of the material.
Consider, for example – the case of mineral wool which is composed of a multitude of tiny, deeply-penetrating and intercommunicating pores. The sound waves enter these pores and are converted into heat by the frictional and viscous resistance in the pores and by the vibration of the fibres of which the mineral wool is composed. By contrast, when sound strikes a panel, the vibrations set up provide the medium through which the sound energy is converted into heat. The greater the vibration, the greater the amount of sound energy which is so converted.
Both these methods of transforming sound energy into heat are of considerable importance to the acoustic consultant when designing suitable sound treatment systems, because, to a certain extent, they are complementary to one another. Absorption by absorbent materials tends to be large at high frequencies but relatively small at the lower frequencies. On the other hand, absorption by panels tends to be small at the higher frequencies but larger at the lower levels.
Absorption Coefficient:
In order to design a sound control system it is necessary to have an accurate indication of the acoustic performance of the absorbent materials which are available for use. This information is obtainable or can be measured, for any given material in the form of an absorption coefficient.
When a sound wave strikes any surface, it is partially reflected and partially absorbed. It is, therefore, possible to arrive at a relationship between the proportion of the sound energy absorbed by the material and the total sound energy of the incident sound.
This relationship provides the absorption coefficient, which can be defined as the fractional part of the total energy of an incident sound wave which is absorbed (and not reflected) by the material. For example – suppose that 55 per cent of the incident acoustical energy is so absorbed, the absorption coefficient of that particular material would be 0.55.
If a certain area of sound absorbent material of a given, uniform absorbent coefficient is taken and the area multiplied by the coefficient, the resultant product of these two quantities is known as the absorbing power (or absorption) of the surface. When the area is expressed in square feet, the unit of absorption is known as the sabin; a sabin is, therefore, equal to 1 square foot of surface with a coefficient of 1.0 or 1 square foot of ‘open window’.
It is now necessary to introduce a further complication because the absorption coefficient varies with the frequency of the incident sound. To deal with this contingency, it is usual for the coefficient to be quoted for frequencies at octave intervals from 125 cycles to 4000 cycles or in other words, at 125, 250, 1000, 2000 and 4000 cycles. This is sufficient for most practical purposes, but, in special circumstances, it may be necessary to obtain values at other frequencies.
All reputable manufacturers of sound-absorbent materials supply a considerable amount of data in respect of their performance, including absorption coefficients, but, if, for any reason, it is necessary to measure these values for a particular material, this can be readily done by the consultant, using the appropriate procedure and equipment.
Other factors apart from the absorption coefficient must also be taken into account when selecting materials for any particular duty. Depending on the situation in which the materials are to be fitted, the ambient conditions and other relevant factors, consideration will have to be given to such characteristics as appearance, wearing qualities in service, ease of maintenance and possible decoration and fire risks.
The acoustic consultant will be able to advise on all these aspects and will bear in mind the need to provide optimum performance from every point of view with minimum cost of materials, installation and maintenance. In very special circumstances, it may be necessary to seek materials with nonstandard properties, such as the capacity to repel exceptional quantities of airborne moisture or to resist insect attack.
Quite apart from special factors such as these, there is as yet no universal, general- purpose absorbent material which can be regarded as suitable for all normal sound insulation problems. It follows, therefore, that each problem must be considered on its merits and each area or even each room in a building may have to be treated with a different material.
Types of Absorbents:
Many different types of absorbent materials and systems are available from specialist manufacturers, but they can all be said to fall into three main classes. The largest and most comprehensive of these is a wide-ranging group of prefabricated units of many different kinds, including acoustic tiles of many materials, perforated units, backed by absorbent materials, wallboards and absorbent sheeting.
A second group includes a range of acoustical plaster and various kinds of sprayed-on acoustical materials. Numerous compounds are commercially available and can be spray-applied by normal equipment or in the case of plasters, applied by trowel and ‘float’. The third group includes various types of acoustical blanket.
Materials can also be classified into types, according to the manner in which they function. Some, for example, depend on porosity and as already mentioned, these absorb sound mainly at the middle and higher frequencies. Into this group fall such materials as mineral wool, glass wool, expanded plastics, asbestos fibre and numerous proprietary acoustic tiles, many of which are made from soft fibreboard, asbestos or mineral-wool-based materials.
Many acoustic tiles have non-porous surfaces (to facilitate decoration) but are drilled with small holes through which the sound waves pass into the internal fibres where they are absorbed. Acoustic plasters and sprayed-on materials are also porous and with these and also with plain porous tiles or boards, care must be taken when decorating, since blocking of the pores will reduce their acoustic efficiency Yet another type into which absorbent materials can be classified is the resonant panel, which absorbs mainly at the lower frequencies, typically in the 50 to 200 cycles range.
It is possible to combine the effects of porous materials and resonant panels, as in a perforated panel backed by an absorbent material. A typical unit of this kind may comprise a panel of plywood, hardboard, plasterboard, metal or other material, perforated with holes or slots and mounted on battens or metal framework in such a way that a suitable porous material (mineral wool or glass wool is general) can be enclosed between the panel and the wall or ceiling.
To obtain optimum results, the porous material should be placed as closely as possible to the panel. The same principle can be applied by using a perforated panel with a slab of porous material fixed to the back of it; and satisfactory results can also be obtained from a perforated panel backed by a sheet of fibrous glass tissue.
When considering absorbent materials or units for any particular installation, special attention has to be paid to the method of mounting, since this can affect the performance. For example, if a panel-type of resonator is fixed to a wall with an air space behind, its performance is improved; similarly, an absorbent sheet fixed on battens will also give improved performance.
Some reference has already been made to the problem of decoration but, in general, it can be said that all materials which depend on their porosity for sound absorbent effect should not be painted, while those which depend on vibration can be decorated without loss of efficiency.
A specialised type of sound absorbent unit, which may be required in certain circumstances, is the functional absorber. This is a particular type of acoustic unit, designed to carry out a specific function and which will vary in shape and in respect of the materials of which it is made, according to the required application.
Functional absorbers can be designed to provide both reflective and absorptive values at specific frequencies and this is achieved by the selection of materials of which they are constructed and by the choice of their geometrical shape, which can be a cube, cone, pyramid or other form. By using acoustic tile material and a suitable shape, a considerable area of absorption material can be provided in a given spot.
They can be fitted where required; for example, functional absorbers can be fixed to walls or suspended from ceilings and are often useful in factories where machinery, overhead cranes, pipes, ducting or other equipment make it difficult to fix acoustic tiles to some of the materials of which the factory roof is frequently constructed, such as asbestos or corrugated iron.
Another advantage of the functional absorber is that it can be placed wherever it is required from an acoustical point of view. For example, in a large factory area there may be a section where some particularly noisy machinery is located and special acoustical treatment is required.
Functional absorbers can then be fitted to reduce factory noise to optimum levels, so avoiding any necessity for applying materials to the walls or ceilings. Alternatively, where noise levels are high throughout a factory area, but where personnel are only working in one part of the area, functional absorbers can be fitted to provide the desired conditions for workpeople.
Functional absorbers also have a number of sophisticated applications, such as in restaurants, night clubs and other locations where exotic conditions are specified. By siting the absorbers in suitable places, the desired acoustical and aesthetic conditions can be obtained in every part of the room or hall. In such circumstances, it may be necessary to employ either the reflective or the absorptive surfaces or even make alternate use of them, according to requirements.
When considering any form of corrective treatment, using any kind of absorbent materials or units, allowance must be made for any incidentally absorptive materials which may be present, such as carpets, curtains and furniture.
Barriers and Enclosures:
Attention has been concentrated on the prevention of reflection, a process which requires the absorption of sound. It is now time to consider the prevention of sound transmission through barriers of all kinds, a process which requires the insulation of sound.
Barriers can be of many different kinds, ranging from the conventional dividing wall between two rooms to the very complex barriers erected in such locations as engine test beds. Enclosures, too, can range from thinly-divided sections in an ‘open-plan’ office to complex covers erected over very noisy machinery. Certain basic principles apply in all these cases.
One distinction which must be made is between airborne and impact sounds, as they affect sound insulation procedures. In this sense, an airborne sound is one which has become airborne before reaching the structure of the room which is being insulated. An impact sound, on the other hand, is one created by a blow on the other side of the partition and which is therefore radiated through the structure and directly into the room concerned.
One way of making this distinction is to visualise an unmounted reciprocating compressor located on the roof of a building. To those in the apartment below, the effect will be mainly impact or structure-borne sound, while those in a detached adjacent building will receive the noise as airborne sound. The methods of insulation differ in each case.
Transmission of sound to the receiving room can be both direct and indirect and the relative amount of sound energy transmitted by these two means is an important factor in the design of a satisfactory insulating system. The direct sound (it may be either airborne or structure-borne in origin) is that which impinges directly on to the surface of the partition, dividing walls or ceiling between the source and the receiving room and is radiated from the other side of the partition or surrounding surface and into the other room. The indirect sound is that part of the incident sound which reaches the receiving room by any other route, such as air paths provided by open windows, ventilation ducts, over the top of partitions and so forth.
Just as the term ‘absorption coefficient’ has been devised as a measurement of the efficiency of absorbent materials, so a need has arisen for an indication of the performance of a partition. The term in this case is known as the ‘sound-reduction index’ (or noise reduction factor) and it represents the insulation value in decibels of the partition material, when the measurement is made under the conditions laid down in British Standard 2750. The problem is usually complicated, in practice, by the presence of a number of conflicting factors, such as the amount of absorption in the room on the receiving side of the partition and other local conditions.
Under laboratory conditions, the sound reduction index of a partition material can be measured, with all indirect sound transmission excluded but, in practice, the amount of indirect sound transmission may well be just as important as the direct value.
It is usually reckoned that the final transmission loss is a function of the sound reduction index of the partition and the acoustics of the room it divides. The amount of sound energy transmitted between rooms, both by the direct and indirect paths, is the level which is subjectively experienced by the room occupants and is, therefore, the most important factor of all.
Where the sound reduction index of the partition is not more than 30 dB, net room transmission loss can usually be regarded as roughly the same as the index figure, if all reasonable precautions have been taken to provide a good installation. As a rough guide, it can generally be assumed that to provide transmission losses progressively above this figure becomes more and more difficult, as regards indirect transmission.
Below that figure, laboratory assessments are likely to be a reasonable guide to figures obtained in practice, while above 35 dB, over the whole spectrum, increasing difficulty is experienced and special techniques must be devised to secure these higher losses.
Sound Transmission by Partitions:
Sound is transmitted through a partition as a result of vibrations of the partition which are set up by the incident sound. The vibrations set up in this way vary according to the material and method of construction of the partition and the frequency of the incident sound. Every partition has a fundamental resonance or natural frequency and this value is mainly dependent on its dimensions, mass and stiffness.
Below this natural frequency, insulation is directly related to stiffness, while above the natural frequency, mass can be the dominant factor. In the case of most normal partitions, the natural frequency is well below 100 cycles per second, so that mass and applied constructional techniques become the controlling factor.
Although the flexural vibration of a partition is the most important mechanism of sound transmission, other modes may be set up, including compressional and bending waves. An important aspect of these other modes of vibration is the possible introduction into the situation of what is known as the ‘coincidence’ phenomenon.
The whole question of coincidence behaviour is a complex one but, for present purposes, it is sufficient to say that it is produced by in-phase vibrations set up in the panel itself which, at specific frequencies, reinforce the existing panel vibrations produced by the incident sound in the normal way.
At such frequencies, therefore, the reinforcement of the normal panel vibrations leads to a reduction in insulation performance as indicated by the sound reduction index, thus resulting in a deviation below predicted behaviour. This is known as the ‘coincidence dip’.
The frequency at which this dip occurs varies from one material to another and the ideal material is that which follows the predicted behaviour to the highest sound reduction index level. Governing factors are the modulus of elasticity and the density of material. A material with a high modulus of elasticity and low density (such as rigid board materials) produces a fairly high coincidence phenomenon, while materials with low moduli of elasticity and high density (lead is the obvious example) give low levels of coincidence behaviour.
Having thus outlined the main factors to be taken into consideration when selecting partitions for particular duties, we can now pay closer attention to the two main types of insulation required – insulation against airborne sound and against impact sound.
Insulation against Airborne Sound:
The three main principles to be considered are the weight, uniformity and stiffness of partition materials. Sound diminishes as it passes through a barrier because it has to overcome the inertia represented by the barrier.
Clearly, the heavier the barrier, the more it will resist vibration and the more effective its insulation properties are, allowing for the reservations already mentioned. It has been shown experimentally that a definite relation exists between insulation values and mass per unit weight of partitions and that when these values are represented graphically; a ‘mass curve’ is arrived at.
Every material follows the mass law, which explains why weight plays such an important part in all sound insulation problems. Though the weight graph is an important aid to the design of partitions, it is not sufficient to look up the relevant point and expect to obtain the performance given. The possible presence of coincidence effects must always be considered, especially with rigid materials.
Uniformity is another important factor in the performance of any barrier, since if there is any hole in the partition, sound energy will flow through it, just as gas will escape through a hole in a cylinder. This is perhaps rather too obvious an example, but there are other possible areas of weaker insulation in the average barrier which sometimes escapes attention.
Ventilators, windows and even slots for the passage of telephone wires are cases in point. Where air paths are absolutely necessary, as with ducts of various types, they must be so designed and acoustically treated that their insulation performance is up to the same standard as that of the remainder of the partition.
The procedure for insulating ducts is to provide as much internal acoustic lining as necessary, possibly using the ‘splitter’ technique. By relating the sound reduction per unit length obtained from the materials to be used to the total sound reduction required, the length of duct treatment can be calculated, remembering that any bends in the ducts can give additional attenuation.
Consider, next, the importance of stiffness of the partition material in relation to its insulation performance. It is here that the coincidence effect has to be taken into account because it is the relationship between stiffness and weight which often produces the coincidence dip in the weight curve, at a frequency within the important 100-3200 cycles range.
To reduce the stiffness without reducing weight will push the dip higher in the frequency range and possibly outside the important range for insulation. For this reason, as a general rule, limp materials are preferred for sound insulation, provided their use introduces no installation problems. Another important advantage of the limp partition is that the fundamental resonance frequency is also lower than with more rigid materials.
At all times, of course, it is necessary to keep installation problems clearly in mind because, except in very specialised circumstances, the role of the acoustic consultant is to select a partition material which gives the optimum results when all relevant factors, including installation and other costs, have been duly assessed.
Thus, while all limp materials are good from an insulation point of view, they may not be practical to install under all circumstances. The problem is likely to be particularly acute in the case of lightweight partitions, which have to be reasonably stiff for installation reasons and are therefore more likely to produce coincidence effects.
Discontinuity can also be an important factor in partition design and in double-skinned partitions the maximum advantage should be taken of the cavity. Obviously, adequate sealing of partition panels to the carcase is vital and it is an advantage, particularly when high transmission loss values are required, to achieve panel discontinuity with respect to the opposite ‘twin’.
Unidentical twin panels are an advantage with respect to weight, though this is sometimes disadvantageous as regards production techniques and costs. Cavities with dead air should be acoustically saturated with absorbents to prevent the build-up of sound.
Impact Sound Insulation:
An impact sound is one created by a blow on one side of a partition which is radiated directly into the room on the other side. The principles which apply here are simpler than those governing airborne sound insulation.
In most cases, impact sounds are likely to be created on floors rather than walls or partitions and the problem is, therefore, largely one of insulating the occupants of a room from impact noise produced by people in the room above. The most frequent causes of this kind of trouble are movements of people or equipment across the floor.
To control the effects of impact noise, the main procedure adopted is to provide a layer of resilient material to kill the force of the blow. In the home or office, for example, the obvious solution is to lay a carpet, but this may not be feasible where the floor is to be subject to heavy use or as in a dance studio or gymnasium, where a carpet will inhibit the activities for which the room is designed.
Here the solution would be to fit the resilient layer beneath a harder wearing surface, so that the actual wearing surface resembles a raft or floating floor. It is worth nothing that this kind of arrangement provides an element of airborne as well as impact sound insulation. On the other hand, a greater degree of protection by resilient material is required for under-floor installations in order to obtain the same result as with a resilient layer placed directly on to the floor.
Another point to be kept in mind when designing the raft-type floor is that if air spaces are left between the raft and the resilient layer, as when the floor is mounted on battens, the result will be a drumming noise in the room whenever a person walks over the floor. This will not only annoy people using the room but may also be transmitted as airborne sound to other rooms, including the room below, for which the impact sound insulation was originally devised.
Types of Barriers:
Obviously, the type of partition selected for any particular application will depend on numerous factors, such as the degree of insulation desired to match predetermined NC curves, the amount and type of noise (whether impact or airborne) likely to be experienced, the design of the building (especially if the partitions are being installed in an existing structure) and economic considerations. Provided a proper survey is mounted and the partition designed in accordance with the results of these investigations, the result should be a satisfactory partition at a reasonable cost, in relation to the urgency of the problem.
Traditional materials are by no means to be disregarded and brick and concrete, where these are otherwise suitable, will often provide adequate performance. Many types of lightweight partition are available, especially in commercial and industrial applications and these may well be adequate, especially if the installation is such as to minimise the number of air paths at floor, ceiling and other joints, through which noise can be transmitted.
In a quiet, open-plan office, for example, a partition about 7 ft. 6 inch in height may provide adequate insulation, while in a light engineering works the same partition may only be satisfactory if it is continued up to the ceiling and properly sealed at that point.
Although conventional methods and materials can often be recommended for many applications, there are an increasing number of specialist installations for which much more complex systems must be designed, such as for providing a quiet area (e.g., a test shop) in an area of high level noise in a heavy engineering department.
This sort of problem calls for some form of discontinuous construction in which the various parts of the structure are separated so that sound vibrations are not easily transmitted from one part to another. Techniques used range from the cavity wall to fully-isolated box structures.
All these methods, in greater or lesser degree, are expensive to design and construct and it therefore follows that materials and methods must be carefully selected to avoid disappointing results from heavy expenditure. If discontinuous structures are to achieve the desired results, it is necessary to be aware of all the paths by which sound can travel through the partitions and to plan to block all these.
Cavity walls are now widely used and in their most simple form, may consist of two walls of brick or other building material, separated by an air space. The continuous air space acts as a cushion against the transmission of sound energy, but a narrow band of air provides only a moderate degree of cushioning and a considerable amount of vibration energy will still be conveyed over the gap, especially vibrations whose wavelengths are longer than the width of the cavity.
When cavity walls are constructed from brick or other conventional materials, it is important to make sure that any arrangements which ‘tie’ the walls together do not themselves permit the transfer of vibrations across the cavity, as would happen if ordinary metal ties were used. Special isolated types are available and should be fitted in accordance with instructions. Improved acoustic performance can also be obtained from cavity walls if a mineral rock wool blanket or quilt is inserted in the cavity.
Construction of Isolated Cabins:
Where a considerable degree of quiet is required in a room in an area of intense noise activity, it may be necessary to install an isolated cabin. This form of discontinuous construction can be designed in various ways, all involving double-skin partitions which may be either of complex or simple construction. The simple form of construction consists of a double-skin partition, while the complex type will have two double-skin partitions, separated by a cavity.
Double-skin construction is normally based on a framework of timber or metal to which carefully selected materials are applied in accordance with specified isolation techniques. In this kind of wall, each skin is formed from materials of different weights. In the case of a complex partition, while the same materials may be used in the two double-skin walls, the weights may differ.
By varying the weights of the materials in this way the performance is changed and if adjacent skins are of different weights, the phenomenon known as ‘sympathetic resonance’ is avoided. Vibrations at a particular frequency are, as it were, cut off by the material concerned and do not set up similar vibrations in sympathy in the adjacent material.
Calculations based on the weights of the materials and cavity widths enable the desired performance of the double-skin wall to be determined and reductions of over 50 dB can be obtained from complex skins. These techniques enable the design of a discontinuous structure to give any transmission loss required and since the cost of building these structures is high and increases very rapidly as they become more complex, it is clearly important to design for precise requirements and no more.
A double-skin construction comprises two outside layers and an interior material. For the outer layers, materials such as gypsum board, hardboard, asbestos or plywood, all of specific weights, can be used. For the interior material, on the basis of mass curve calculations, lead is widely regarded as one of the most suitable materials.
Since it is now mainly used in sheet form, in which condition it is non-stiff, the chief problem it poses is the method of mounting in the double-skin framework. If circumstances permit, the sheet of lead of the desired weight can be freely mounted between the other two skins, being suspended from the framework and fixed at top and bottom, using appropriate techniques to prevent the passage of sound vibrations above or below the lead.
This is normally quite a satisfactory procedure for double-skin walls in permanent installations, but there are problems when sheet lead is used in this way in demountable partitions. It is also difficult to use sheet lead as an isolating material in double-skin roofs and floors, due to its lack of rigidity. Research is, therefore, taking place into the possibility of laminating lead to more rigid skin materials.
Investigations are proceeding into the effects of laminating lead to a material with a low coincidence performance and studies are being made of bonding methods and the effects of different proportions of lead and bonding material in the composite product. As would be expected, the greater the proportion of lead and the more loosely it is bonded to the other material, the better the resultant material.
Another interesting line of research is the possible use of lead dust, which would be incorporated as an ingredient in another material whose acoustic properties would correspondingly be enhanced.
Importance of Partitions:
Partitions, in their widest sense, as barriers to the transmission of noise in all directions, are important factors in the design of every type of building. Though many complex problems are involved, a great deal is now known about their nature and solutions are available for almost all of them. Acoustic consultants, in conjunction with the manufacturers of materials, are now able to design systems to give the specified transmission loss, on the basis of well-proven techniques.