Everything you need to learn about controlling noise.
Introduction:
The world around us is becoming an increasingly noisy place and whether at work in office or factory, at home or in a place of entertainment or even in hospital, unwanted noise is seldom entirely absent. Depending on the circumstances, the noise may be merely a nuisance, wasteful of time and effort; or a positive hazard to health and safety.
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It follows, therefore, that freedom from unwanted noise is one of the most important features sought by the owners of any factory, office, hospital or indeed, any other building. And it can be achieved if, when designing new buildings or reconstructing or adapting old premises, all who are concerned will keep the problem of unwanted noise continually in the forefront of their thoughts.
So far as industry is concerned, the problem is not a new one, although the introduction of mechanisation and larger-scale machinery has undoubtedly accentuated the problem in recent times. Obvious examples include the need to ‘run in’ heavy diesel engines under test-bed conditions, the noise of high-speed presses and machine tools and the mass production of ball bearings.
Noise produced by industrial plant, equipment and processes poses two sets of problems. On the one hand, there are the effects of industrial noise on people living and working in the neighbourhood, but not directly concerned with the source of the noise; and on the other hand, the effects of that noise on those who work in the plant or factory.
In some cases, though not always, the same set of noise control measures will meet both the external and internal requirements and in any event, both these aspects have to be kept in mind when designing suitable noise control measures or recommending acoustical equipment.
So far as people living in the vicinity of industrial premises are concerned, the effects will depend upon the makers of the noise, the frequency with which it is emitted and the times at which it is heard. A noise which is not unduly disturbing during the day may cause a considerable nuisance during the night, while such a process as drop forging or saw-milling will be objectionable to people in its proximity at any time of the 24 hours.
These are extreme examples and modern ideas of town planning generally preclude this type of noise nuisance from arising in the first place. There are, however, many other industrial processes which, though less dramatic in their impact, nevertheless produce sufficient noise to be detrimental to the amenities of a district. The problem which then faces the architect and acoustical expert is to control the amount of noise which escapes from an industrial plant for the benefit of the population at large.
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Old ideas that a factory must be expected to be a noisy place and those operatives should regard noise as an occupational hazard, have now gone by the board. The spread of humanitarian ideas in this direction has been accompanied by the realisation that a quiet atmosphere in a factory or workshop will not only add to the welfare of the employees but will also result in greater efficiency.
Even quite low noise levels can interfere with the hearing of speech and instructions, while moderately loud noises can produce auditory fatigue. At the other extreme, very loud noise, especially if persistent, can sometimes result in a permanent loss of hearing.
It has been shown that excessive noise in a factory can result in loss of output, particularly towards the third period of a working day. Conversely, there are numerous cases where reduction of noise and a shortening of its ‘decay’ time in a factory or office area by the fitting of absorbent materials, have resulted in increased output. And in almost every such case, the cost of purchasing and installing the sound-deadening materials has been more than offset by increased output and efficiency.
One of the difficulties facing industry in relation to the problem of factory noise is that accurate information about its incidence is scarce. The position is becoming clearer, however, as acoustic engineers and specialist institutions concerned with the problem extend their investigations into all aspects of the subject. It is known, for example – that noise levels in such locations as boiler making shops, textile- weaving sheds and foundries can be as high as 100 dB, or even above, compared with the generally accepted danger level of 85 dB at 1000 cps.
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It is widely believed that workpeople continuously subjected to noise levels of this order throughout a full working life acquire a certain degree of occupational deafness—sufficient, perhaps, to dull their appreciation of music and make it difficult to listen to the radio at normal volume. Tentative estimates suggest that as many as one-tenth of all factories can be considered to be dangerously noisy and that between 5 and 10 per cent of all workers in factories are suffering from some form of hearing loss as a result of industrial noise.
Although the effects of industrial and other noise on the community at large are less dramatic than on workers in factories, this aspect of the noise problem in everyday life is rightly now receiving increasing attention. Here again, the problem is complicated by the lack of any exact methods of measuring the degree of annoyance, nuisance or injury caused by various types of noise.
The known constants, as applied in acoustical investigations of a specific noise, do not always satisfactorily express the nuisance value of a noise compounded of an industrial plant, aircraft, nearby road traffic and even the sound of radios and other noises from neighbours. In such cases, subjective criticism may be the only way of arriving at a judgment.
When considering community reaction to noise, it is therefore necessary to assess the environment in which the noise is being heard and using suitable bases of judgment, to determine whether or not a definite nuisance exists. Does the noise interfere with sleep or with study or with the enjoyment of normal amenities of modern community life?
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It may have to be accepted at the present state of knowledge and under present-day economic and social conditions that many industrial workers have to tolerate reasonably noisy conditions, during working hours. On the other hand, however, it is at least logical to suppose that they should be able to enjoy reasonable quietness during their leisure hours. This is one of the important social aspects of noise which especially concerns the acoustical consultant.
Besides seeking to control noise because of its effects on their employees or on the amenities of surrounding residential areas, industrialists must also bear in mind the possibility of legal action, either from those who are working in the factory or from householders or other sufferers outside.
Whilst it is obviously preferable to forestall any such legal action by adopting suitable noise control systems in the first place, objectors may come forward and it will then be necessary to determine whether, in fact, a nuisance does exist and if so, the steps which must be taken to abate it.
In the case of factories, power stations, railway marshalling yards, docks and other installations, the main problem is to so control the noise produced on the site that it does not adversely affect those employed there or living nearby. Where public buildings are concerned, the problem is generally the reverse: that of how to prevent the entry of noise from outside.
This particular problem is often the more acute because judicial buildings, local government offices and commercial and administrative buildings are frequently located in the centres of busy cities. Yet it is in these very buildings, where the nature of the work done calls for a high degree of mental concentration, that quietness is essential and can only be achieved under good acoustical conditions.
Again, in hospitals, where the need for quietness for patients and staff has always been generally recognised, no really scientific efforts have yet been made to analyse and secure the degree of quietness which patients should have. The problem is particularly acute in older hospitals, many of which are situated in busy urban areas.
It is in these hospitals, too, that the application of modern noise control techniques to old-style buildings poses particular problems. In new hospitals and hospital extensions, adequate acoustical arrangements can be made at the design stage.
To prevent outside noise from entering hospitals or other buildings, windows and suchlike means of noise entry have to be closed, which necessarily involves installation of a ventilation system. This, in itself, can easily produce a noise problem, unless proper steps are taken to control noise which is entrained through the system from outside or from fans, motors and other equipment. A combination of suitable sound barriers with absorbent materials within them, will, however, solve most sound problems of this kind effectively and economically.
Control of Noise:
If the measures undertaken to control noise are to achieve their optimum effect, the materials selected, the way they are utilised and the quantity applied should be determined in accordance with a technically correct acoustical scheme. This should be based on a competent sound survey from which an accurate specification can be drawn up and the installation should then be carried out by qualified technicians and workmen.
An example of how incorrect application of an otherwise acceptable technique can lead to unsatisfactory and/or uneconomic results can be found in the use of several barrier materials to solve a noise problem. The proper use of such barrier materials in combination with sound absorptive treatments is frequently misunderstood and the one confused with the other.
Even a difference of 10 per cent in the absorptive coefficients of two different materials used in such a system, may make a difference of perhaps no more than 1 dB in the average noise level of the room. Ignorance of this fact alone could, therefore, lead to the selection of a material or amount of material, altogether out of proportion to its beneficial acoustical effect, resulting in increased costs without any additional efficiency.
Over-treatment of rooms or factory areas is thus as much to be guarded against as the reverse danger of providing less than the required amount of insulation. In both cases, the prior institution of a properly-mounted acoustical analysis of the problem would have exactly determined the amount and type of materials necessary to bring about the desired conditions.
In new buildings, planning of the acoustical arrangements at the initial design stage will avoid the need for modifications later on. In all cases, in buildings old and new, it is usually necessary to consider a number of alternatives or combination of alternatives, to attain the desired noise reduction in the cheapest and most acceptable manner.
Aesthetic considerations are also important, providing another dimension to be considered along with effectiveness and cost, though obviously their degree of importance will depend to some extent on the location and purpose of the building or structure.
Approach to a Noise Problem:
Once it has been established that a noise problem exists and that action must be taken to solve it or at least to alleviate its effects, the question arises as to the correct approach to determining its nature and extent and devising methods for dealing with it.
That a noise problem should exist at all may be due to any one of a number of factors or to a combination of several. In industry, the problem may arise as the result of the installation of new plant and equipment which brings a previously acceptable noise level up to a point where action has to be taken. The introduction of a new process on existing plant or the use of higher speeds on machine tools or other equipment may have the same effect.
Higher noise levels may result from the re-siting of plant and equipment, in relation to machinery which is already producing noise, so that the combined result produces a noise level which cannot be tolerated. The erection of a new production department close to, say, the office and administrative block, may yield unacceptable noise levels in the offices, even though the production workers may not find the noise to be greater than is normally encountered in that type of work.
Nor is it always realised that the introduction of such otherwise desirable facilities as air conditioning systems, ventilation schemes and even central heating can have side effects on noise levels, through the operation of fans, pumps and other associated equipment. All these factors and others which will readily come to mind, can produce a noise problem which did not previously exist, but now calls for remedial action.
It would probably be no exaggeration to say that a noise problem of some kind or another exists, at some time or another, in almost every building in the country and that this situation is likely to prevail for as long as the general desire remains for a reduction in the amount of noise which exists in modern life. Fortunately, the necessary acoustical knowledge and equipment are now available to diagnose a noise problem and to produce a satisfactory solution.
Nevertheless, though the necessary techniques are widely available, only in comparatively few cases is a competent acoustical consultant called in when a new building is being designed; the old attitude that the acoustical conditions will ‘be all right on the night’ still applies in far too many cases. Even when an industrial client or public authority is aware of the implications of noise and the steps which can be taken to control it, the feeling still persists that the structure can be put up now and the acoustics, if necessary, applied later.
As a result of this attitude, it can and does happen that bad acoustical conditions are actually built into a new building, although they could have been quite easily avoided if proper acoustical advice had been taken at the outset. Just as in the case of bad plumbing, heating and ventilating, which can only be rectified later at considerable expense, so built-in bad acoustic conditions require subsequent alterations, the cost of which far outweigh the initial expense of building-in good conditions in the first place.
That this should be so is due partly to lack of knowledge and experience on the part of industrialists and others concerned with the contracts for new buildings and partly, no doubt, to questions of capital cost. Fortunately, the position is changing and an increasing number of clients are accepting the advice of their architects and calling in an acoustical consultant when commissioning plans for a new building.
In consequence, a growing proportion of new buildings can be expected to have good, instead of bad, acoustic conditions built in at the design stage. Similarly, in the case of many existing buildings which are being altered or improved, acoustical experts are now, increasingly, being called in to solve noise problems which have arisen from some reason and which it is now desired to control.
A procedure which is standard in main outline, but which will naturally differ in detail from one case to another, has been evolved and is now followed by acoustic consultants when approaching a particular noise problem. This involves undertaking a comprehensive survey of the problem, crystallising the problem into a report and producing recommendations for dealing with it.
A description of the steps which would be required to deal with a hypothetical noise problem will provide an indication of the latest techniques which are now employed in this field, whether the problem concerns a new building in which either no measures at all or inadequate measures were taken to control noise or an existing building whose acoustics it is desired to improve.
Recommendations for Controlling Noise:
It cannot be emphasised too strongly that the efficacy of the solution proposed for a particular noise problem will depend on the thoroughness with which it is investigated and the care with which recommendations, based upon this investigation, are framed. This, in turn, depends upon the employment of a competent and experienced acoustic consultant, using up-to-date techniques and equipment.
An investigation of the problem must be made on the spot, using the necessary instruments, which may include a third-octave band or octave-band sound-level meter and analyser and a high-quality tape recorder. The latter enables recordings of sound levels to be made on the spot and analysed later. Where investigations require readings to be obtained inside buildings, it will be possible to make use of normal power sources while, for outside investigations, the equipment can be run from a rotary converter off a standard car battery.
To illustrate the procedures adopted, let us consider a simple hypothetical case of an acoustic investigation for a report and recommendations on conditions in a typing pool, covering both the noise inside the building and the noise entrained from outside.
Using an octave-band analyser, readings are taken of noise levels outside the building (the environmental noise) and inside, while the building is unoccupied (the ambient noise level). In each case, an average of perhaps half a dozen octave-band readings will be taken. Environmental noise levels will depend on the position of the site in relation to road traffic and other factors and the ambient level will also be considerably affected by these factors.
By comparing the average readings obtained, it is then possible to determine the transmission loss, which, since the difference determines the amount of noise reduced by the walls, windows, doors and roof of the building, is an indication of the acoustical performance of the structure. Any reduction in the level of the ambient noise can thus only be achieved by changes to the buildings.
Additional noise inside the building is also produced by the occupants and the activities they perform. In this case, it may include the noise of typewriters, conversation, the operation of other equipment, such as duplicators, the opening and closing of drawers in desks and filing cabinets and other activities. This general noise level can also be measured by means of a third-octave or octave-band analyser.
Basic information is now available concerning the existing noise level, consisting of the noise inside the building and outside entrainment. The problem now is to provide control of the noise to produce the desired conditions and the next stage is the assembly of data about the room in which the typing pool is housed.
The information required includes the physical dimensions of the room (its volume), the surface areas (walls, floor and ceiling), the nature of these surfaces (e.g., whether plaster or other materials), the area of glass, the area of timber (desks, tables, etc.) and the area of other equipment, such as filing cabinets. It is also necessary to know the number of persons normally working in the room.
All these items—desks, cabinets and human beings—are capable of absorbing sound and by reference to known data, the number of absorption units actually in the room can be calculated. This total represents another important stage in the approach to a sound control problem.
It is next necessary to relate this figure to the optimum condition which it is desired to attain. This is done by reference to a series of curves which have been devised by acousticians as a result of a worldwide survey and which are now accepted as a standard basis of comparison.
These are based on the concept of optimum reverberation times, the times being plotted against the volume of the rooms, according to the use to which they are put. Curves have, thus, been established for rooms where speech is the main factor (lecture rooms for instance), offices, concert halls and so forth.
From this table it is possible to select a curve which is appropriate to the application under consideration, in this case a typing pool and by applying the volume of the room to the appropriate curve, the number of acoustical units of absorption required to produce these conditions in a room of that size can be obtained.
To produce optimum conditions, therefore, it is necessary to provide this number of absorption units. From previous investigation and calculation, the number of absorption units already in the room (in the form of wall surfaces, ceiling, floor, windows, equipment, people, etc.) is known. It is, therefore, only necessary to subtract the number of absorption units present from the number required to produce the desired result, in order to arrive at the number of additional units which must be provided to yield optimum conditions.
There are many different ways in which these ‘unwanted’ units of noise can be absorbed and it is here that the skill of the acoustician can determine the best way of doing this, having regard to the techniques available and the characteristics of the building. Various types of acoustic tiles are available for fixing to walls and ceilings; for example – noise-deadening pads can be placed under typewriters, partitions can be erected and curtains fitted, to mention only a few such measures.
Many different kinds of acoustic tiles are now on the market and the manufacturers in very many cases specify the acoustical performance of their products. The effect of fitting a certain type of tile to a ceiling, for example – can be readily calculated in terms of number of absorption units added, using data provided by suppliers. Where such data are not available the absorption coefficient of any particular material can be measured and determined.
Preparing a Scheme:
Only in comparatively rare cases will a noise problem have a straightforward solution, such as fitting one kind of acoustic tile to a ceiling. In most cases, a number of alternative tiles will be available and the places in which they can be sited may also offer different possibilities.
Much will depend on the amount of noise entrained from outside, due to road traffic, other nearby industrial premises or even proximity to an airport. Even if the maximum number of acoustic tiles and other absorption units are installed, calculations may show that the desired conditions will not be attained; which means that other methods of noise control must be employed to supplement the use of tiles and other materials.
Other methods of increasing the building’s power to keep out environmental noise require attention to be paid to the roof, windows and doors, which are the most frequent source of noise ‘leakage’. The roof may have to be double-skinned and double windows may have to be installed.
All these points will be incorporated in a comprehensive scheme, if this has been called for and included in the full report which the consultant prepares for the client’s consideration. The report will contain a statement of the consultant’s findings, including an acoustical analysis of the situation, supported by graphical evidence and the results of noise level measurements, covering both noise inside the building and noise entrained from outside.
Arising out of this, the report will then present a suggested scheme for providing comfortable acoustical conditions, to the level requested by the client and will be accompanied by appropriate drawings, details of any necessary interior treatments or ventilation schemes.
On receipt of the complete report, the client can then form an opinion as to the amount of work which will need to be done to produce the conditions he has specified. He may accept the scheme as it stands or call for its revision on particular points.
Assuming that the scheme is accepted, the work to be done is then put out to tender. If the consultant is working directly for the client, the consultant will himself call for tenders; if the consultant is working for an architect who is responsible to the client, it is usual for the architect to undertake the call for tenders.
In either case, the consultant will supervise the work and when it is completed, submit a final report on the performance of the building after the alterations have been completed. This will include noise level readings similar to those originally taken, so that the client has factual information on which to assess the success of the scheme.
Constructing a Noise Persistent Building:
All too often in the past, the acoustic consultant has found that the greater number of his remedial contracts have concerned the rectification of entirely new buildings. This is the direct result of the ‘built-in’ noise problems which are inevitable if the acoustical aspects of a new building are not properly considered when the original plans are prepared. It is not until the building is completed and occupied that the acoustical problems emerge and consideration has to be given to their solution.
This situation is, however, tending to become less frequent as the importance of calling in the acoustician at the very earliest planning stages of a new building is increasingly recognised. The significance of this argument cannot be too strongly stressed, because not only is it often virtually impossible entirely to offset the results of built-in noise problems, but even when complete solutions can be found, the cost is invariably far greater than if an acoustic consultant had been employed and his advice accepted during the design stage.
When this precaution is not taken, the owner of a brand new factory may find himself faced, either with complaints from nearby residents about the noise emitted from the factory or with objections and possibly even indirect loss of production, from staff, who find noise levels inside the factory to be excessive.
To prevent noise from disturbing local residents, it might be thought that it is only necessary to close all windows and doors. Even if this could be done without creating internal ventilation problems, there would still remain other ways in which noise can be transmitted, apart from airborne noise.
Structure-borne noise, for example, can be transmitted from vibrating machinery in the factory through the intervening strata to domestic dwellings nearby. Walls of buildings can transmit noise and so can roofs, unless steps have been taken to prevent this at the design stage.
Consider the case of an industrial building, comprising workshop and office accommodation, situated in an area containing a fair amount of domestic dwellings. Let us further assume that the building will house such industrial plant as plastic moulding equipment, aluminium die-casting plant or even engine test-beds. All these are examples of industrial equipment in common use and all can cause considerable noise problems, through the amount of vibration regularly produced when in operation.
When the original plans are prepared, the architect may not be briefed as to the use to which the building is to be put, his only instructions being to design a building of a specified size and type. The floor, for example, may consist of an ordinary screed concrete surface, the windows may be single glazing, the walls constructed of 9-inch brick and the roof of asbestos sheet.
Machines, such as automatic core-making machines, hydraulic accumulators, moulding machines, tumbling machines and the like, are then brought into this building and bolted down on to a solid concrete base, the inevitable result being excessive vibration.
Electric motors for driving the equipment are also brought in on plinths and similarly bolted directly on to the concrete floor. And if a considerable amount of electricity is to be used, even a transformer substation may be installed, possibly in a small shed somewhere on the site and again, bolted down to a concrete base.
At one end of the building the office and administrative accommodation may be erected, divided from the production areas by only a thin wall. To add to the noise from the machines, the building itself may front on to a busy traffic route.
No sooner is the factory in production than it is realised that the built-in noise is unacceptable, both to the occupants of the building and to nearby residents, who begin to look into the position in the light of the Noise Abatement Act. Now the services of an acoustic consultant will be in demand, but what can he do in these circumstances?
He can recommend provision of special mountings for all machines, suggest fitting some form of double glazing to the windows and indicate ways in which other structural changes could be made to walls and roof.
All these modifications, however, are expensive, even when it is physically possible to introduce them at this late stage. Moreover, the interruption to the normal work of the plant while the alterations are taking place will add considerably to the already heavy costs of the changes.
Understandably, therefore, more and more architects, particularly those who specialise in industrial buildings, are bringing these points to the attention of their clients when initial discussions take place. Acoustic consultants are being increasingly called in at this early stage and the time may not be far distant when an acoustician will join the heating specialist and the ventilation engineer as indispensable partners of the architect during all stages of the planning and construction of buildings of all kinds.
Building-in Good Acoustics:
Assuming, therefore, that the ideal method of approach is to build-in good acoustical properties during the planning stages of a new building, what should be the role of the acoustic consultant at this time? Ideally, he should be present with other specialists, such as the heating and ventilating engineers, at the preliminary discussions between architect and client.
At this time, the general plan of the proposed new building should be outlined and its purpose explained, especially where any special factors, such as noisy machinery, will have to be considered. An approximate schedule of accommodation can be prepared, showing the position of machinery and equipment relative to office and administrative accommodation. The structural principles on which the proposed building is to be built will also be laid down.
Armed with this information, the acoustician can now undertake a preliminary survey of the site and carry out tests to determine environmental noise levels. An area which appears to be quite satisfactory when examined on a map or even following a brief visit to the site, may be found to pose considerable environmental noise problems when accurate tests are conducted over a period of time.
It must also be remembered that a noise level which will now worry the production departments may prove a nuisance to administrative or research and development staff unless measures are taken to prevent noise entrainment into those sections of the plant.
This information is one of several factors which have to be taken into consideration when the architect, in due course, supplies the acoustician with a copy of the outline drawings of the building. On the basis of these drawings, noise criteria curves (called NC) are prepared for all areas of the building and provide the datum to which all must work. Noise criteria will vary from one area to another, depending on the activities carried on and may range from NC35 in office areas to possible NC80 in areas of high industrial activity (Fig. 3.1).
To attain and control these conditions in practice involves collecting a great deal of data as to acceptable noise levels within the building. At this stage, this information can only be obtained by predicting the noise levels which may be expected from the known facts, such as the number of people to be employed in the various rooms, the type and quantity of machinery, the services, which may produce noise (e.g., ventilation, electricity transformer-stations, etc.) and other factors.
Clearly, the more accurately the client is able to specify the equipment to be installed, the more accurate will be the acoustic consultant’s predictions. In fact, experience has shown that such predictions can be made with considerable accuracy and certainly, sufficiently accurately to form the basis for reliable acoustical planning.
Where standard machinery is to be employed, noise tests can be conducted in similar environment in other factories. If there are no comparable installations where tests can be conducted, noise levels can be predicted from engineering data, such as horse power, revolutions per minute and other factors. From known data of this kind it is possible to calculate sound pressure levels and dominant frequencies.
These predictions and calculations can take a considerable amount of time and it is a great help to the acoustician if the plant manufacturers are able to supply noise spectra for the equipment they produce. An increasing number of manufacturers are now providing this information in respect of their products and all who make equipment of any kind would be well advised to follow suit since scientifically-determined information as to the noise levels of their products would not only be of assistance to the acoustician in controlling noise, but would also encourage manufacturers themselves to design quieter machinery and plant in the first place. Meanwhile, it can be said that the position is steadily improving and some leading fan manufacturers, in particular, have an exceptionally good record in this respect.
All the information regarding environmental and ambient noise can be tabulated and will form the basis of discussions with the architect and other specialist consultants, since these factors exert a strong influence on design features, such as partitions, doors, windows, ceilings and floors.
The type of outside wall, for example, will depend on a number of factors. Environmental noise, if this is a serious factor, will affect wall structure while, from the opposite point of view, a noisy machine-shop will also affect the construction of outside walls, which must be suitably designed to prevent noise becoming a nuisance to the neighbourhood.
Similar considerations apply to windows, doors and roofs. With regard to the interior of the building, where production requirements may not allow for separation between noisy and quiet areas, considerable thought will have to be given to the design of partitions. In the case of a multi-storey building, it may be necessary to site machinery areas on one floor and administration on another. The design of floors and ceilings is, therefore, important, taking into account the desired noise transmission loss between floors.
It is equally important that the type of machinery to be installed and its exact location should be made known to the acoustic consultant at the earliest opportunity. Such machinery may have to be located on a floating floor, which must be properly designed as an integral part of the building and structural engineers will have to be briefed for the special work involved, which will require specialised materials and techniques.
The siting of machinery and the methods of mounting must be known at the planning stages because these factors can determine so many other aspects of the construction, including floor levels, stairs and position of lifts and other services.
At all times, it is necessary to consider not only airborne noise but structure-borne noise and the latter will be a particularly important factor where a great deal of heavy machinery is to be installed. It will, for instance, affect the design of the outer skin of the building, possibly demanding a double wall with special isolating ties linking the two skins of the wall.
Some form of double glazing may also be necessary for the windows and a special form of double roof. Once again, the point must be made that if these decisions are taken and acted upon when the building is being planned, the cost is far less than if the same decisions have to be taken by force of circumstances after completion.
When all these decisions have been agreed between the client, architect and specialist consultants, the acoustician draws up detailed plans of his recommendations, which the architect may wish to convert and incorporate in his own detailed drawings, apart from any special acoustical work.
As the work proceeds, it is customary for the acoustic consultant to inspect those aspects of the building which fall into his province, offering advice on such points, for instance, as the best procedure for constructing sound-proof partitions or the installation of sound-proof treatments. The acoustic consultant will also advise on the acoustical aspects of the work of other specialists, such as the ventilating engineer and heating consultant.
When the work is completed, the acoustic consultant undertakes a final survey, including noise level tests, which can then be compared with the predictions previously made and the noise level criteria which were set. If there has been a considerable element of prediction in the preliminary survey, there may be the need for some minor modifications to the final building but these will present little difficulty, since the basic problem has been solved.
In the majority of cases, however, it will be found that the final survey and report show a remarkably close resemblance to the original planned acoustic conditions. In this event, the client can accept the building with the assurance that those who are employed in it and those who live in the neighbourhood will experience only those noise conditions which were predetermined and built-in to the structure.
Choosing the most Economical Scheme:
Many clients have hitherto been discouraged from calling in an acoustic consultant to help in designing a building by considerations of cost. Building work of all kinds is expensive enough; the client is inclined to say, without adding the cost of a consultant and that of the changes he will suggest.
This argument is a fallacious one because the cost of removing built-in bad acoustics (assuming that this is technically possible) far outweighs the initial expense of building-in good acoustics. Moreover, one of the main concerns of the acoustic consultant is to ensure that the most economical methods are employed, taking all the relevant factors into consideration.
He is strongly supported in these endeavours by the architect and quantity surveyor, so that all the specialists concerned in the project are united in their intention to keep costs down to the minimum consistent with fulfilling the conditions specified by the client.
It is particularly important to determine the precise treatment required to provide the desired optimum conditions; anything which fails to achieve this is an abdication of the consultant’s role. On the other hand, it is not less undesirable to go beyond optimum conditions, ‘just to be on the safe side’, since, once the optimum point is reached, any additional acoustical advantages obtained may and probably will, be disproportionately costly.
These are all points which the acoustic consultant will explain to his client, when advising him on the treatment required to yield the desired results. Consider, for example, the case of a factory in which the machinery installed produces noise levels which will prove unacceptable to people living in neighbouring houses.
At first sight, the solution would appear to require the complete sealing of all windows and the installation of a ventilation system, all of which would add considerably to the construction costs of the building, quite apart from the running costs of the ventilation system. The consultant will consider whether it is not possible to obtain satisfactory conditions by means of a less comprehensive and therefore less expensive, solution.
There may well be a number of alternatives which will produce the required result without the need for complete sealing of windows. The windows themselves can be so designed that the necessary noise transmission loss is achieved, while still permitting the ingress of sufficient air for adequate ventilation Or a design may be devised which permits the window to be opened for periods, without unduly increasing the amount of noise transmitted to outside areas. Similar considerations apply when the problem is one of controlling the entrainment of noise into quiet areas inside the building.
In designing an installation such as this, capable of providing the optimum solution to a noise problem, it is essential to start with an accurate picture of the position, such as can only be obtained from a properly-mounted acoustical analysis. Once the consultant has obtained this analysis, he can then deploy his knowledge of materials and techniques in designing a system which will achieve the required noise levels in the various departments of a building at the least possible expense.
It may not always be possible to avoid recommending a comprehensive system of double windows but, even in this extreme case, the experience and know-how of the acoustic consultant can suggest the most economical procedure to adopt. The ventilation system which is necessary in such a case can be designed to provide the facilities required at minimum cost, especially with regard to the acoustic steps needed to control the noise produced by the ventilation equipment. The type of ventilation units recommended and their position, as well as the attenuation equipment, will be selected with care and with due regard to such factors as the amount of trunking required and so forth.
Consultant’s Advice on Noise Problem:
Although the acoustic consultant is now called in more frequently to advice on noise problems in all types of building, there is still widespread ignorance of his precise role, his terms of reference and of other practical details associated with the engagement of his services.
As a general rule, an acoustic consultant can either be called in direct by whoever is undertaking the construction of a new building or the modification of an existing structure or he can be retained, indirectly, on the client’s behalf by the architect engaged on such work.
Whilst there are cases when it may be advantageous for the client to call in the acoustic consultant direct, there is much to be said for the practice of working through an architect. In this case, appropriate terms of reference are agreed between the architect and the consultant covering the precise role of the latter.
This will vary with individual cases and in addition to the provision of advice and assistance on the acoustical aspects of the work and the preparation of acoustical reports, may also include the preparation of drawings for inclusion in the architect’s final plans and the recommendation of contractors for specialist acoustical work.
It is the usual practice, when the acoustic consultant is nominated and approved by the architect, for him to be appointed and paid by the client. Fees have, therefore, to be negotiated between the consultant and the architect in the first place and subsequently put forward to the client for possible further negotiation. The fees will vary according to the amount of work specified in the terms of reference, having particular regard to the amount of time to be spent on the project and including a reasonable allowance for travel.
When the consultant is appointed direct by the client, he occupies the same position as an architect vis-a-vis his client. The consultant takes the client’s instructions which, in addition to calling for an acoustical report, will almost certainly also include preparation of sketch designs, calculation of cost estimates and submission of details of the proposed work to the appropriate local authority.
The plans having been officially accepted, the consultant will then prepare working drawings and specifications which will be passed to the quantity surveyor for quotation. Once the work has been put in hand, the consultant will supervise the installation, certify the accounts of subcontractors and in general, act in the client’s interests from the beginning to the end of the project. Finally, on completion of the work, acoustical tests will be carried out and a report submitted to the client.
Whether the acoustic consultant is appointed direct by a client or engaged on his behalf by an architect, his professional services are made available on a clearly-defined basis, both as to terms of reference and remuneration, before the contract is agreed.