A great deal of the unwanted noise in all buildings, from factories and workshops to administrative buildings and hospitals, is caused by machinery, plant and equipment. It is, therefore, of the utmost importance that all such equipment should be carefully selected with this in mind and installed in such a way that structure-borne noise is kept to the minimum and if possible, matched to the predetermined NC curves for the various areas of the building.
The problem can conveniently be considered under two main headings, namely – the initial selection of quiet-running machinery and the employment of appropriate installation procedures to reduce the noise level to acceptable standards. Similar general principles apply to all equipment, from heavy machine tools to domestic warm-air central-heating systems.
Selection of Quiet Equipment:
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It can be argued that it is the duty of the manufacturer to design and produce quiet-running machinery and equipment and undoubtedly some industries are doing a great deal in this direction. On the other hand, manufacturers in a highly competitive field are unlikely to undertake the considerable research necessary unless pressed to do so by the demands of their customers. If quietness of operation becomes a selling point of a particular item, there are immediate commercial reasons for looking into the noise aspects of design and manufacture.
It has been pointed out that in many cases the designers of machines had never even been asked to consider means of quietening them. Machines which were so noisy as to constitute a deafness hazard to those working on them had, of course, been considered, but with the majority of machines, noise was regarded as an incidental nuisance of no particular significance.
It can fairly be claimed that the continual efforts of acoustic consultants to seek out the quietest equipment for any particular application has been an important factor in the creation of this new attitude of manufacturers towards the noise levels of their plant.
Two examples of industries which have good records in this respect can be quoted. Leading fan manufacturers pay special attention to noise, especially in the case of fans which are used in ventilating and air-conditioning systems. Many promising lines of research are being followed, a typical example being the use of new designs of components and the introduction of new materials. In small fans for domestic or commercial systems, the conventional propeller and shaft type is now being replaced by the rotor type, which is less noisy, while further noise reduction is being achieved by using plastics instead of metal for the rotors.
These designs and materials can be equally well applied to all types of plant where fans are employed, such as heat treatment and other furnaces where the circulation of air and products of combustion is induced by fans which, on large installations, have very high noise levels. Similar research is being conducted into compressors by some leading manufacturers, with a view to reducing the amount of noise produced at source by this type of equipment.
Though it is unlikely that noise from machines will ever be completely eliminated, it is certain that much more could be done. In the case of the transformer, for instance – it is thought that the noise from these traditionally noisy pieces of equipment could be considerably reduced at source by such techniques as selective core building and the use of special steels in core manufacture.
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This is a complex problem, involving steel manufacturers as well as those concerned in the building of the transformer and at least one leading British transformer manufacturer claims that by paying close attention to the treatment of the steels used in core laminations, marked improvements in noise level have been achieved.
Fans, compressors and transformers stand out as obvious targets for noise reduction exercises, but attention is equally necessary and perhaps even more overdue, for all other types of machinery.
Possible lines of approach are numerous and only a few of the more likely trends can be briefly mentioned here-
Bearings, for example – are a common source of noise and quieter-running materials can be considered. With the introduction of self-aligning bearings as standard engineering components, the former complete rigidity of bearings is no longer so necessary and bushes of compliant materials can be employed. A great deal of research is taking place into new forms of rubber for higher duty applications and this work is relevant here.
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Silencer design has made considerable strides under the impetus of the automobile industry and suitable types should always be fitted to machinery whose exhausts or air intakes are a cause of noise. Machinery which incorporates various forms of prime-mover should be investigated from the point of view of the mounting of the prime-mover on the machine itself.
More often than not, the variable-speed drive for a section of conveyor line is bolted solidly to the framework of the line, whereas it could just as easily be isolated from the equipment by any one of a number of available mounting techniques. This situation is repeated widely throughout industry, frequently reaching its climax in the machine-tool industry where a complex machine may have a considerable number of electric motors performing specific tasks, all firmly bolted to the framework and so contributing to the general noise level.
Admittedly, machine design is a complex business and fundamental changes in design cannot be made at the dictate of a particular customer, however desirable from the acoustical point of view. What is urgently required, however, is for acoustic factors to be taken into consideration from now onwards whenever a new machine or piece of equipment is being designed. The noise aspect of design should be given just as much attention at the design stage as the machine’s efficiency, ease of maintenance and all the other factors which are rightly given pride of place in the designer’s plans.
One such problem which should be considered when bringing out a new model is the type of drive and its relation to the production of unwanted noise. The selection of gears, as well as the shafts on which they are mounted, can also be made with acoustical values in mind. New materials, such as nylon and fibre gears, offer good prospects here and the noise properties of gears made from the increasing number of sintered materials should also be investigated.
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Many other mechanical parts which come into contact with each other or with the main structure of the machine should also be studied afresh. These include such components as push rods, cams and driving quadrants, which modern techniques often permit of being bushed with rubber or other compliant material where they come into contact. Many such components can also now be made of nylon.
Once manufacturers of machines and equipment include quiet-running as one of the essential qualities to be built into their products, the acoustic consultant will be able to recommend the exact machine required from the performance point of view, in the knowledge that its acoustic characteristics will be no less satisfactory. As a corollary of this, the manufacturer should be able to furnish data concerning the acoustic performance of the equipment he supplies.
Finally, the acoustic consultant has to carry out his own investigations into the noise levels of the various equipment under consideration before selecting the machine which offers the best combination of acoustic and performance characteristics for the application in question. Measurements have to be taken when the machine is in operation, preferably in similar installations, but if this is not possible it may be necessary to make a prediction of the possible noise level in order to arrive at a likely figure for the particular installation.
Machinery Mounting:
Having selected the machine which performs the required duty with the minimum amount of noise at source, the next question is the method of mounting to be employed. And as it is unlikely that completely noiseless machines will ever be designed, machinery-mounting problems will always constitute an important part of the acoustic consultant’s task.
Many machine-mounting methods are available and the final choice will depend on numerous factors, including the nature of the machine and the noise it emits, the design of the building, the noise levels which it is desired to attain in the building, the layout of the area and relevant economic considerations. Clearly these factors can best be balanced against one another and a sound decision taken if the question of mounting is considered during the initial planning of the building.
Although the actual methods of machinery mounting may vary with circumstances, the basic aim of the exercise is to ensure that the vibratory energy produced by the action of the machine is dispersed and not injected into the building structure to be transmitted through the fabric to the discomfort of people both in the machine room and in other areas.
Factors to be taken into account include the forcing frequency (or weight-disturbing frequency which will usually be a function of the r.p.m), the measured amount of vibration given off and what proportion of the vibration, in octave bands, is otherwise injected into the building fabric.
The consultant works to a specified percentage of isolation; for example, if the aim is 80-90 per cent isolation over the frequency spectrum, most noise will be kept out of the building structure. Most machinery mounts are made from rubber or rubber-based materials and a standard type can generally be found which will be suitable for any normal mounting problem.
Manufacturers usually provide enough data about their mounts to enable the consultant to select the type and size most suitable for the application. In exceptional cases, it may be necessary to design a ‘one-off’ mount for a particular machine, in which case manufacturers will often agree to design a suitable mount in accordance with information supplied.
When rubber is used as the compliant medium, it is better to employ a mount of the type which incorporates rubber in shear rather than in compression, since the former arrangement can often give much greater resilience. Different values can also be obtained by varying the rubber ‘mix’.
With fairly simple machines, it may be possible to use the same type of mount at all mounting points. For instance, with a machine built on a square base-plate and having its centre of gravity exactly at the centre point of the plate, it would probably be sufficient to place identical mounts at each corner.
In more complex machinery, it is necessary to consider each mounting point in relation to the position of the centre of gravity and to select mounts suitable for each position. In extreme cases, when dealing with very complex machinery, it may even be necessary to vary the rubber ‘mix’ from one mount to another.
Other Isolation Techniques:
It is not always realised that it is not sufficient to isolate a machine from the floor if it is also connected to the building structure at other points. Vibrations can also be transmitted through pipes, ducts and other points at which equipment, such as compressors, pumps and prime-movers, are in contact with the fabric of the building. At all such points, therefore, structure-borne noise must be controlled by means of suitable flexible connections.
Various types of flexible connection are manufactured and a wide range of standard materials is available which will meet most normal requirements. Provided the material is sufficiently compliant, a high percentage of isolation can be attained at connecting points.
There are three main types of flexible connection in common use, the selection of one type in preference to another depending on the nature of the liquid or gas flowing through, the degree of isolation required in relation to the forcing frequency and the amount of space available for the flexible connection. Other factors of a non-acoustical nature, such as the compatibility of the material with the liquid passing through it, must also, of course, be taken into account.
Convoluted types are particularly suitable for use in restricted spaces, since a high degree of isolation can be obtained in relatively short length of connection, provided the pressure level is not too high. The other two types are spiral connections and bellows-type connections. Rubber is probably the most common material for all flexible connections, but modern techniques permit the use of many different materials, either on their own or in conjunction with rubber.
When selecting a flexible connection for a particular application, allowance must, of course, be made for the presence of any liquid or other material likely to damage the connection. For example, while the connection itself may only have to contain air or water, there may be a possibility of oil or some harmful chemical dripping on to it from other plant nearby.
Isolation of Ventilation and Air-Conditioning Systems:
With the increasing use of ventilating and air-conditioning plant in industry, not only to provide comfortable working conditions for staff but also to meet operational requirements, the problem of the noise created by such systems has rightly been given a great deal of attention by heating and ventilating engineers and acoustic consultants.
There will be a number of different environments in the typical factory or other building and the noise environment condition, specified for each area, has to be matched by taking appropriate action against the entrainment of noise in every such area.
It is the primary task of the acoustic consultant to produce the required noise criteria curves for each environment and if necessary, the ventilation and air-conditioning system must be designed and treated to match these requirements. Taking each area or room on its merits, the consultant will be faced with the specified NC curve and opposed to it, a given level of noise from the plant with, perhaps, some entrainment of outside noise through the system, via the ducting, for example.
Other machinery noise, from various process departments in the factory, may also have entered the system through pipes and ducting passing through the process areas. It is thus necessary not only to take out the fan noise from the air-conditioning plant, but also to examine the possibility of other noise entering the system.
Nevertheless, the first step to be taken is to control the noise from the air-conditioning and ventilation plant and this process should begin with the selection of the quietest possible fans and other equipment. A considerable amount of useful data on which to base the selection is available from fan manufacturers but it may be necessary to take readings.
When a suitable fan has been selected, it must be carefully mounted to ensure that the energy which is developed is dispersed and not injected into the fabric of the building. Where equipment is joined to ducts or pipes, flexible connections must be fitted, as already described. Many faults arise through incorrect mounting and connections.
Attenuators:
Even when every precaution has been taken to control the noise at source by mounting and connection techniques, there may still be some residual noise passing through the system. Additionally, there may be noise entrained from outside the building and noise picked up as the pipes and ducting pass through the factory. This noise can be further controlled by fitting attenuators at strategic points in the system.
As the name implies, an attenuator is a device which thins down or reduces the noise passing through a duct or other aperture. Noise attenuation takes place automatically as it passes along ducting and the amount of reduction due to this cause can be calculated from the appropriate formula.
This is related to the cross-sectional area of the duct, its perimeter and the sound absorption coefficient of the internal surface of the duct. Further noise reduction is effected by each bend in the duct and also by the grille or opening to the room concerned.
By making this calculation, the figure obtained in decibels can be matched against the specified noise level for the room concerned and it can then be determined what further attenuation is necessary. This can then be provided by inserting into the duct an attenuator of the type which will give the required degree of noise reduction.
A typical attenuator is a box-shaped structure, lined with absorbent material and fitted longitudinally with strips of perforated metal; these are also lined with absorbent, but have a certain amount of ‘free’ air between them. It is important that the area of the free air is equal to the cross-sectional area of the duct in which the attenuator is to be fitted. The lengths of perforated metal inside the attenuator, known as ‘splitters’, can be either straight or of the chevroned type.
Standard attenuators are manufactured by a number of firms and it is usually possible to obtain a suitable attenuator for all purposes, direct from stock. In exceptional circumstances, however, it may be necessary for the acoustic consultant to design an attenuator for a particular purpose. Manufacturers of attenuators supply all the necessary performance data with respect to their products, thus enabling the consultant to select the exact type required.
When designing a special-purpose attenuator, the most important variables to consider include the length of the attenuator and the thickness of the splitters. Care must be taken to obtain the precise amount of dB reduction, over specific octave bands, since it is clearly uneconomic to provide a greater reduction than is required and equally undesirable to provide less.
In a complex system, where the ducting passes through a large number of rooms, each with its own specified noise level and through areas where additional noise may be entrained, the selection of points at which to install attenuators is critical. The attenuator selected must be such as to provide the precise level required at the next discharge point and in some cases, it may be necessary to install attenuators in parallel or in series.
Besides installing attenuators in this way to control noise from fans and other sources, it may be necessary to include secondary attenuators in some areas to control speech and other noise transmission from one room to another. Such attenuators, known as cross-talk attenuators, are also available in standard form and can be installed at points between rooms, irrespective of the position of the main attenuators.
Thus, whereas one main attenuator may provide sufficient noise reduction to meet the noise level requirements of a number of rooms in a block, there may have to be a cross-talk attenuator between each of these rooms, especially if speech privacy is an important consideration.
Thus, selection of quiet-running machinery and equipment is an important aspect of the control of noise in any environment. It is one of the factors which must be taken into account at the earliest possible stage in the planning of a new factory or any other building where plant and machinery will be in operation.
Many buildings were, of course, erected and fitted out with machinery before the importance of acoustical control was recognised, but even in such cases successful remedial steps can usually be taken. Managements will find that even if it is not possible to produce optimum noise level conditions in such buildings, considerable improvements can be effected by calling in an acoustic consultant to advise on mounting or other arrangements which can be economically applied.
In any case, when new equipment is being installed in an old building or a new layout if existing plant is under consideration, it is essential that the new equipment and layout should be examined from the acoustical point of view and installed with this aspect of performance prominently in mind.