Noise, Vibration and Acoustics Consultancy from the Institute of Sound and Vibration Research
Written by Bob Davis
Original version © 2002, ISVR University of Southampton. All rights reserved.
The most effective way of reducing noise is usually to avoid making noise in the first place - to use a quieter machine or process. Unfortunately this
is often not practicable. For example it would, in theory, be possible to design a 'quieter' forging hammer. Forging presses are less noisy than
hammers (although by no means quiet). However, most forges and foundries will be using existing machinery and processes for the foreseeable future.
However, there is some scope for noise reduction at source, even in an existing forge or foundry. When new ancillary equipment such as an air compressors or a ventilation system is installed, there is often a choice between noisy and quiet equipment to do the same job. It is not unusual for a company to buy new equipment without consideration of noise levels and create a 'new' noise problem.
Always obtain information on noise levels when buying new machinery or tools. This is particularly important for equipment to be located outside or in a plant room which is directly ventilated to outside through louvres, such as dust collectors, heat exchangers or compressors. Suppliers are legally obliged to provide such information. You may need to take specialist advice to interpret the information, and on whether you should specify a maximum noise limit for new machines, and what this limit should be. A visit to see and hear the same machine working on another site is often a good way to assess the likely noise levels.
Equipment is usually noisier if it is defective. Worn fan motor bearings, or a badly-maintained clutch on a drop hammer, can produce distinctive noise which might not seem like a problem on the working site, but might be very annoying to neighbours.
Many noises in forges and foundries arise from hard metal-to-metal impacts. The main problem is the noise caused by small forgings or castings falling
into hoppers. on to chutes and tables, or into bins ('stillages'). The resulting 'crashes are frequent causes of complaint from neighbours.
These noises can often be reduced by low-cost methods. For example:
Reducing the number of transfer operations. In most forges and foundries, pieces are tipped from stillage to hopper or bin and returned to stillage several times between production and the end of finishing operations. A review of working methods and re-design of fixtures can often reduce these operations.
Reducing the height from which pieces fall on to hard surfaces. Sometimes this is difficult to control - a crane driver moving scrap metal using a magnetic crane has considerable freedom. However, some measures are effective: for example, raising stillages at fettling stations or at the discharge of shot-blast machines, using a simple fixture to reduce the drop distance, can be effective.
The main problem with steel stillages, hoppers, chutes and benches is that they 'ring' when struck by a metal object. This 'ringing' can be reduced by reducing the impact force using a resilient surface covering, by 'deadening' or 'damping' the response of the structure being hit, or by making it a less efficient radiator of noise. Examples of these techniques are illustrated in the case studies in Section 5.
There is no great problem in producing a 'quiet' stillage, which does not 'ring' when metal pieces are thrown or tipped into it. However, there are thousands of stillages in use, which circulate between final customers, forges and foundries, heat treatment and machining companies. The universal introduction of quiet stillages (which would invariably cost more than the standard steel stillage) is therefore an unrealistic aim. However, a company might find it practicable to have a number of 'quiet' stillages dedicated to a particular task or area, to resolve a particular problem For example, it might be possible to use a small number of 'quiet' stillages to take scrap from an external stockyard to the furnaces.
Always be on the lookout for ways to reduce noise.
Never buy a new machine or tool without considering its likely noise levels.
Noise reduction should be a key element in process engineering - quieter methods can often be introduced in conjunction with changes designed to improve efficiency or quality.
Where you have houses near your site. especially if noise has caused problems in the past. make a regular 'patrol of the site boundary to identify new noises and possible defective equipment.
Identify operations where there are frequent metal-to-metal impacts. Eliminate unnecessary material transfers. Reduce impact forces by reducing drop heights. 'cushion 'impacts using resilient linings, make stillages, chutes, and tables less effective noise radiators.
Noise emitted from machines and operations can be reduced by enclosing them. Enclosures present some problems - they may obstruct access, they take up
floor space, and if personnel have to work inside they may present a hazard. However, sometimes enclosures are effective solutions to noise problems. To
be effective. enclosures intended to reduce noise must have good sound insulation properties. To provide a high degree of sound insulation, as explained
in para. 3.2, enclosures must be reasonably heavy and well-sealed to eliminate direct air paths. However, lightweight enclosures, with some openings, often
provide useful attenuation. For example, an enclosure made from overlapping PVC strip curtains can provide a noise reduction of around 10 dB(A). Such enclosures
have been used with some success around foundry shake-out stations, to control dust as well as noise. They have the advantages of providing ready access
and reasonable vision, although they are not very durable and obviously cannot withstand contact with hot castings.
For noise reductions greater than about 10 dB(A), careful design is needed to deal with features such as access doors and openings for materials. There are numerous specialist suppliers of 'acoustic' enclosures. Proprietary enclosures are built up using modular sheet steel panels with an internal lining of mineral wool faced with perforated steel sheet. This sound absorbent lining reduces noise reflections within the enclosure, which would otherwise lead to a 'build-up' of noise inside and resulting inferior performance. Effective enclosures can also be built (at a lower cost) using brick or concrete blocks, timber, plasterboard or materials such as wood-wool cement slabs, although without specialist advice results may be disappointing.
Forges and foundries can be tough environments. Enclosures round machines need to be well-engineered and robust to withstand assault from fork lift trucks, for example. If buying an enclosure from a specialist supplier, find a supplier with specific experience in these industries.
Examples of acoustic enclosures fitted to shot-blast machines are described in the case studies in Section 5.
Enclosing individual machines or processes to reduce noise can be effective. but the drawbacks (access. space. creation of workplace hazards) should
be carefully considered.
The effectiveness of an enclosure in reducing noise depends on the detailed design. Seek specialist advice.
Enclosures in forges and foundries must be robust. A badly-engineered enclosure of unsuitable materials will require regular repair during a limited life.
The buildings which house a forge or foundry can be thought of as a large acoustic enclosure - the sound insulation provided by the building determines
how much noise escapes to annoy neighbours. Most forge buildings are of lightweight construction, with many openings for ventilation. The noise reduction
afforded by such a building is poor, usually no better than 10 - 15 dB(A). (Foundry buildings tend to be rather better than forges in this respect, because
more stringent environmental controls have led to the widespread introduction of mechanical ventilation, with cleaning systems to remove dust and fumes.
There is therefore less reliance on natural ventilation and fewer openings in the buildings to provide escape paths for noise).
Inside an enclosed space with solid surfaces the level of noise builds up because of the repeated reflections of sound from the walls, floor and roof - this effect is termed 'reverberation'. Treating the internal surfaces with sound-absorbent (less reflective) materials will reduce the reverberant noise. Reducing the noise level inside a building reduces the level outside by the same amount. However, the installation of internal sound-absorbent treatment does not have a dramatic effect -- a reduction of around 5 dB(A) in average noise levels might be achieved by lining the inside of the roof. The most widely used material is semi-rigid boards of mineral wool, 50mm thick, which can have a woven cloth or very thin plastic facing to resist dust contamination. For new buildings (or re-sheeting) some proprietary double-skin steel sheeting systems have a perforated inner skin which exposes the mineral wool between the skins and provides sound absorption, although the actual sound insulation of these systems (the resistance to sound passing through) is inferior to that of two solid skins.
The obvious way of reducing noise from a building which contains numerous noise sources (for example, a forge with a number of drop hammers) is to improve the sound insulation of the building itself, to keep the noise in. This is rarely a simple task. Improving the sound insulation of the actual cladding of the building (often single-skin metal sheeting) means adding considerable weight, which may not be possible without strengthening the structure. More fundamental is the need to limit the escape of noise through openings, which have to be closed off or greatly reduced in area. This immediately presents a problem with ventilation: high air change rates are needed to maintain reasonable working temperatures, particularly during hot weather. Access to and from the building is required, and an open doorway will provide a large noise escape path. Doors must therefore be kept open for a minimum time, and when closed must provide sound insulation to match that of the rest of the building. At an existing forge or foundry it is unlikely to be possible to suspend operations whilst major building works are carried out. which further limits the scope of practicable works.
It is tempting to believe that treating only part of a building, perhaps just the wall facing nearby houses - will reduce noise significantly. As explained in 3.3, noise 'goes round corners'. The noise which reaches neighbours is coming from all parts of the building, including the parts which cannot be seen. The roof of a building is usually of much greater total area than the walls, and is often of lighter construction. Improving the sound insulation of the walls of a building, without treating the roof, is almost always ineffective. An acoustics specialist with experience of industrial buildings can identify the contribution of each part of the building to the noise level at any point outside, and design a suitable 'balanced' package of insulation works.
One of the pilot projects involved extensive modifications to a forge building. involving the installation of a second external 'skin', easily-operated acoustic doors. and a system of mechanical ventilation which allowed other ventilation openings to be closed off. Adding a second skin outside the existing sheeting has several advantages - the building can be made more weatherproof, appearance can be improved, and the work can often be carried out without disrupting production. This project is described in para. 5.2.
If a building contains many sources of noise, improving the sound insulation of the building could to be the only practicable means of reducing noise
escaping to outside.
It will usually be necessary to treat large areas of the building - at least 3 walls and the roof - to achieve a significant noise reduction.
Holes and openings must usually be closed off. and a mechanical ventilation system installed.
Assistance from an acoustics specialist, structural engineer and heating/ventilating engineer is generally essential. Planning consent may be required.
Building modifications are expensive. However, a scheme has been demonstrated in the course of this project which has proved to be effective and practicable. and was installed without disrupting normal production to any appreciable degree.
It is often assumed that hiding a source of noise from view will reduce or eliminate the noise. Because sound goes round corners (unlike light) the effect
of placing a physical barrier ( a wall or fence) between a source of noise and a listener is quite limited. This is particularly true for low-frequency
noise (such as a dull rumble) which passes round a barrier very readily, whereas high-frequency noise (such a high-pitched whistle) behaves more like light
and can be screened more effectively.
Screens can be effective in reducing noise from a small source (such as a cooling tower or an open doorway, for example) escaping in a particular direction. A screen is most effective if placed near the source or the receiver (see diagram), because the noise reduction is higher if the angle 'a' is greater. For the same reason, a large screen is more effective than a small one, even though both may hide the noise source from view.
Because the noise reduction provided by a screen is limited by the noise which passes over the top and round the edges, the screen does not need to provide very high resistance to noise passing through it. As a screen, a close-boarded fence will provide the same noise reduction as a brick wall or earth mound of the same height and width. However, on a factory premises a robust form of screen is needed. Timber sleepers slotted into vertical steel columns can serve as a useful and durable screen, and can also form (for example) material bays in a foundry stockyard.
In practice, a screen is unlikely to provide a noise reduction of more than 5 - 10 dB(A). A screen may have a psychological benefit as well as an acoustic
one: for example, a screen which conceals a lighted doorway at night, or which prevents local residents observing truck movements in a factory yard, may
satisfy complaints even though the actual noise reduction is quite modest.
Screens can be expensive and are rarely a cost-effective solution to a noise problem. Proprietary 'acoustic screens', primarily intended to be used as noise barriers alongside trunk roads and motorways, are widely promoted by the manufacturers for more general use but in many applications are no more effective than a simple close-boarded timber fence (although they are constructed to a higher specification and would be more durable). Specialist advice should always be obtained before erecting a screen in an attempt to reduce noise - there is often a better way of achieving the same effect.
A barrier or screen in the form of a wall or fence can provide a modest noise reduction, and may be useful in concealing a noise source from view.
The noise reduction provided by a screen depends mainly on the width and height. Cheap forms of construction - timber fencing or reclaimed timber sleepers - are usually adequate. Expensive 'acoustic screens' often provide little or no advantage.
Obtain specialist advice before erecting a screen as a noise reduction measure -the results are often disappointing, and there may be a better solution.
Ventilation and dust-extraction equipment are regular causes of noise problems. The main noise source is the fan powering the system. Ventilation openings
such as louvres to compressor houses can also transmit noise to outside. Any passage. duct or opening which permits the flow of air will also transmit noise.
Note that the transmission of noise along an air passage or duct is not affected by the direction of air flow - it is a common misconception that noise
travels 'with the flow', so that no noise escapes from an air inlet opening such as a fan intake. The reason is simple - sound travels at about
330 metres per second in air, and it is hardly impeded by the flow of air in a ventilation duct which would rarely exceed 20 m/s.
Noise emitted from fan intakes or exhausts, or from exhaust stacks and louvres, can be reduced using silencers. These are of various types, but they generally contain passages lined with porous sound-absorbent material, which 'take out noise whilst permitting air to flow through. Silencers almost always restrict the airflow and may reduce the performance of a system - this has to be taken into account.
The positioning of a silencer can also be important. A silencer too close to a fan can disturb the air flow and make the fan noisier. A silencer too far from a fan, connected by a duct, may allow noise to 'break out' through the duct before it reaches the silencer. Selection of silencers, and choosing where to put them, is a specialist job.
Beware of making fans and equipment containing fans (such as cooling towers) too quiet. A continuously running fan, as long as the noise it makes is 'smooth' in character, may actually be useful in masking intermittent noises from other sources. There have been many cases where factories have spent a great deal of money and effort in reducing noise from fans and have provoked more complaints because bangs and crashes, previously masked by fan noise, became audible. (Some neighbourhood noise problems have actually been solved by introducing continuous noise - and hence actually raising average noise levels -to conceal intermittent impact noises, although this is not a recommended approach in most circumstances).
Noise travels against as well as with the direction of air flow
Silencers reduce noise travelling along a duct or through an opening whilst permitting the flow of air. Specifying silencers and where to put them is a specialist job. Incorrectly sized or wrongly-positioned silencers may be ineffective and can seriously reduce air flow.
Before reducing noise from a fan, make sure that it is not doing a useful job by masking other more intrusive noises - noise control does not always mean noise reduction.
Some industrial sites are laid out in a way which almost invites complaints from neighbours, with the noisiest (and most visually intrusive) areas and buildings close to a boundary shared with a residential area. This is often historical ("the factory was here before the houses ) and major reorganisation of a site is not often feasible. When designing a new site, the major sources of noise and visual impact can be identified and located away from a sensitive boundary, perhaps using 'quieter' buildings such as a warehouse or office block as a 'buffer'. This is usually a matter of common sense, not acoustics.
Even on existing sites, some changes in layout may be possible. One Black Country foundry had a long-standing problem caused by noise from the stockyard
which faced houses across a residential road. Deliveries of scrap iron and coke, and movements of the crane and trucks in the yard, were regular causes
of complaint. As part of a programme of general improvements to the site the stockyard was relocated to land previously used as the employees' car park,
further from the houses, and screened from view with a barrier of timber sleepers. This has successfully reduced the problem of stockyard noise.
Even without a change in processes or site layout, noise problems can sometimes be resolved by relatively minor changes in the way an industrial site is operated and managed. Such 'non technical' measures might include:
The need for such measures can only be established by carrying out regular assessments of noise from the site, as experienced by neighbours. Some problems
do not need the services of an acoustics specialist to find a solution - an employee with a good knowledge of the site operations can sometimes define the
problem and identify the solution better than the 'expert'.
People living close to forges and foundries in the Black Country are generally extremely tolerant of noise from these premises, at least during reasonable daytime working hours. Residents are most likely to complain if they experience some noise which they believe to be unnecessary - a door to a noisy shop left open, shouting and revving of a truck engine in the yard, or a radio being played loudly. These problems can be avoided by adequate management control and the development of greater 'noise awareness' amongst employees.
Last, but by no means least, any company should have a proper method of dealing with complaints from local residents. Experience shows that if a company deals with complaints in a courteous and organised way and makes efforts to reduce 'avoidable' noise, then residents are far more tolerant of other noise (for example, from drop hammers) which they appreciate is difficult for the company to reduce whilst staying in business.
On many sites with noise problems. some benefit could be obtained by relatively minor changes to the layout of the site. Such changes should always be
considered before embarking on 'acoustic' remedies.
Consider how noise can be reduced by management measures. Avoid unnecessary noise, make employees 'noise aware'.
Develop good relationships with neighbours. Respond quickly to complaints.
Noise from an industrial site often arises from a number of separate sources. There are many ways of controlling noise, but each noise problem is different.
Many attempts at reducing noise from industrial sites have failed because the problem was not properly identified, and inappropriate control measures were
carried out. Before attempting to reduce noise affecting neighbours, it is essential to develop a strategy to address the following key points:
What are the noise levels at the houses affected? What noise levels would be acceptable?
What are the noise sources which contribute to the problem?
Sometimes this is obvious. However, often it is necessary to make detailed measurements, perhaps with equipment operated individually or in groups, to find out how much each source contributes to the overall noise. Remember that noise measurements do not tell the whole story - some noises cause far more annoyance than might be expected from the actual measured noise levels they produce. A visit to a complainant's house to listen to the noise from your site can be most instructive, and sometimes reveals an obvious solution to a problem.
Some sources of noise will be more important than others - they will not all need the same degree of reduction. However, the difficulty and cost of reducing noise from each source must also be considered. As an extreme example, if there are ten equal sources of noise, a reduction of 10 dB(A) could be achieved by reducing the noise of each source by 10 dB(A). Alternatively, you could reduce the level of nine of the sources by 30 dB(A) and leave the 'difficult' source unchanged, to achieve the same effect. The objective is to develop the most effective and economical package of works to achieve the overall result.
Solving noise and vibration problems often needs specialist assistance, from a consultant or noise control equipment supplier. Trade Associations
can usually help to locate a source of advice. However, do not rely on an outside specialist to arrive at the bets strategy. He/she will not be fully
conversant with your operations, and noise control equipment suppliers may be inclined to concentrate on solutions which use their products. The close
involvement of company employees (perhaps the Works Engineer or one of his staff) is essential to reaching a practicable solution to any industrial problem.
Original version © 2002, ISVR University of Southampton. All rights reserved.