After reading this article you will learn about Factory Noise:- 1. Introduction to Factory Noise 2. Factory Noise Affecting the Neighbourhood 3. Measurement 4. Affecting Machine Operators 5. Control 6. Vibrations 7. Foundry Noise 8. Vibratory Conveyors 9. Enclosures 10. Nature.
- Introduction to Factory Noise
- Factory Noise Affecting the Neighbourhood
- Measurement of Factory Noise
- Factory Noise Affecting Machine Operators
- Control of Factory Noise
- Vibrations Produced by Factory Noise
- Foundry Noise
- Vibratory Conveyors
- Enclosures to Prevent Factory Noise
- Nature of Noise Generated in a Factory
1. Introduction to Factory Noise:
It is well known that factories and workshops tend to be quite noisy due to the noises and vibrations arising from the operations of various machines. Noise levels in the work areas of even production industries may be as high as those found in typical workshops.
However, there may be quitter areas at locations far away from the noise sources in the case of production industries. But even at such relatively quitter sites, the noise levels may still be above speech interference levels, or even potentially harmful levels. Typical noise levels in some production industries are shown in Table 1, while those found in various types of workshops are listed in Table 2.
Since the problems of noise encountered in various types of factories and workshops are so diverse, it is almost impossible to make precise recommendations regarding the control of such noise.
This problem, however, consists of the following two major aspects:
(a) The escape of factory noise and its effects on the neighbourhood; and
(b) Noise generated and contained within the factory itself.
2. Factory Noise Affecting the Neighbourhood:
There is no specific legislation in most countries regarding the noise escaping from factories and affecting adversely the people in the neighbourhood. The local authorities may, however, set limits for the noise near plant boundary to avoid complaints from the residential areas near factories.
Such measures are usually applied more stringently in the case of new plants. People living in the neighbourhood of noisy plants in industrial areas become “conditioned” to fairly high levels of neighbourhood noise during daytime working hours and usually accept it.
Typical industrial noise is a fairly steady low-pitched hum, when heard at a distance. A wide variety of other individual noises may be superimposed on this steady hum. These superimposed noises are often intermittent and usually high in pitch.
The steady low-pitched hum arises from the general clatter of work in the plant, modified by passing through the light building structure of the plant. Such structures offer far better insulation against high frequencies, but are not very effective in insulating against the low-frequency clatter.
The following are some examples of individual sources of noise in the neighbourhood of a noisy factory:
(a) Doors, windows and ventilators emitting noise from the interior of the factory;
(b) Fans, compressors, power saws, steam hammers and other similar equipment which may be placed outside a building, either in the open or merely under a roof; and
(c) Loading bays and access roads leading to them.
Reaction of Neighbours:
The reaction of people living in the neighbourhood of a noisy factory must be assessed in terms of the following:
(i) General annoyance;
(ii) Effect on people living within earshot; and
(iii) Long-term effects.
Tolerance of noise in the neighbourhood of a factory can be expressed in terms of an addendum to the usual residential noise levels. This addendum is the increase in the general background noise level which would normally be regarded as realistic and acceptable over the usual or recommended noise levels for typical residential areas.
In urban areas near industrial sites, for example, an increase of 10 dB in the background noise level could be regarded as reasonable. Near heavy industries, however, this reasonable and acceptable increase could even be as high as 15 db.
3. Measurement of Factory Noise:
For single reading measurement of noise in the neighbourhood of a factory, a general noise level can be recorded. The average value of neighbourhood noise can be estimated from such records, provided that the fluctuations in the general noise level is less than 10 dB (A).
The recorded noise levels are subjected to the following corrections:
(a) For a continuous tone, subtract 5 dB (A), and
(b) For impulsive or basically irregular noise, add 5 dB (A).
A basic noise level criterion of 50 dB (A) has been suggested for factory noise as a reasonable noise level. However, 55 or even 60 dB (A) may be acceptable in the case of older, well-established factories in primarily industrial areas. This criterion is then subject to various corrections shown in Table 3.
People living in the neighbourhood of a factory are likely to complain if the corrected background noise level criterion is exceeded by 5 dB (A). It has been found that a 5 dB (A) difference is subjectively noted as a “considerable” difference in the noise level. Similarly, a 10 dB (A) difference is noted by most people as a “very noticeable” difference.
Tolerance to noise is also affected by exposure time, though this is not used in establishing the basic noise level criterion. Relatively high noise levels produced for short periods of time, for example, may be tolerated by most people.
On the other hand, more continuous generation of lower noise levels may produce many complaints. Specific correction factors based on the duration of noise during normal working hours are shown in Table 4.
4. Factory Noise Affecting Machine Operators:
All machine operators in factories are primarily affected by the noise produced by their own particular machines. Other noisy machines are, in general, at distances such that their noise does not add appreciably to the noise produced locally by the machines they operate. This is evident from the standard corrections for background noise, as shown in Table 5.
It is apparent from Table 5 that if an individual machine is appreciably noisier than the background noise from other machines, say by 4 to 7 dB (A), the total noise level is not appreciably increased as far as the operator is concerned.
On the other hand, if one machine in a shop is considerably noisier than all the other machines running simultaneously, the total noise level will not be increased appreciably by the presence of other machines.
However, if two equally noisy machines operate simultaneously, they produce a 3 dB (A) rise in the total noise level. Similarly, three equally noisy machines will produce a 5 dB (A) increase in the total noise level, when operating simultaneously.
5. Control of Factory Noise:
Work area noise levels, comprising the cumulative noise of several machines, are generally lower than those of individual machines (as felt by their operators), and with a broader spread. The overall or background average noise level can usually be reduced by standard acoustic treatment of walls and ceilings.
This, however, will not be of much benefit to individual machine operators. To an operator, an appreciable decrease in the average sound level will only become noticeable further from the sound source (the machine he operates) than he normally stands.
It follows, therefore, that noise reduction at source is the only efficient method of treatment as far as the individual machine operator is concerned. From this point of view, the most effective treatment is the complete enclosure of the machine, with the operator performing his normal work from a position outside the enclosure.
If such an enclosure provides adequate and complete sealing and if sheet metal panels are adequately damped, noise reduction of the order of 25 dB (A) is possible, by such means, in the high frequency bands. Much, however, depends on the type and size of machines and the nature of the working area. Complete enclosure, for example, may not be feasible in many cases.
In particular cases, no method of noise reduction may be effective in reducing the noise to an acceptable level at the location from where the operator normally operates the machine. In such cases, ear protectors may be the only answer as far as the operator is concerned.
6. Vibrations Produced by Factory Noise:
In the case of factory noise, it is mainly the air-borne noise, radiated from machines, which has to be treated to reduce the noise. In some plants (e.g., printing presses), however, high levels of noise may be caused by the structure-borne vibrations and these may be predominant in adjacent areas.
The treatment of airborne noise may quite the printing shop, for example; but still result in objectionable noise levels elsewhere in the building transmitted through the floor.
Structure-borne vibrations can only be cured by reducing the vibration of the machine and/or isolating one part of the building from the remainder. Machines which inherently generate high impact shocks may require rigid mounting on massive concrete blocks which are themselves suitably isolated.
Although vibration can be effectively treated by isolation, this will generally not have any appreciable effect on reducing air-borne noise.
The development of vibration during the period of use of a particular machine, however, may be an indication of its deterioration, or an inherent (or impending) fault in the machine, calling for a remedial action. Sometimes, this may show up as a change in the machine operating noise.
Where the deterioration is gradual, however, this may pass unnoticed. It follows, therefore, that periodic vibration checks, or vibration analysis, may be valuable, both to limit the extent of developing damage and, indirectly, to control the noise level of the machine.
Nearly all machines produce some form of vibration. In order to reduce the transmission of vibrations to the minimum (or at least acceptable) levels, therefore, the use of supporting elements known as “anti-vibration mounts” should be encouraged.
This may appreciably and effectively reduce the overall noise levels in factory buildings, where machine noise (or essentially machine vibration) can be transmitted through the floors, or other structural members, to other parts of the building.
7. Foundry Noise:
Foundry noise is one of the most difficult types of factory noise to treat for noise control. The various processes involved in the foundry operators are invariably carried out at high noise levels. The most obvious noise sources in a foundry are the ventilating system shakeout and finishing areas, and vibratory conveyor systems.
The exhaust fan noise level can be substantially reduced using a number of approaches. The initial step involves the consultation with the supplier of the exhaust fan, and the criterion of prime importance for this is, of course, the movement of air.
A reduction in the number of revolutions per minute of the unit and/or a change in the blade pitch may possibly offer substantial noise reduction with a minimum of air flow reduction.
In most cases, moreover, an engineered acoustical silencer can be attached to the exhaust fan unit to offer a 10-15 dB (A) reduction with minimal reduction of air flow. If the silencer route appears to be the most applicable, it is advisable to consult a reputable acoustical engineering (or acoustical materials) firm.
In order to design an effective silencer system for an exhaust fan unit, the following information is required:
(a) An octave band analysis of the fan noise source,
(b) The rate of air flow being generated by the fan; and
(c) The tolerable pressure drop.
With this information, a silencer can be designed to give the appropriate reduction in the noise level. The silencer should then be applied to a single fan unit to ascertain its technical effectiveness.
Once the effectiveness of the silencer has been proved on one unit, the same procedure would apply to the remaining exhaust fan units. The noise from the ventilation system can thus be controlled fairly easily and at a minimal cost.
Regarding the noise produced in the shakeout areas of foundries, several aspects have to be considered.
Some of these are:
(a) The noise generated by the shakeout only;
(b) The noise generated by the shakeout with parts; and
(c) The noise at the operator positions.
It has been found that the shakeout itself; without products, is a prime source of noise. When the product is introduced, the noise level is slightly increased. Of course, the situation varies from factory to factory, and also with various types of equipment. The prime concern here is the reduction of noise at the operator position. The objective should be a reduction of at least 15 dB (A) in this case.
In the systems approach for shakeouts, the design of a solution for noise control is a critical step. Several elements are necessary to reach the desired 15-20 dB (A) reduction in the shakeout noise level. This requirement can be met by proper consideration of transmission loss, absorption, damping, and silencers.
The ideal solution, of course, is to reduce the noise of the equipment by modifying its design, so that it makes less noise. The may be technically feasible. In most cases involving shakeouts, however, redesign or equipment modification is not feasible because of the loss of production and high costs involved. If the shakeout cannot be modified to reduce the noise level, it must be isolated from the operators.
This can usually be achieved by one or more of following methods:
(a) Moving the machine into another area;
(b) Isolating the operator from the machine; and
(c) Putting a properly engineered acoustical enclosure around the shakeout. The last alternative, in most cases, is the most feasible one.
Acoustical enclosures for shakeouts perform another useful function. It has been found that these enclosures not only reduce substantially the noise levels throughout the shakeout area, but they also confine the dust. The shakeout area is one of the prime sources of dust pollution within a foundry.
Since the acoustical enclosures must be properly sealed to prevent the leakage of noise, they also contain the dust. In most instances where acoustical enclosures are employed, the enclosure is attached to the appropriate duct work and fed to the dust collecting system. In most cases, a silencer has to be used at the ventilation intake on the enclosure to attenuate any serious noise leakage.
The check-list for the main requirements of an acoustical enclosure is shown in Table 6.
From the point of view of noise control, finishing area is probably the most difficult area of a foundry. Here the noise is an inherent part of the operation. The actual grinding or clipping operation requires the operator to be in the direct field of the noise. The problem is not totally hopeless, however.
In order to reduce the noise level in the finishing areas of a foundry, the same basic steps of acoustical analysis are followed. Each noise source must be analysed as a separate unit. In addition, it is imperative to determine the operator’s total exposure time to his individual noise source.
Each hand-tool operation is a multiple source of noise since the noise is generated by the tool itself, and also by the actual implementation of the tool. Obviously, the actual hand-finishing operation will always create noise. The ultimate solution of this problem would come through the elimination of these hand-tool operations through process engineering.
For most applications, however, this is not yet possible in practice. The manufacturer of the tool may be consulted to find out of a quieter tool is available. Quieter pneumatic tools, for example, have been developed for some finishing operations.
Measuring and defining the problems in the finishing areas of a foundry is easy enough; but to design, test and implement a solution are difficult tasks. The designing of systems solutions to this complex noise problem becomes more difficult due to presence of a large number of variables.
Since each operator is a contributing source of noise, the noise generated by one operation has an accumulative effect on other operators in the finishing area of a foundry. The first objective, therefore, is to isolate each noise source from the other sources.
In most cases, a reorganization of the finishing area to isolate each source of noise from other sources is the first step in a long-term, cost-effective solution. This, however, requires a great deal of co-ordination between the production and plant engineering divisions.
In most cases, the finishing and grinding areas of a foundry contain cement floors, steel ceilings, and cement walls. Merely isolating each operator from the other by means of a barrier would not be adequate due to the reverberation of noise.
It follows, therefore, that absorption must also be applied, preferably on the walls, ceilings and inside the individual barriers. Acoustical modular panels with appropriate absorption on all surfaces facing the operators will generally serve the purpose. The barriers should extend as high as possible over the operator’s head.
Wall absorbers are imperative since the noise reflected from the walls reverberates to other areas. These wall absorbers perform basically the same function as foam or any other absorptive material. Moreover, they have a durable perforated surface to ensure long-term performance.
Noise control in the finishing areas can also be augmented by unit absorbers. Due to relatively high frequency of the noise sources, and the reflective characteristic of the ceiling, unit absorbers help the absorption of noise and cut down reverberation. Unit absorbers can be used in two ways.
In the first place, they can be used merely as absorptive units. In this case, they are hung from the ceiling and prevent the reflection of sound. Secondly, unit absorbers may also be deployed in a silencer configuration, as close to the top of modular panel or curtain barriers as possible.
In this case, the unit absorbers are spaced relatively close together and thus perform the same basic function as an engineered silencer.
Optimum spacing, size and number of the unit absorber for a particular finishing area may be determined by the standard methods of acoustical engineering. One should keep in mind, however, that the unit absorbers will do absolutely nothing for the noise sources themselves. Their primary purpose is to reduce the additional noise created by reverberation from the walls and ceilings.
8. Vibratory Conveyors:
In the operation of vibratory conveyors, the problem of excess noise arises from two sources, viz., the noise generated by the conveyor itself, and the noise generated by the impact of the parts on the conveyor. The noise of the second type is of prime concern in most cases.
Typically, the difference between the noise levels of a conveyor without parts and that with parts is 8-10 dB(A). The metal surface resonates due to the repeated impact of the parts.
A large amount of noise is radiated from the structure to the surrounding environment. In order to reduce the air-borne noise, the vibration of the metal must be controlled. This can be achieved by bonding to the metal surface an appropriate damping material to dissipate the vibration.
A quick and simple method to determine if the resonance of metal surface is a problem (and whether or not damping will be effective in reducing the noise) is to press very hard against the surface suspected to be resonating. If a decrease in the noise level is perceived or measured, the damping of sound will be effective.
For the determination of specifications of the damping material, the prime factors to be considered are the temperature and thickness of the resonating surface. A consultation with a reputable supplier can help to determine the type and thickness of the damping material.
9. Enclosures to Prevent Factory Noise:
Enclosures are erected in order to secure quite areas in factory spaces and workshops. The enclosures are constructed with sound insulating surfaces chosen to provide the attenuation required. The noise level inside the enclosure, therefore, can be reduced to a suitable figure, regardless of the normal background noise level of the factory.
Works offices for use by supervisors, for example, are commonly erected at some suitable site on the main floor space of factories and workshops. With the help of suitable enclosures, the noise level inside such offices can be reduced to 55-65 dB(A), i.e., just below the level of speech interference.
Even if it is reduced to 70 dB (A) for a noisy office, it is still acceptable in a noisy factory, since reasonable speech and telephonic communication is possible at this noise level. A particular advantage in the case of such works offices is that they are single, complete rooms and can be readily designed and constructed as efficient enclosures. In fact, they can even be isolated from the floor-borne vibrations, if necessary.
Other offices in a factory should, as far as possible, be separated from noisy works areas by substantial barriers or, ideally, be located in a separate structure.
Particular attention must be paid in this case to avoid direct transmission paths for air-borne sound, e.g., through the cracks or other small openings. Attempt should also be made to eliminate the structure-borne sound from the offices. The basic problem in this case is largely that of proper building construction.
10. Nature of Noise Generated in a Factory:
In the case of factory noise, some attention may also have to be given to the nature of the noise generated by various machines, particularly if they generate impulsive noise.
The true peak noise levels in the case of impulsive noise may be appreciably higher than those given by simple sound level meter readings. For satisfactory analysis of factory noise, it is necessary to take into account the nature of the noise and determine the true noise levels and their significance accordingly.
For this purpose, noise may be classified under the following headings:
(a) Continuous broad-band noise;
(b) Intermittent noise;
(c) Impact noise; and
(d) Narrow-band noise.
Intermittent may be broad-band or impulsive noise, or even a combination of both the types. These differences modify the “exposure time” of intermittent noise. In the case of impact noise with high peak levels of specific duration and frequency, there is a general agreement that the maximum permissible exposure is 140 dB (A). The narrow band noise, on the other hand, is particularly distressing at higher frequencies.
Analysis of factory noise with respect to noise spectra may indicate the need for and method of treatment of individual machines, with a view to obtaining a reduction of the noise at source. In a large number of cases, for example, the major source of noise may be the work piece rather than the machine itself. This is frequently true in the case of clipping and riveting machines.
Often much can be done to reduce the level of such noise and vibration at source, e.g., by the use of vibration-damping clamps holding the work machines. In the case of machine tools, “squeal” is often present.
In such cases, the operation may be modified to eliminate the squeal, and this may result in a reduction of 6 dB (A) in the overall noise level of the machine, with a particular reduction in the significant audio-frequency range.