This article throws light upon the top seven ways of reducing noise in factories. The ways are: 1. Noise Reduction at Source 2. Vibration Isolation 3. Noise Reduction and Layout 4. Enclosures to Reduce Noise 5. Sound-Absorbing Materials 6. Partial Enclosures and Screens 7. Ear Protection.
Way # 1. Noise Reduction at Source:
(i) Process and Machine Tools Selection:
One of the basic principles of noise control is that noise should be reduced as near the source as possible. The greatest number of people are protected from the noise by this means, and moreover, the noise control treatment is less expensive.
The noise that is likely to be produced should be considered at the very beginning of the planning of a factory, even before the orders are placed for machines and tools. The process used for production determines, to a very large extent, the noise problem in a factory. When a factory is planned, the planners should keep in mind that many of the very noisy processes have alternatives that produce far less noise.
Some of the common examples of this are:
(a) Welding instead of riveting;
(b) Pressing instead of forging; and
(c) Grinding instead of chipping.
Noise can hardly be a major consideration in the choice of an industrial process; but it must be taken into account as one of the economic factors. The reason is that, once a process has been selected and introduced in a factory, no amount of noise reduction treatment inside the factory is likely to reduce the noise level of one of the noisy processes to that of its quieter alternative.
To a lesser extent, the machine tools and other equipment used in the factory will also influence the noise level in the factory. It may so happen that one make of a machine tool may have particularly noisy gears, compared with another of similar performance.
Since very few machine tool manufacturers give noise levels for their products at present (in India and other developing countries, at least) factory engineers should compare the noise produced by different makes of machine tools in operation. One should also keep in mind that excessive noise from a tool, for its type, size and power level, generally indicates low mechanical efficiency.
(ii) Reducing the Potential Noise Energy:
It is well known that the amount of noise radiated from a surface depends on the amplitude of the vibration and on the area of the radiating surface. Among these two factors, the amplitude is determined by the resistance of the surface to oscillatory motion, and the power available to drive it.
It follows from the above discussion that there are two basic methods of reducing the noise at source, viz.,
(a) By reducing the amount of energy communicated to the vibrating surface; and
(b) By reducing the efficiency of the surface as a radiator of noise.
Unless the way in which the power is applied is changed, however, reducing the energy is only likely to be effective if the operation generating the noise is not an essential part of the process.
The sources of noise in a factory can be grouped under:
(c) Friction; and
(d) Air turbulence.
We discuss now the precautions that should be taken to keep the noise energy of a source at a minimum, for each of these groups of noise sources. Where impact is essential to the process (as in hammering and riveting, for example), the possibilities of reducing noise at the source are usually limited to using no more power than is absolutely necessary, and preventing unnecessary impacts.
If the impact can be spread over a short period (and thus converted into more of a squeezing or shearing operation), an appreciable noise reduction is obtained. Riveting, punching, and pressing are processes where this is sometimes possible. On the other hand, impact that is not essential to the process can often be quietened.
For example, noise caused by the handling and dropping of materials on hard surfaces can be reduced by the following methods:
(a) By covering surfaces with resilient materials;
(b) By using resilient materials for containers; and
(c) By fitting rubber tyres on trolleys.
Machine rattle can be minimised by proper maintenance. Often a rattle can be eliminated merely by securing a loose panel. If reciprocating or vibrating movement is part of the work process, the amplitude of such a vibration should be kept to a minimum.
In cases where the rate of reciprocation is not important, decreasing this rate will reduce the noise, provided that loose parts are not excited to vibrate at a higher frequency. Vibration from rotating machinery can usually be reduced by dynamic balancing. If practicable, a reduction of speed will also prove to be effective.
Frictional noise generated by the cutting action of tools and saws can be reduced by keeping them sharp. Changes in the shape of cutting tools may also be beneficial.
Other noises caused by friction in machines, conveyors, and roller trolleys can be minimised by proper lubrication. Often the noise excites a high-frequency resonance. Such a resonance can be prevented by substituting a highly damped, material for the resonating component.
Air-turbulence noise from air and steam exhausts can be simply and effectively reduced at the source, with a silencer that lowers the escape velocity of the exhaust. In the case of small pneumatic tools, for example, this can be achieved by incorporating in the tools an exhaust collecting sleeve.
Another method, suitable for larger portable tools, is to conduct the exhaust air away, through a second line, to a remote silencer. When a jet of air is used for cleaning off, or for lifting parts from dies, the air pressure should be kept at the minimum required for the operation.
Noise caused by turbulence at outlets, valves, and bends in pipes and ducts can be reduced by careful streamlining as well as by lowering the velocity of air or gas passing through them.
(iii) Reducing the Radiation of Noise:
For a given amplitude of vibration of a source, the intensity of noise produced by it will be roughly proportional to the area of the radiating surface, if the dimensions of the surface are large compared with the wavelength of the sound generated.
This means that if the surface area is halved, the intensity of the noise will be reduced by 3 dB; and at lower frequencies (i.e., larger wavelengths), the reduction will be much greater. To radiate sound effectively at 100 Hz, for example, a source must be of the order of a square metre in area; but at 1,000 Hz intense sound can be radiated from a source of only a few cm2.
The directional properties of a source are also determined by the size of the radiating surfaces. If the dimensions of the radiating surface are much greater than the wavelength of the sound generated, the sound will be emitted primarily in one direction. Thus, even quite small sources will be directional at high frequencies (i.e., small wavelengths).
Supporting structures for vibrating machines and other equipment will radiate less noise if they are frames than if they are cabinets or sheeted enclosures. An enclosed machine may be noisier in fact (unless proper precautions are taken to isolate its housing) than if it were not enclosed.
The reason for this is the larger surface area in the case of an enclosed machine. The noise radiated by machinery guards can be minimized by making them of perforated sheet or of wire mesh.
(iv) Resonance and Damping:
As a general rule, the noise radiated from metal plates and other metal parts is made more intense by resonance in such parts. The well-known phenomenon of resonance occurs in this case when the natural frequency of the metal plate, which depends on its stiffness, is equal to the frequency of the source driving it.
The amplitude of vibration is then limited only by the damping in the material. For example, a steel sheet (which has very little damping) will vibrate freely at resonance; but a lead sheet (with high damping) will not.
Resonance can be prevented in machine parts by:
(b) Increasing the damping; or
(c) A combination of both.
Stiffening, which is generally easier, is satisfactory when the frequency range of the driving source is narrow and constant. Where the panel concerned is a permanent part of the equipment, it can be stiffened by corrugations or by adding ribs. If the work-piece is resonating, clamping of it to a suffer structure will reduce the noise.
On the other hand, increasing the damping will be more effective if the exciting force covers a wide range of frequency. A permanent increase in damping (in resonating parts of equipment, for example) will be achieved if the surface concerned is coated with a chemical compound of the kind used for under-sealing cars.
To be effective, however, the coating should be at least equal in weight to the panel. A temporary increase in damping, to reduce the noise from the riveting of steel plates for example, can be obtained with sand bags, or even by ensuring that the plate is continuously supported over its whole area.
Way # 2. Vibration Isolation:
(i) Basic Principles:
A source of sound usually does not itself have a large enough area to radiate much noise. However, the vibration is conducted along a mechanically rigid path to a surface that can act as an effective radiator.
The vibration can be transmitted, in this way, for long distances with very little reduction. If the rigid connecting path is interrupted by a resilient material of the correct characteristics, however, the vibration transmitted (and the noise radiated) will be greatly reduced.
The reduction of vibration obtained in this way depends on the ratio of the driving frequency of the source to the natural frequency of the resiliently supported system. The natural frequency depends on the stiffness of the system.
The higher the ratio between the two frequencies, the greater the noise reduction; but it is difficult to achieve a very large reduction in practice. Table 1 gives approximate values of the transmissibility (i.e., the ratio of the amplitude of vibration transmitted to the amplitude of the driving vibration), and the equivalent noise reduction, for various ratios of these frequencies.
However, the values given in Table 1 are theoretical values. In practice, it has been found that a ratio of not less than about 3:1 between the driving frequency and the natural frequency is satisfactory for most purposes. Such a ratio will reduce the vibration transmitted by 87% (equivalent to a noise reduction of about 18 dB).The discussion given above applied to steady-state vibration (for example, the vibration from an electric motor); but the vibration can also be impulsive (for example, the vibration from a punch press). The isolation of impact vibration (or shock) is based on a different principle.
In this case, the vibration energy is not absorbed in the amount. It is stored for a short time and released at a slower rate. This requires a reasonably stiff mount.
(ii) Resilient Materials:
The resilient material used for vibration isolation may be in the form of a pad, or of a proprietary mounting.
The materials commonly used for this purpose are:
a. Felt, cork, and glass wool;
b. Natural or synthetic rubber; and
c. Steel springs.
Felt, cork, and glass wool are often used for resilient mats or pads under machine bases. The total area of such a mat is important; and so is its thickness. The load per unit area must be high enough to give deflection adequate for the isolation required.
Similarly, the thickness of the material should be such that, at this deflection, it is not loaded beyond its elastic limit. In many cases, a large area of material has been used with negligible effect because the mounting has been too stiff.
Natural or synthetic rubber is used occasionally as a mat, but more frequently as part of a proprietary mount. It must be used either in shear or in compression, and is generally bonded to metal in order to provide connections.
Coil springs of steel are very useful in giving large deflections for isolating low frequencies, but high frequencies may be transmitted along with the coils unless there is another resilient material in series with the spring.
(iii) Isolation Methods:
In addition to resilient materials, pneumatic suspension has also been suggested as a method of vibration isolation, although it would be more expensive than other forms of suspension.
In this method, the stiffness of the mounting system could varied to suit the characteristics of the source, and automatic leveling devices would make it possible to use very “soft” suspension without impairing the stability of the machine. The advantages of this method may make its extra cost justifiable in special cases.
The normal position for vibration isolation is between a machine and the floor; but it can be applied as well between any energy source and radiating surface. Isolating machines from the floor will reduce the radiation of low-frequency noise, and also prevent its transmission to remote parts of the factory.
This type of isolation will also prevent vibration from being transmitted to delicate machinery. In multi-storey factories (where the floors are more liable to vibrate, and to radiate noise both upwards and downwards), such isolation is an essential part of noise reduction.
Most machine tools are not likely to generate low-frequency vibration, and the resilient mount may then be placed between the machine casting and the floor. Where there is low-frequency vibration, however, the deflection necessary for adequate isolation may cause too great a movement of the machine.
When this is the case, the resilient mounts may be placed under an independent concrete base to which the machine is bolted. This arrangement increases the mass and reduces the amount of movement. A similar arrangement is necessary if a machine needs to be bolted down to achieve sufficient rigidity.
Besides reducing noise, vibration isolation between the machine and the floor has other important advantages. For example, the reduction in vibration transmitted to the floor will enable the live-load allowance for vibration to be reduced, and the reduction of shock loading within the machine may increase its useful life.
Some of the proprietary mounts have built-in leveling devices, and since vibration mounting stops the machine from “walking”, holding down bolts are not required. This gives greatly increased flexibility to the plant layout.
(iv) Other Considerations:
For reducing the noise in a factory, equally important is the isolation of a vibrating source within a machine from other parts that can radiate the noise. Noise may, for example, be radiated by the sheet metal enclosing the moving parts. In such cases, the cover should be isolated from the source by resilient fixings.
The frequencies involved here are generally higher than with floor isolation, so that the deflection required in these fixings need not be so large. In other cases, the source of noise may be isolated as a separate part within the machine, with the power transmitted through belts or shafts incorporating resilient couplings.
The factory engineer should ensure that vibration is not transmitted to objects fixed to machines. Sheet metal ducts and chutes, for example, should be attached to machines only through flexible canvas couplings.
In factories, the vibration isolation of service equipment is often neglected. However, the noise from service equipment can often be as intense as that from the production machinery. A very common example of this is an axial-flow fan rigidly mounted in the sheeted side of a factory. Other potential sources of vibration and noise are unit heaters, dust extraction equipment, conveyors, cranes, and transformers.
An otherwise satisfactory vibration-isolating installation can often be made useless by rigid pipe and conduit connections, which “short-circuit” the isolators. All such connections should, therefore, be flexible and loose. Even a flexible connection will transmit vibration if it is taut.
Where flexible connections are impracticable, the introduction of bends into a pipe will reduce its efficiency as a conductor of vibration. Alternatively, the pipe itself should be supported on vibration mounts for a considerable distance from the source.
Way # 3. Noise Reduction and Layout:
(i) Site Layout:
If the factory under consideration is located in a built-up area, or in an area that is likely to be developed in the future, consideration should be given to the position of noise sources on the site.
Management should take care to place the noisiest part of the factory as far away as possible from the neighbouring buildings, since the noise level decreases by 6 dB as the distance from the source is doubled. For example, if a noise source is placed at the centre of a 400 x 400 m2 site, the noise level at the boundary of the site will be at least 18 dB lower (at all frequencies) than the noise source were 25 m from the boundary.
Buildings and high walls between the noise source and the listeners will act as screens. This is particularly true for the high-frequency noise, which is more directional. For such a screen to be effective, however, its height above the level of noise source must at least equal half its distance from the source.
Moreover, the directional characteristics of high- frequency noise should also be taken into account. Factory engineers should see to it that open windows, doors, and other openings that allow high-frequency noise to escape from the factory, should not face neighbouring buildings.
Frequently, the designer of factory building faces the problem of deciding what part of a factory is likely to be noisy. The manufacturers of equipment’s should be consulted in case of any doubt regarding this. Potential noise sources that are often placed on the perimeter of factories include loading bays, dust or air extraction plant, compressor houses, boiler houses, and transformers.
(ii) Factory Layout:
Small factories (in the case of small-scale industrial units, for example) generally consist of only two main groups of accommodation viz.,
(a) A production unit; and
(b) Office space.
Large factories, on the other hand, may have separate enclosed areas tor different parts of the production process, and thus may offer more scope for noise control. Wherever possible, the office space should be a completely separate building, and it should not share a common wall with the production area.
The office should, in any case, be situated away from the noisiest part of the factory. Where it is not possible to avoid a common wall between the office space and the production area, the wall should be heavy, with as few connecting doors as possible and no permanent openings.
If the production area is divided into separate compartments (in the case of larger factories), it may be possible to grade these compartments in the order of noise produced, and to separate the very noisy areas from the relatively quieter ones. In multi-storey factories, the division of production area will help to reduce the airborne noise if the very noisy machines can be kept together, preferably on the ground floor.
In factories where there is no such internal subdivision, not much noise reduction can be achieved by the layout of the machines alone. In very large undivided areas with relatively low ceilings, the noise level decreases continuously as the distance from the source increases. The actual reduction in this case depends on the amount of sound absorption in the space between the noise source and the listener.
Even in small areas, the noise level decreases for a short distance from the source. It follows, therefore, that if the factory contains only a few sources of loud noise which can be grouped together and positioned away from the densely populated areas, some benefit in noise reduction can be obtained.
Way # 4. Enclosures to Reduce Noise:
(i) Introductory Remarks:
Once the noise has been generated and radiated, the most effective way of controlling it is to contain it within the enclosure. A reduction in noise level of about 50 dB can be readily achieved by an ordinary building enclosure.
The region enclosed may be that of the whole factory (to reduce the noise reaching residential areas in the neighbourhood), or only a small area of it (a single machine, for example).
The smaller the region, the greater the number of people who benefit from noise reduction. On the other hand, the noise level within the enclosure itself will be higher than it would have been if uncontained, unless some extra noise absorption treatment is applied.
When airborne sound reaches an impervious partition, this (i.e., the partition) must be set in vibration before the sound can be radiated by the other side of it. Consequently, the resultant noise reduction will depend on the resistance of the partition to vibration (i.e., on its weight per unit area) and also on the frequency of the noise concerned (since it requires more energy to move the partition at a faster rate).
The sound reduction factor of a partition increases by about 5 dB for each doubling of the weight, or of the frequency of sound.
(ii) Double-Leaf Enclosure:
For a given weight of material, a higher sound reduction can sometimes be obtained by using a double-leaf enclosure. A double- leaf enclosure, in its simplest form, is a cavity wall with no rigid ties across the cavity. A more elaborate form of this is the completely isolated “box within a box” that may be required for a quiet listening test or other similar uses.
A double wall will give twice the reduction of a single leaf. This, however, would only be so if the cavity between the walls were impracticably wide, and there were no indirect transmission through floor or ceiling.
For walls with cavities up to 5 cm. wide, the noise reduction obtained at low frequencies is no better (and may even be worse) than for a solid wall of the same total weight. But the sound reduction factor of a double wall increases more rapidly with frequency than that of a solid wall, giving relatively greater reductions at high frequencies.
The double wall may give a higher average noise reduction than a single wall, though it is more effective only at high frequencies. To increase the effectiveness of the double-leaf enclosure at low frequencies, the cavity must be made 15 to 30 cm wide.
(iii) Enclosure Efficiency:
In addition to weight of the wall and frequency of the noise concerned, the efficiency of an enclosure depends also on its completeness and uniformity. A direct air leak (through a hole or through porous material) will transmit the air pressure fluctuations, propagating the noise without reduction.
Holes amounting to as little as 0.01% of the total area of the enclosing construction will transmit more than half the sound energy, thus lowering the total sound reduction by 3 db.
For this reason, porous sound-absorbing materials are in fact poor sound barriers. The best partitions, for the purpose of noise reduction, are generally of “wet” construction, since they can be more easily made airtight.
Prefabricated partitions, and those constructed of sheet materials, require special precautions to ensure that all the joints are properly sealed. Any parts of the enclosure construction that, although airtight, transmit sound more easily than the rest will have much the same effect.
Where the “weaker” part of the construction makes up half its total surface area, the total sound reduction factor cannot be more than 3 dB above the lower value (corresponding to the “weaker” part of the enclosure). Thus, in a building of heavy construction (such as a multi-storey factory), most of the noise escapes through the windows, which act as the “weak” parts of the enclosure.
The noise transmission paths round a partition must also be considered. There may, for example, be a direct air path over the top of a partition (as through a porous sound-absorbing ceiling). Besides ceiling, other direct air paths may be from one open window to another (by-passing the partition), or along a duct through the enclosure.
Similarly, the sound transmission through flanking floors may be critical if the noise reduction required is more than about 45 db. A flanking wall may transmit the sound from one side of the partition to the other by vibration caused by the airborne sound on the noisy side. This is the main reason why the cavity needs to be continuous all round a double-leaf enclosure for noise reductions above 50-55 db.
Access openings will generally be weak points in an enclosure. Doors should be provided with (and, if possible, give) the same degree of sound reduction as the rest of the enclosure. This is particularly needed if the doors form a large part of the total wall area. If a high degree of noise reduction is required, double doors with a large airspace between them will be necessary.
Doors must be equipped with gaskets or some other means of providing an airtight seal when closed. Door-closers may be an advantage if the doors are used frequently. Permanent openings may be required for ventilation or for conveyors.
The noise transmitted through these openings can be reduced by adding a duct to the opening and lining the inside with a sound-absorbing material. To obtain an appreciable amount of noise reduction, the duct should incorporate several right- angle bends; otherwise, it needs to be very long.
(iv) Noise Source Covers:
Another form of enclosure is a cover for a noise source or a machine. An example of this is a casing for noisy gears (where the noise is radiated from the face of the gears). To contain the noise, the cover should be heavy and aright. The cover should, moreover, be coated with a layer of damping compound of equal weight to reduce the effects of resonance.
The junction of the cover and the machine should be sealed with a gasket Soft enough to stop vibration being transmitted to the cover. The fastenings also should be isolated, or they will “short-circuit” the gasket. In addition, some sound-absorbing material will be necessary inside the cover to prevent the build- up of sound energy.
Way # 5. Sound-Absorbing Materials:
(i) Reverberant Sound:
When a source of noise is enclosed in a room, the direct sound will decrease as the distance from the source increases (as is the case in open air). This reduction in the intensity of sound will continue up to the boundaries of the enclosure.
At the boundary surfaces, some of the sound will be reflected back, and back again indefinitely. In this way, the sound level continues to build up, if the source is continuous, to a total level appreciably higher than that of the original direct sound. The total intensity of sound at any point, therefore, will consist of a direct component and a reflected (or reverberant) component.
The build-up of reverberant sound within an enclosure can be controlled by the use of sound absorbing materials. Such materials reduce the amount of reflected sound by absorbing it. Sound-absorbing materials, however, do not reduce the direct component of the sound level, which is predominant near the source.
The reason is that the sound must reach the absorbing material before it can have any effect on the former. But as the distance between the source and the listener increases, the total sound level decreases to a point where the reflected component alone determines the noise level.
Fig. 1 shows the typical decrease in sound pressure level with increasing distance from the source for an untreated factory with normal hard inside surfaces, and for a similar factory of same size with a lining of sound-absorbing material at the ceiling.
(ii) Noise Reduction:
The reduction in noise level obtained by sound-absorbing treatment increases with the size of the enclosure, and with the area and sound-absorbing efficiency (i.e., sound absorption coefficient) of the absorbing material, as well as with the distance from the source. For any one constant noise source, the reflect sound level will always be lower in a large enclosure than in a small one.
The reason is that the acoustic energy is spread over a larger volume in the case of a large enclosure. Sound-absorbing material should be used inside an enclosure designed to reduce noise. If this is not done, the noise level inside the enclosure will be higher than before (due to reflected sound), and this will reduce the efficiency of the enclosure besides making the noise worse for its occupants.
In very large spaces where the smallest horizontal dimension is more than ten times the height, the total (i.e., direct plus reflected) sound level tends to decrease continuously as the distance from the source increases.
The reason is that in the case of such large spaces, the direct sound that reaches the wails is insignificant compared with that reflected between the floor and ceiling, with some sound being absorbed at each reflection, as it moves out from the source.
In this case, the reduction in sound level will obviously increase with the absorption coefficient of the reflecting surfaces. If the absorption coefficient of the ceiling or floor is close to unity, the reduction of sound level with distance will be almost equal to that in open air.
(iii) Noise Levels:
In a factory where most of the machines produce roughly equal amounts of noise, the average distance from the source (and, consequently, the average noise level) will be determined by the spacing of the machines.
The typical noise reduction achieved by lining the roof of a factory with a sound absorbing material is shown in Fig. 2 as a function of average machine spacing. With densely packed machinery in a factory, the reduction in a factory, the reduction in sound level due to the absorption is very small.
Consequently, sound absorption treatment does not attain its maximum effectiveness until machine spacing’s of more than 10 m are used. Although the reduction in measured sound level with close spacing of machinery is small, the subjective quality of the noise environment is improved by the acoustic treatment.
The reduction in the reflected noise level from distant machines makes the direct noise stand out, so that its source is more readily apparent. The noise in a factory with close machine spacing and sound absorbent treatment is, in fact, less confusing.
This will be an advantage if the machine operators have to be able to locate “information” sounds from their own machines. In addition, the increased directional characteristics of the noise environment will also improve speech communication a little.
We consider now the case of an impulsive noise source. In this case, the decay of the noise level (once the source has stopped) is directly proportional to the amount of sound absorption in the space concerned. If the noise emission from an impulsive source is not too frequent, an appreciable reduction in the average noise level can be obtained by sound-absorbing treatment.
The typical decay of the noise level for an impulsive emission of sound every second is shown in Fig. 3 for the treated and untreated factories. Since the average noise near the source is also lower, there is some relief for the machine operator in the treated factory, though the reduction in the peak noise levels is negligible.
(iv) Sound-Absorbing Materials:
It is a well-known fact that all building materials absorb sound to some extent. On the other hand, porous materials, and thin sheet materials mounted in panels over an airspace, are much more efficient as sound absorbers than heavy, hard-surfaced materials. Moreover, sound absorption characteristics of materials vary with the frequency of sound.
For example, porous materials absorb high-frequency sound more efficiently; but their low-frequency absorption can be improved by increasing the thickness of the material, or by mounting them over an airspace. The peak absorption of a panel occurs when the panel vibrates in resonance, and this condition generally occurs at a low frequency.
The absorption coefficients of some common sound-absorbing materials are given in Table 2. The absorption of a heavy, hard-surfaced building material (concrete) is also included in this table for comparison. It will be noticed from Table 2 that most of the porous sound- absorbing materials are also effective heat-insulating materials.
A specific degree of thermal insulation for the roof structure is required for certain factory buildings, and the material used for this purpose may also reduce the noise in the building at no extra cost, provided it is correctly installed.
(v) Sound-Absorption Treatment:
Obviously, the absorbing material must be exposed to the direct noise. It should not be shielded appreciably by projecting parts of the building or by other materials. Even a coat of paint, for example, will considerably reduce the sound-absorption efficiency of some porous materials.
The greatest benefit from the sound-absorbing treatment is realised when it is placed near the source, so as to intercept as much as possible of the direct sound before it has travelled far. If there are only a few noisy machines in the factory, placing the absorbing materials on screens or nearby surfaces will be the most effective solution.
If, on the other hand, the sources of noise are distributed throughout the factory, the ceiling is the most useful area to treat. The sound absorption is likely to be a little higher if the absorbing material is installed vertically in the roof space, as a series of baffles, than if it is applied as a roof lining.
But the latter type of application will certainly be necessary, if it is to provide thermal insulation as well. In an overall treatment, the walls should not be covered at the expense of ceiling unless the space is high and narrow, though local absorption on the walls will reduce the noise from nearby machines.
In existing factories, there may be no clear surface that can be lined with sound absorbing material; or its installation may greatly interfere with production. In such cases, the material can be hung from the roof members in sheets, or in hollow fabricated shapes (which give increased absorption for the same quantity of material).
Way # 6. Partial Enclosures and Screens:
When combined with sound-absorbing treatment, the shielding of a noise source by a partial enclosure is a very useful method of noise reduction in factories where complete enclosure cannot be achieved (because of large openings required for continuous access, or other reasons).
A small partial enclosure around a single machine is one of the few forms of noise control that protects the operator from the noise of his own machine. The machine is then operated through the opening of such a small, partial enclosure.
Tunnels, open-sided boxes, hoods, and combinations of screens are a few examples of partial enclosures. As a general rule, partial enclosures are specially designed for each situation; but they may also be made up of standard panels when such enclosures are not required for long periods.
As most of the noise that escapes is radiated from the opening of a partial enclosure, the noise reduction obtained depends on the degree of enclosure. Their shielding effect is mainly limited to the more directional high frequencies. With fairly complete enclosure (such as that provided by a tunnel), a reduction of about 20 dB above 500 Hz may be achieved.
Partial enclosures may also be used to create a quiet area; for example, a cover may be provided over a conveyor to form a tunnel, so that a listening test may be made on the product without removing it from the conveyor.
The enclosing structure should have a sound reduction factor equal to the noise reduction expected. Since this is limited to about 20 dB, almost any sheet material having the strength to stand up to industrial use will be adequate. The enclosure must be lined on all surfaces with an effective sound absorbing material to prevent the noise being reflected around the inside and escaping out of the opening.
Wherever it is possible, the opening should face a wall covered with sound-absorbing material, or should be baffled with a screen of the same construction as the enclosure. If the top of the enclosure is open the noise reduction will be increased by placing the sound-absorbing material on the ceiling overhead.
Way # 7. Ear Protection:
In some particularly noisy industries, the methods of noise reduction described in the preceding sections may not be adequate to reduce the noise to safe levels.
Even in factories where the noise has been reduced to acceptable levels for the majority of workers, some operators of noisy machines may still be exposed to noise levels that are much higher. In such cases, the noise in the ear must be reduced to the safe level by individual ear protection if hearing damage is to be prevented.
The two most common forms of commercially available ear protectors are earplugs and earmuffs. Earplugs are inexpensive, small, and inconspicuous when worn. Some workers find them uncomfortable at first, but soon become used to them, and can wear them for long periods.
Earplugs are available in various sizes, and must be correctly fitted to the ear to give a good seal. In some cases, a different size may be required for each ear.
Dry cotton wool is often used as a makeshift earplug; but, being porous, it does not provide an adequate seal, and the reduction of noise is slight. Waxes cotton wool, on the other hand, will give more reduction; but it still does not compare with a well- designed earplug.
Earmuffs fit over the ears. They are much heavier and more expensive than earplugs; but they give greater noise reduction. The earmuffs having a liquid-filled, ring-shaped cushion to form a seal between the head and the muff are very comfortable to wear even for long periods. The typical noise reduction obtained in the ear from well-designed earplugs and earmuffs is shown in Fig. 4.
In the case of extremely high levels of noise, it may be necessary to use a combination of earplugs and earmuffs. The total noise reduction, however, is not equal to the sum of their individual noise reductions.
The reason is that there is an upper limit to the amount of noise reduction that can be obtained locally at the ear. Beyond this, the noise is conducted directly by the bone structure of the head to the inner ear (by-passing the hearing protection).
We note here that the hearing of speech and warning signals in a noisy factory is not affected by ear protection, since both the speech level and the noise level are reduced in the same ratio. In fact, the speech communication may even be improved because the sound levels are reduced to a range in which the ear is more sensitive.
Ear protection, however, is not likely to be popular with factory workers unless they understand why it is necessary. An educational programme, therefore, on the nature of industrial hearing damage and the role of ear protectors is desirable when the hearing protection programme is introduced. Subsequently, there should be proper supervision to ensure that the ear protectors are worn regularly.