For the solution of noise problems, it is very convenient to view the system in terms of a source, a path and a receiver, as shown in Fig. 3 in the form of a block diagram.
The source is the point (or points) in space where the noise apparently originates. The path is the line (or lines) in air along which the noise can be considered to propagate to the receiver (or the ear). Of course, more complicated situations would have multiple sources, paths and receivers with multiple interaction among them; yet the complexity is one of degree only and the approaches to solution are the same.
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Acoustic sources may be described in terms of fluctuating force acting on a fluid medium. The origin of these fluctuating forces could be surface motion of solid objects or fluctuating fluid properties such as turbulent flow.
Energy from time-dependent forces flows along a direct path or by one or more indirect paths. The objective of noise control is to reduce the noise at the receiver. This may be accomplished by modifications of either the source, the path, or the receiver or of all three.
The design engineer has the task of specifying the mechanical construction of the noise control system to limit the levels of noise at the receiving point to acceptable values.
In general, modifications at the source are considered to be the best solution of noise-control problems. The reason is that noise-producing energy is concentrated at the source. Consequently, if the cause can be sufficiently reduced or eliminated, one need not worry about relatively cumber and often more expensive treatments of the path and receiver.
However, there are many instances where source modifications are not practicable or even possible.
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Controlling the noise along the path involves some kind of modification of the space enclosing both source and receiver. For the direct path, a barrier constructed of impervious material is necessary to reflect the noise back towards the source.
For indirect paths, it is necessary to use an absorptive material on the reflecting surfaces to absorb the sound energy. The fundamental differences in these two concepts and the materials used in implementing them cannot be overemphasized.
Some of the common techniques for control of noise in the transmission path are:
(a) Construction of enclosures;
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(b) Use of mufflers;
(c) Vibration isolation;
(d) Breaking mechanical paths;
(e) Lengthening of transmission paths;
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(f) Absorption of sound energy with acoustic treatments; and
(g) Construction of heavy, air-tight enclosures.
In some application , more than one of these techniques must be applied. In cases of extremely noisy equipment, for example, it may be necessary to apply all of them. The usual receiver of sound is the human ear and, unfortunately , there is nothing that can be done to alter response to noise.
As a results, if all other efforts have failed to lower the noise levels at locations humans beings must occupy , there are only a few resources remaining. If noise levels at locations human beings must occupy, there are a few resources remaining.
If noise levels are excessive, one solution is to remove human workers from the area and use remote control devices to operate noisy equipment.
Another method of receiver control is the use of an operator booth with viewing windows. (An operator booth is a three or four-sides barrier or enclosure in which the operator stays when the operation of his machine requires monitoring only). Operator booth is a means for reducing the noise exposure time of the operator without need for wearing personal hearing protectors.
Controlling noise directly at the ear by use of ear plugs or ear muffs can be very effective in most industrial environments; in some environments, it may well be the only practical solution. Nevertheless, there are potential problems in their effective use.
The main reasons for objections to the use of ear protectors are:
(a) Masking of warning signals;
(b) Hygienic reasons (in some environments); and
(c) Difficulties in enforcement of their use.
When ear protectors must be used, an educational programme should also be instituted to make sure that people will use the protection devices.
Machinery Noise Control Methods:
The techniques for noise control were presented in a general way and their specific applications to various machinery were not mentioned. The most desirable noise-control technique, of course, is to alter the source by redesigning the mechanical elements to reduce forces, or by altering the sound-radiating surfaces.
Based on case histories of industrial noise control, recommended machinery noise treatments (excluding modification of the source) are listed in Table 3.
