1.3 PRINCIPLES OF NOISE CONTROL

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There are three basic elements in any noise control system, as illustrated in

Fig. 1-1:

1. The source of the sound

2. The path through which the sound travels

3. The receiver of the sound (Faulkner, 1976).

In many situations, of course, there are several sources of sound, various

paths for the sound, and more than one receiver, but the basic principles of

noise control would be the same as for the more simple case. The objective

of most noise control programs is to reduce the noise at the receiver. This

may be accomplished by making modifications to the source, the path, or

the receiver, or to any combination of these elements.

The source of noise or undesirable sound is a vibrating surface, such as

a panel in an item of machinery, or small eddies with fluctuating velocities in

a fluid stream, such as the eddies in a jet stream leaving an air vent pipe.

The path for the sound may be the air between the source and receiver,

as is the case for machinery noise transmitted directly to the operator’s ears.

The path may also be indirect, such as sound being reflected by a wall to a

person in the room. Solid surfaces, such as piping between a vibrating pump

and another machine element, may also serve as the path for the noise

propagation. It is important that the acoustic engineer identify all possible

acoustic paths when considering a solution for a noise problem.

Introduction 7

FIGURE 1-1 Three components of a general noise system: source of noise, path of

the noise, and the receiver. The path may be direct from the source to the receiver, or

the path may be indirect.

Copyright © 2003 Marcel Dekker, Inc.

The receiver in the noise control system is usually the human ear,

although the receiver could be sensitive equipment that would suffer

impaired operation if exposed to excessively intense sound. It is important

that the acoustic designer specify the ‘‘failure mode’’ for the receiver in any

noise control project. The purpose of the noise control procedure may be to

prevent hearing loss for personnel, to allow effective face-to-face communication

or telephone conversation, or to reduce noise so that neighbors of

the facility will not become intensely annoyed with the sound emitted by the

plant. The engineering approach is often different in each of these cases.

1.3.1. Noise Control at the Source

Modifications at the source of sound are usually considered to be the best

solution for a noise control problem. Components of a machine may be

modified to effect a significant change in noise emission. For example, in a

machine used to manufacture paper bags, by replacing the impact blade

mechanism used to cut off the individual bags from the paper roll with a

rolling cutter blade, a severe noise problem was alleviated.

Noise at the source may indicate other problems, such as a need for

maintenance. For example, excessive noise from a roller bearing in a

machine may indicate wear failure in one of the rollers in the bearing.

Replacement of the defective bearing may solve the noise problem, in addition

to preventing further mechanical damage to the machine.

There may be areas, such as panel coverings, that vibrate excessively

on a machine. These panels are efficient sound radiators at wavelengths on

the order of the dimensions of the panel. The noise generated by large

vibrating panels can be reduced by applying damping material to the

panel surface or by uncoupling the panel from the vibrating force, if possible.

Making the panel stiffer by increasing the panel thickness or reducing

the panel dimensions or using stiffening ribs may also reduce the amplitude

of vibration. In most cases, reducing the amplitude of vibratory motion of

elements in a machine will reduce the noise generated by the machine element.

In some cases, using two units with the same combined capacity as one

larger unit may reduce the overall source noise. To determine whether this

approach is feasible, the engineer would need information about the relationship

of the machine capacity (power rating, flow rate capacity, etc.) and

the sound power level for the generated noise from the machine. This information

is presented in Chapter 5 for several noise sources.

A change in the process may also be used to reduce noise. Instead of

using an air jet to remove debris from a manufactured part, rotating clean-

8 Chapter 1

Copyright © 2003 Marcel Dekker, Inc.

ing brushes may be used. A centrifugal fan may replace a propellor-type fan

to reduce the fan noise.

1.3.2 Noise Control in the Transmission Path

Modifying the path through which the noise is propagated is often used

when modification of the noise source is not possible, not practical, or

not economically feasible. For noise sources located outdoors, one simple

approach for noise control would be to move the sound source farther away

from the receiver, i.e. make the noise path longer.

For noise sources located outdoors or indoors, the transmission path

may be modified by placing a wall or barrier between the source and receiver.

Reduction of traffic noise from vehicles on freeways passing near residential

areas and hospitals has been achieved by installation of acoustic

barriers along the roadway.

The use of a barrier will not be effective in noise reduction indoors

when the sound transmitted directly from the source to receiver is much less

significant than the sound transmitted indirectly to the receiver through

reflections on the room surfaces. For this case, the noise may be reduced

by applying acoustic absorbing materials on the walls of the room or by

placing additional acoustic absorbing surfaces in the room.

A very effective, although sometimes expensive, noise control procedure

is to enclose the sound source in an acoustic enclosure or enclose the

receiver in a personnel booth. The noise from metal cut-off saws has been

reduced to acceptable levels by enclosing the saw in an acoustically treated

box. Provision was made to introduce stock material to the saw through

openings in the enclosure without allowing a significant amount of noise to

be transmitted through the openings. If the equipment or process can be

remotely operated, a personnel booth is usually an effective solution in

reducing the workers’ noise exposure. An air-conditioned control booth is

also more comfortable for the operator of a paper machine than working in

the hot, humid area surrounding the wet end of the paper machine, for

example.

The exhaust noise from engines, fans, and turbines is often controlled

by using mufflers or silencers in the exhaust line for the device. The muffler

acts to reflect acoustic energy back to the noise source (the engine, for

example) or to dissipate the acoustic energy as it is transmitted through

the muffler.

1.3.3 Noise Control at the Receiver

The human ear is the usual ‘‘receiver’’ for noise, and there is a limited

amount of modification that can be done for the person’s ear. One possible

Introduction 9

Copyright © 2003 Marcel Dekker, Inc.

approach to limit the noise exposure of a worker to industrial noise is to

limit the time during which the person is exposed to high noise levels. As

discussed in Chapter 6, a person can be exposed to a sound level of 95dBA

for 4 hours during each working day, and encounter a risk of ‘‘only’’ 10%of

suffering significant permanent hearing loss, if the person remains in a much

more quiet area during the remainder of the day. The 95 dBA sound level is

typical of the noise fromprinting and cutting presses for folding cartons, for

example (Salmon et al., 1975).

Hearing protectors (earplugs or acoustic muffs) can be effective in

preventing noise-induced hearing loss in an industrial environment. In

some cases, the use of hearing protectors may be the only practical means

of limiting the workers’ noise exposure, as is the case for workers who

‘‘park’’ airplanes at large air terminals. Because of inherent problems with

hearing protectors, however, it is recommended that they should be used

only as a last resort after other techniques have been reviewed. For example,

the worker may not be able to hear warning horns or shouts of co-workers

when wearing earplugs. One can get accustomed to wearing hearing protectors,

but the earplugs are often less comfortable than wearing nothing at all.

This characteristic of earplugs and people introduces some difficulty in

enforcement of the use of hearing protection devices. In cases where earplugs

are the only feasible solution to a noise exposure problem, an education,

training, and monitoring program should be in place to encourage

strongly the proper and effective use of the protective devices.

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