1.1 NOISE CONTROL

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Concern about problems of noise in the workplace and in the living space

has escalated since the amendment of the Walsh–Healy Act of 1969. This act

created the first set of nationwide occupational noise regulations

(Occupational Safety and Health Administration, 1983). There is a real

danger of permanent hearing loss when a person is exposed to noise

above a certain level. Most industries are strongly motivated to find an

effective, economical solution to this problem.

The noise level near airports has become serious enough for some

people to move out of residential areas near airports. These areas were

considered pleasant living areas before the airport was constructed, but

environmental noise has changed this perception. The airport noise in the

areas surrounding the airport is generally not dangerous to a person’s

health, but the noise may be unpleasant and annoying.

In the design of many appliances, such as dishwashers, the designer

must be concerned about the noise generated by the appliance in operation;

otherwise, prospective customers may decide to purchase other quieter models.

It is important that noise control be addressed in the design stage for

many mechanical devices.

Copyright © 2003 Marcel Dekker, Inc.

Lack of proper acoustic treatment in offices, apartments, and classrooms

may interfere with the effective functioning of the people in the

rooms. Even though the noise is not dangerous and not particularly annoying,

if the person cannot communicate effectively, then the noise is undesirable.

Much can be done to reduce the seriousness of noise problems. It is

often not as simple as turning down the volume on the teenager’s stereo set,

however. Effective silencers (mufflers) are available for trucks and automobiles,

but there are other significant sources of noise, such as tire noise and

wind noise, that are not affected by the installation of a silencer. Household

appliances and other machines may be made quieter by proper treatment of

vibrating surfaces, use of adequately sized piping and smoother channels for

water flow, and including vibration isolation mounts. Obviously, the noise

treatment must not interfere with the operation of the applicance or

machine. This stipulation places limitations on the noise control procedure

that can be used.

In many instances, the quieter product can function as well as the

noisier product, and the cost of reducing the potential noise during the

design stage may be minor. Even if the reduction of noise is somewhat

expensive, it is important to reduce the level of noise to an acceptable

value. There are more than 1000 local ordinances that limit the community

noise from industrial installations, and there are legal liabilities associated

with hearing loss of workers in industry.

The designer can no longer ignore noise when designing an industrial

plant, an electrical generating system, or a commercial complex. In this

book, we will consider some of the techniques that may be used by the

engineer in reduction of noise from existing equipment and in design of a

quieter product, in the case of new equipment.

We will begin with an introduction to the basic concepts of acoustics

and acoustic measurement. It is important for the engineer to understand

the nomenclature and physical principles involved in sound transmission in

order to suggest a rational procedure for noise reduction.

We will examine methods for predicting the noise generated by several

common engineering systems, such as fans, motors, compressors, and cooling

towers. This information is required in the design stage of any noise

control project. Information about the characteristics of the noise source

can allow the design of equipment that is quieter in operation through

adjustment of the machine speed or some other parameter.

How quiet should the machine be? This question may be answered by

consideration of some of the design criteria for noise, including the OSHA,

EPA, and HUD regulations, for example. We will also consider some of the

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Copyright © 2003 Marcel Dekker, Inc.

criteria for noise transmitted outdoors and indoors, so that the anticipated

community response to the noise may be evaluated.

A study of the noise control techniques applicable to rooms will be

made. These procedures include the use of acoustic treatment of the walls of

the room and the use of barriers and enclosures. It is important to determine

if acoustic treatment of the walls will be effective or if the offending noise

source must be enclosed to reduce the noise to an acceptable level.

The acoustic design principles for silencers or mufflers will be outlined.

Specific design techniques for several muffler types will be presented.

Some noise problems are associated with excessive vibration of portions

of the machine or transmission of machine vibration to the supporting

structure. We will consider some of the techniques for vibration isolation to

reduce noise radiated from machinery. The application of commercially

available vibration isolators will be discussed.

Finally, several case studies will be presented in which the noise control

principles are applied to specific pieces of equipment. The noise reduction

achieved by the treatment will be presented, along with any pitfalls or

caveats associated with the noise control procedure.

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