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nnoise control

The control of noise is possible at three different stages of its transmission:

1. Reducing the sound produced,

2. Interrupting the path of the sound, and

3. Protecting the recipient.

When we consider noise control in industry, in the community, or in the home, we should keep in mind that all problems have these three possible solutions.

 Industrial Noise Control

Industrial noise control generally involves the replacement of noiseproducing machinery or equipment with quieter alternatives. For example, the noise from an air fan may

be reduced by increasing the number of blades or the pitch of the blades and decreasing the rotational speed, thus obtaining the same air flow. Industrial noise may also be

decreased by interrupting the path of the noise; for example, a noisy motor may be covered with insulating material.

A method of noise control often used in industry is protection of the recipient by distributing hearing protection devices (HPDs). These HPDs must be selected to have enough noise attenuation to protect against the anticipated exposures to noise. Great care must be taken to avoid interference with the ability to hear human speech and warning signals in the workplace.

Community Noise Control

The three major sources of community noise are aircraft, highway traffic, and construction. Construction noise must be controlled by local ordinances (unless federal funds

are involved). Control usually involves the muffling of air compressors, jackhammers, hand compactors, etc. Since mufflers cost money, contractors will not take it upon

themselves to control noise, and outside pressures must be exerted. Regulating aircraft noise in the United States is the responsibility of the Federal Aviation Administration, which has instituted a two-pronged attack on this problem.

First, it has set limits on aircraft engine noise and will not allow aircraft exceeding these

limits to use the airports, forcing manufacturers to design engines for quiet operation as well as for thrust. The second effort has been to divert flight paths away from populated areas and, whenever necessary, to have pilots use less than maximum power when the takeoff carries them over a noise-sensitive area. Often this approach is not enough to prevent significant noise-induced damage or annoyance, and aircraft noise remains a real problem in urban areas.

Supersonic aircraft present a special problem. Not only are their engines noisy, but the sonic boom may produce considerable property damage. Damage from supersonic

military flights over the United States has led to a ban on such flights by commercial supersonic aircraft.

The third major source of community noise is traffic. The car or truck creates noise in a number of ways. The exhaust system, tires, engine, gears, and transmission all contribute to a noise level, while the very act of moving through the atmosphere creates wind noise. Elevated highways and bridges resonate with the traffic motion and amplify traffic noise. The worst offender on the highways is the heavy truck, which generates noise in all of these ways. The total noise generated by vehicles may be correlated directly to the truck traffic volume. Figure 22-15 is a typical plot showing sound level as a function of traffic volume (measured in number of trucks per hour). Clearly, truck volume is of great importance. It is important to note that this graph is plotted as “sound

level exceeded 10% of the time.” Peak sound levels could be a great deal higher.

A number of alternatives are available for reducing highway noise. First, the source could be controlled by making quieter vehicles; second, highways could be routed away from populated areas; and third, noise could be baffled with walls or other types of barriers.

Vegetation, surprisingly, makes a very poor noise screen, unless the screen is 50 yards or more deep. The opposing lanes of the