There are three basic approaches to controlling noise:
Containment reduces the transmission of sound waves by confining them. Almost any material can help contain sound; the denser it is, the better it works. Many economy mufflers silence exhaust noise by containing the flow of exhaust gases. This is typically not desirable. When exhaust gases are contained, the backpressure increases and vehicle power, performance and fuel economy suffer.
Absorptive devices convert sound energy to heat energy. Sound waves collide with material that flexes and vibrates easily. The material absorbs the sound rather than passing it on as resonance, or bouncing it back toward the source. Sound is reduced by a loss of energy when the absorptive material is moved. This device is more effective on high frequencies. Glasspack-type or glass-filled mufflers are an example of sound reduction by absorptive control.
Reactive devices reduce, change, or eliminate noise by reflecting sound waves back toward the source. One common application of a reactive sound device is the louvered tubes found on the inside of most mufflers. The main purpose of these tubes is to steer sound waves against each other and to use their energy to cancel each other out.
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