8 Silencer Design

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A silencer is an important noise control element for reduction of machinery

exhaust noise, fan noise, and other noise sources involving flow of a gas. In

general, a silencer may be defined as an element in the flow duct that acts to

reduce the sound transmitted along the duct while allowing free flow of the

gas through the flow passage. A silencer may be passive, in which the sound

is attenuated by reflection and absorption of the acoustic energy within the

element. An active silencer is one in which the noise is canceled by electronic

feedforward and feedback techniques. In this chapter, we will examine

several types of passive silencers, also called mufflers. The detailed design

procedures for mufflers are available in the literature (Munjal, 1987).

Passive silencers may be of the reactive or dissipative type. In this

chapter, we will consider two types of reactive mufflers—the side branch

muffler or resonator chamber muffler and the expansion chamber muffler—

in which the main mechanism for attenuation of sound passing

through the muffler is reflection of the acoustic energy back to the source.

The other passive muffler considered in this chapter is the dissipative muffler,

in which the primary mechanism for acoustic energy attenuation in the

muffler is absorption of acoustic energy within the lining of the muffler.

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