8.1 SILENCER DESIGN REQUIREMENTS

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The optimum design of a silencer involves several requirements, some of

which may be in conflict with others; consequently, the muffler design will

Copyright © 2003 Marcel Dekker, Inc.

involve consideration of the interactions of the various design criteria. The

design requirements are now considered.

1. Acoustic requirements. The minimum reduction of the noise by

the muffler is usually specified as a function of frequency, either

in octave bands or in 1/3 octave bands. The most frequently used

acoustic performance parameters include (a) the insertion loss,

IL, which is the difference in sound pressure level for the surroundings

due to the insertion of the silencer into the system; (b)

the noise reduction, NR, which is the difference in sound pressure

level between the point immediately upstream and the point

immediately downstream of the muffler; and (c) the transmission

loss, TL, which is the change in sound power level across the

muffler, if there were no energy reflected back to the muffler in

the tail pipe. The insertion loss and the noise reduction usually

depend on the characteristics of the tail pipe, in addition to the

muffler parameters. The transmission loss usually depends only

on the characteristics of the muffler.

2. Aerodynamic requirements. The maximum allowable pressure

drop of the gas flowing through the muffler is usually determined

by the application. The pressure drop for air-handling systems is

usually limited to a few inches of water: 4 in H2O or 1 kPa or less,

for example (McQuiston and Parker, 1994). On the other hand,

internal combustion engines may operate with back pressures as

high as 25–30 kPa (3.6–4.4 psi) (Heywood, 1988).

3. Geometric requirements. In many cases, such as in automotive

applications, there are limitations on the physical size and the

shape of the muffler. This requirement often interacts with the

acoustic requirements.

4. Mechanical and material requirements. Although the internal

gauge pressure within most mufflers is relatively small, the

mechanical design of the muffler must be considered. In applications

involving high-temperature gases or corrosive gases, the

materials selected for the muffler must be compatible with the

fluid handled. If there are suspended particles (soot, for example)

in the gases, the mechanical design must be such that these particles

are not easily trapped within the muffler. This requirement

may interact with the geometric requirements.

5. Economic requirements. Although this requirement is listed last, it

is often the most critical one. The muffler must be designed for

minimum cost, subject to the constraints of the other require-

Silencer Design 331

Copyright © 2003 Marcel Dekker, Inc.

ments. Economic considerations include both the initial (purchase)

cost and the operating (maintenance) costs.

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