Two terms, insertion loss, IL, and transmission loss, TL, are commonly used to describe the
effectiveness of a muffling system. The insertion loss of a muffler is defined as the reduction (in
decibels) in sound power transmitted through a duct compared to that transmitted with no
muffler in place. Provided that the duct outlet remains at a fixed point in space, the insertion
loss will be equal to the noise reduction which would be expected at a reference point external
to the duct outlet as a result of installing the muffler. The transmission loss of a muffler, on the
other hand, is defined as the difference (in decibels) between the sound power incident at the
entry to the muffler to that transmitted by the muffler.
Muffling devices make use of one or the other or a combination of the two effects in their
design. Either sound propagation may be prevented (or strongly reduced) by reflection
(generally as the result of using orifices and expansion chambers), or sound may be dissipated,
generally by the use of sound absorbing material. Muffling devices based upon reflection are
called reactive devices and those based upon dissipation are called dissipative devices. A duct
lined with sound absorbing material on its walls is one form of dissipative muffler