Expansion Chamber

The expansion chamber can be considered as a simple low pass filter. The transmission loss performance is determined by the cross-sectional area ratio between the inlet and outlet ducts and the volume. With the single expansion chamber the transmission loss falls to zero when a half wavelength or multiple thereof equates to the length of the chamber.

Extending the inlet and outlet pipes into the volume will give a more abrupt impedance change and hence a larger transmission loss.


The transmission loss is given by:

where
f = frequency [Hz]
fn = resonant frequency [Hz]
c = speed of sound [ms-1]
S = cross-sectional area of inlet and outlet ducts [m2]
Sc = cross-sectional area of expansion chamber [m2]
Lc = length of expansion chamber [m]
dB

Frequency (Hz)

Reference

P. O. A. L. Davies, "Plane wave reflection at flow intakes", Journal of Sound and Vibration, Volume 115, Issue 3, 22 June 1987, Pages 560-564

P.O.A.L. Davies, "Practical flow duct acoustics", Journal of Sound and Vibration, Volume 124, Issue 1, 8 July 1988, Pages 91-115

See also: Acoustic Duct End Correction, Acoustic Filter Elements, Helmholtz Resonator, Quarter Wave Tube.

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Subjects: Noise & Vibration