Attenuation measurements inside and at the output of a passive silencer equipped with parallel absorbing baffles

The mock-up of a great passive silencer used for noise attenuation in industrial applications has been
designed and tested in laboratory. This mock-up consists of three metal casing containing the noise source
and several removable rails and supports, allowing the test of different configurations of parallel absorbing
baffles. The output of the silencer radiates in an anechoic chamber in order to simulate free-field conditions.
The acoustic attenuation has been measured not only at the output, but also inside the silencer, with a mobile
microphone located at several positions along the axis. Also, the tested configurations include three types
of absorbing pillows and several geometrical arrangements. The results show that the maximum insertion
loss (dB) measured at the output of the silencer corresponds to frequencies between 800Hz and 2.5kHz and
its value strongly depends on the air gaps between baffles. The measurements with the mobile microphone
show a linear decrease of the sound pressure level with the distance along the axis (mainly between 100Hz
and 1kHz), which corresponds to the depth of absorbing material involved in the attenuation. Finally, a
comparison with a real-scale silencer shows a fairly good correspondence.