Noise control is essential in an enclosed machine room where the noise
level has to comply with the occupational safety and health act. In
order to overcome a pure tone noise with a high peak value that is
harmful to human hearing, a traditional reactive muffler has been used.
However, the traditional method for designing a reactive muffler has
proven to be time-consuming and insufficient. In order to efficiently
reduce the peak noise level, interest in shape optimization of a
Helmholtz muffler is coming to the forefront.
Helmholtz mufflers that deal with a pure tone have been adequately
researched. However, the shape optimization of multi-chamber Helmholtz
mufflers that deal with a broadband noise hybridized with multiple tones
within a constrained space has been mostly ignored. Therefore, this
study analyzes the sound transmission loss (STL) and the best optimized
design for a hybrid Helmholtz muffler under a space- constrained
situation. On the basis of the plane wave theory, the four-pole system
matrix used to evaluate the acoustic performance of a multi-tone hybrid
Helmholtz muffler is presented. Two numerical cases for eliminating
one/two tone noises emitted from a machine room using six kinds of
mufflers (muffler A~F) is also introduced. To find the best acoustical
performance of a space-constrained muffler, a numerical assessment using
a simulated annealing (SA) method is adopted. Before the SA operation
can be carried out, the accuracy of the mathematical model has been
checked using the experimental data. Eliminating a broadband noise
hybridized with a pure tone (130 Hz) in Case I reveals that muffler C
composed of a one- chamber Helmholtz Resonator and a one-chamber
dissipative element has a noise reduction of 54.9 (dB). Moreover, as
indicated in Case II, muffler F, a two-chamber Helmholtz Resonator and a
one-chamber dissipative element, has a noise reduction of 69.7 (dB).
Obviously, the peak values of the pure tones in Case I and Case II are
efficiently reduced after the muffler is added.
Consequently, a successful approach in eliminating a broadband noise
hybridized with multiple tones using optimally shaped hybrid Helmholtz
mufflers and a simulated annealing method within a constrained space is