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 spaceconstrained 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 AโผF) 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 onechamber 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 demonstrated.Keywords: multiple tones, hybrid, Helmholtz, four-pole transfer matrix method, SA method. p -acoustic pressure (Pa), p% -the porosity of the perforated tube in the dissipative element, Q -volume flow rate of venting gas (m 3 s โ1 ), r k -the radius of the resonating tube in the Helmholtz resonator (m), Rres -the acoustic resistance, SA -simulated annealing, S i -the section area at the i-th node (m 2 ), Archives of Acoustics -Volume 37, Number 4, 2012 S k -the section area of the resonating tube in the Helmholtz resonator (m 2 ), STL -sound transmission loss (dB), SWL T -overall sound power level inside the muffler's output (dB), T * ij -components of a four-pole transfer system matrix, TS ij , TSC ij , TSE ij , THR ij -components of four-pole transfer matrices for an acoustical mechanism with a s...