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. 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 will analyze the sound transmission loss (STL) and the best optimized design for a hybrid Helmholtz muffler under a space-constrained situation. The acoustical elements of a hybrid Helmhholtz muffler include the acoustical elements of a straight tube, an internally contract/expanded extended tube, a perforated chamber filled with sound absorbing wool, a sudden contraction/expansion element, a perforated plug-tube, and a Helmhholtz...