Acoustic Attenuation in Exhaust Muffler with the use of Granular Material

IIn the present electronic document, a characterization of the granular material's equivalent properties of density and velocity for a dissipative type of exhaust muffler system and a simulation to evaluate the response of Finite Element (FE) noise levels have been evaluated been developed. In the initial stage, we conduct experimental measurements within an acoustic testing facility at the Department of Mechanical Engineering, Polytechnic University of Valencia. The employed methodology involves using a transfer matrix derived from four microphone recordings. During the second stage, theoretical models consider comparable acoustic characteristics, such as the density and velocity of propagation of the granular material, which vary depending on the frequency. Ultimately, a commercially available Finite Element (FE) software is utilized to analyze the sound reduction capabilities of silencers incorporating granular material. The software's findings are compared with experimental measurements, providing additional insights to enhance the study. Comsol's methodology and experimental tests are validated based on the comparison of several measurements and simulations of individual expansion chambers of square, circular, and triangular shapes, which provide different sizes of spheres and filling levels inside. As a result, a sphere size below 4 mm in diameter favors sound attenuation, and as the sphere size increases, the attenuation is noticeably reduced. A 0.5 m length of the chamber and a circular cross-section would provide higher attenuation levels close to 30 dB.