Development of Light-weight, Low-noise Exhaust Muffler Using a Laminated Structure for the Muffler Shell

For motorcycles, the exhaust system is one of the major contributors to the overall acoustic performance. Above all, the radiated noise from the muffler surface needs to be considered sufficiently for rider acoustical comfort. In many cases, countermeasure parts need to be applied to reduce the radiated noise, which, consequently, hinders weight and cost reduction. This paper reports our study on reducing the radiated noise from the muffler surface by applying to the muffler shell a laminated panel structure in which two steel sheets contact each other. We identified the major factors affecting noise radiation from the muffler surface and used the dynamic stiffness of the outer body surface as a physical quantity to evaluate the damping effect of the laminated steel plate. Based on the idea that the damping effect of laminated steel plates mainly depends on the friction force generated by fine relative displacement between the layers, the surface dynamic stiffness was evaluated with various thickness combinations of the square plate, and it was found that the surface dynamic stiffness was maximized when the thickness ratio was about 2:1. In light of this finding, we produced two prototypes of laminated shell mufflers with circular and non-circular cross-sections, and evaluated their noise reduction performance on actual motorcycles. The results showed that the radiated noise reduction performance of the new mufflers was equivalent to that of a conventional structure muffler. Using the laminated muffler shell of the proposed thickness ratio, we achieved 13% weight reduction in a newly developed muffler.