Experimental study of structure-borne sound transmission loss of mechanical joints

A mechanical joint is one of the most effective ways to reduce the transmission of structure-borne sound. In order to increase the transmission loss, heavily damped joints are often used, which, in many cases, will reduce the structure integrity and hence can only be used in limited cases. In this study attention is focused on a type of resonant joint, i.e., a joint which will increase the transmission loss but will not reduce the structure integrity. The study is based on experiments in a one-dimensional structure. It is found that by adjusting the overlap of the joint, the transmission loss of 30 dB can be obtained at a certain frequency range without adding any dissipative materials. The mechanism of this high transmission loss is the cantilever-type resonance. The resonant frequency can be predicted precisely. The influence of extra dissipative material is investigated. The performance of the same joint in a finite structure is also examined by using the concept of vibrational insertion loss. When there is a certain damping in a finite system, a rather high insertion loss can still be achieved by using the above-mentioned joint, but the resonant frequency is shifted to higher end. It seems that the effective length of the cantilever is shortened by the finiteness.