Sound transmission loss of expansion chambers with staggered upper and lower chamber

The sound transmission losses across expansion chambers with the upper and lower chambers offset (staggered) inside a duct were studied experimentally in the present investigation. The sound transmission losses were measured using the four microphone method (one pair upstream and one pair downstream of the chambers). Pressure transducers were used to record the pressure fluctuations within the chambers. An anechoic termination was included in order to minimize the acoustic reflection at the exit of the test rig. Compared to the conventional expansion chamber, the staggered chamber setting results in a higher sound transmission loss at higher frequencies (below the first duct cutoff) and such increase in the sound transmission loss increases with the degree of chamber offset. Such rise in the sound transmission loss is abrupt showing that there exists a critical frequency at which the change is excited. The increase in the sound transmission loss is also relatively broadband, though it drops as the forcing frequency is further increased. The critical frequency appears weakly dependent on the degree of offset. On the low frequency side, the increase in the chamber offset does not result in significant change in the sound transmission loss, except that a larger fluctuation of the latter can be observed at increased offset.