Noise reduction via three types of acoustic back holes

In the last decades, several types of ABHs, i.e. circular, annular, and rectangular ABHs, have been used for vibration attenuation. However, only the circular ABH has been studied for sound radiation. In this paper, we have thoroughly compared the radiation damage of the three types of ABHs and presented the reduction mechanism. Both the sound radiation into free space and into a cavity has been studied. The former is achieved by developing the impedance matrix method, together with the supersonic intensity to unveil the mechanism of sound radiation reduction. The latter is managed by the mid-high frequency statistical modal energy distribution analysis (SmEdA) method to fast predict the response of the plate–cavity system. Both methods are based on the Gaussian expansion method (GEM). Results show that for free space the radiation efficiency loss is caused not only by the damping effect but also by the transonic behavior of the flexural wave in the ABH region, while for cavity noise, the reduction of coupling strength, in addition to the former two reasons, is also responsible for noise reduction. By comparison it is found that the annular ABH and the rectangular one have better radiation reduction performances than the traditional circular one.