Broadband acoustic liner based on the mechanism of multiple cavity resonance

In this paper the mechanism of multiple cavity resonance is proposed to broaden the absorption bandwidth of an acoustic liner. The basic idea is that, when the back cavity of the liner is divided into sub-cavities of different size, many resonances relating to different lengths are produced so that the peaks of sound absorption become more closely distributed and take effect jointly to afford high attenuation over a large bandwidth. To study this mechanism, a cavity-resonance theory is first established to describe the coupling effect of the acoustic modes inside the sub-cavities and arrives at a general formula for the multiple resonance frequencies. Furthermore, a more rigorous propagation model is developed that is capable of predicting the effect of a multi-resonance acoustic liner on the sound attenuation in a duct. A verification experiment is also carried out in the situation of a rectangular duct without mean flow. When compared with the experiment, the cavity-resonance theory well predicts the resonance frequencies, while the propagation model also gives reasonably good prediction for the magnitude of the transmission loss. Both the theoretical and experimental results demonstrate the potential of the present method in improving the broadband performance of an acoustic liner.