On low frequency sound propagation across closely coupled narrow cavities along an infinite duct and the similarity in stopband cut-on frequencies

Abstract

The propagation of sound across closely coupled narrow rectangular cavities in an infinitely long duct is studied numerically using finite-element simulation and the method of mode matching in the present investigation. Sound frequencies below the first cut-off frequency of the main duct are the focuses. Results show that the middle region of the coupled cavity section plays the most important role in affecting the spectral characteristics of sound transmission across the coupled cavities. It is also found that there exists a critical frequency above which a coupled cavity section can offer reasonably good sound transmission loss. In addition, similarity in this stopband cut-on frequency across coupled cavity sections of different dimensions is established empirically after the introduction of a new length scale. The corresponding frequencies of the symmetrical non-offset coupled cavity sections are also a vital parameter in the scaling. Two frameworks are developed to predict the stopband cut-on frequencies. Both of them are able to give predictions with accuracy to within engineering tolerance.