Broadband duct noise attenuation by a middle-ear mechanism

Abstract

Low‐frequency duct noise is difficult to control passively by any device limited by its total volume due to system stiffness. When a side‐branch device is used cavity air becomes incompressible toward the dc frequency. One way to tackle the relative incompressibility for low to medium frequencies is to introduce acoustic phase difference in the cavity air oscillation which otherwise behaves like the low‐order room acoustics modes. The introduction of a light plate covering the cavity in our previous studies [J. Acoust. Soc. Am. 119 2628–2638 (2006)] does essentially that by enhancing the non‐volume‐displacing modes. Optimization shows that the required stiffness‐to‐mass ratio is much beyond what is available for existing bulk materials. In the present study an alternative device that resembles the ossicular structure of the mammalian middle ear is introduced. The acoustic signal is passed along through a rigid pin which simulates the ossicular bones via its longitudinal motion. Contrasting with the usual flexural waves in plate or membrane structures the bone transmission has almost infinite speed and it creates the phase difference needed to overcome the system stiffness. Theoretical predictions show that the performance of such device is comparable and better than the idealistic plate silencer with interesting comparisons of features. © 2008 Acoustical Society of America