Far Field Acoustic Radiation From an Expansion Chamber

A method is presented for the assessment of the effect of several system parameters on the radiated and transmitted acoustic fields of an expansion chamber with a flexible side wall. The approach is based on a previously developed model of a three dimensional cavity in an otherwise one dimensional acoustic transmission line. The acoustic field within the cavity is represented in terms of the cavity acoustic normal modes. The flexible side wall is represented in terms of its natural modes of free vibration. Computations are made and results presented to assess the effect on the acoustic output of parameters such as side wall thickness, side wall stiffness, inlet pipe length, and inlet and outlet pipe placement. It is concluded that radiation from the side wall can occur at significant levels at certain plate resonances and at cavity internal resonances. Reduction of radiation is achieved by thickening of the side wall and by adding appropriate stiffeners. Success in reduction of radiated noise requires an understanding of the entire coupled system.