A theoretical and experimental investigation of sound intensity distribution within a splitter silencer

Results are presented of an investigation into the sound intensity distribution in a ventilation duct splitter silencer subject to plane waves incident in the axial direction, under conditions of zero flow. A theory is developed which accounts for the generation of nonplane fields at the cross-sectional discontinuities of the duct-silencer-duct system. A local reaction model of the splitter material is employed, and analytical solutions and numerical results are presented for the sound intensity distribution within the silencer. Experiments were conducted to measure the distribution of sound intensity in a silencer subject to incident plane waves. The conventional cross-spectral method of measuring sound intensity could not be used because of the practical difficulties in manoeuvring the two-microphone assembly through the narrow air passage and obtaining measurements at positions close to the wall of the splitters. Instead, a modified cross-spectral method was devised which employs only one microphone for measurement. In this method, the transfer functions between the loudspeaker input signal and the microphone output signal were measured at an array of microphone positions within the air passages of the silencer. The divergence equation for sound intensity was employed to infer transverse intensity distributions from the computed axial distribution. Reasonable qualitative agreement is obtained between theoretical and observed sound intensity distribution, but the local reaction model of the splitter is found not to be satisfactory at the lower frequencies.