Insertion Loss of Clamped Perforated and Non-Perforated Poroelastic Plate Silencers with Flow

Abstract

Idealized silencers consisting of cavity-backed clamped porous elastic plates mounted on fairings parallel to the direction of flow can result in low frequency attenuation of noise generated in a duct. The insertion loss (IL) of such silencers has been measured without and with perforations in the plates, without and with mean air flow and without and with noise generated by loudspeakers placed downstream in the flow duct system. Whether the plate is perforated or not but without noise generated by loudspeakers in the duct, the IL due to the poroelastic plate silencer is found to increase in the presence of mean air flow at speeds of 37.65 m/s and 5.5 m/s. With noise from the loudspeakers and a mean air flow of 37.65 m/s, the IL when the plate is perforated is higher than that achieved with the non-perforated plate at frequencies less than 100 Hz. To investigate the extent to which there may be structure-borne components of the plate deflection, the displacement of the duct wall has been measured and plotted versus frequency. The displacement of the non-perforated plate is found to be higher than the displacement of the duct wall. However the displacement of the perforated plate is less than that of the duct wall which suggests that low frequency improvement in the performance in the presence of mean air flow and perforations is a consequence of the aeroacoustic effects of the perforations rather than increased plate deflection.