Measurement of perforate insertion loss with bias flow

Perforated pipes are commonly used in mufflers with grazing flow; however, configurations involving bias flow have received less attention in the literature. In this study, perforated plates with varying hole diameters and open area ratios are positioned at the center of an expansion chamber. The acoustic performance of each plate is evaluated. Care is taken to minimize blower-induced flow noise, ensuring that insertion loss measurements are based on two upstream loudspeaker sources. Insertion loss is defined as the difference in outlet sound power without and with the perforated plate in place. Additionally, a flow-induced insertion loss is measured to quantify the acoustic contribution of the bias flow alone; this is defined as the difference in outlet sound power without and with flow, with the acoustic sources turned off. Results show that bias flow can enhance insertion loss, but excessive flow may introduce significant aeroacoustic noise at the perforation.