Acoustic behavior of orifice plates and perforated plates with reference to low-frequency sound absorption☆

Abstract Analyses of impulse time history data from acoustic transmission tests for conical nozzles attached to a pipe show that the internal reflections from the solid contraction and the open area tend to cancel. To gain understanding of the opposing reflections, tests were conducted by replacing the conical nozzles with orifice plates and perforated plates. The primary variable was the open-to-solid area ratio. Internal reflection coefficient data reveal that, for an area ratio of 10–12%, the low-frequency internal reflection is reduced from unity to about 0·2. On the basis of comparisons of far field and internal data, acoustic energy is not conserved. Results are presented for a complex reflection coefficient and the far field noise for a series of orifice and perforated plate configurations.