Measurements of orifice impedance with grazing flow

Perforated interfaces are commonly used in silencers for automotive applications. In these silencers, the acoustic behavior of the perforations, typically described in terms of a perforate impedance, can have a significant effect on silencer performance. The perforate impedance, in turn, is affected by the mean flow in the silencer. The focus of the present study is mean flow tangential to the perforate face (grazing flow), which is often predominant. In most cases, the perforate impedance cannot be determined analytically, and therefore experimental data and empirical models are necessary. Yet, experimental data suitable for use in models of perforated tube silencers are relatively scarce. The objective of the present investigation is to measure the impedance of perforated plates subjected to grazing flow. A description is given of the experimental apparatus, which has been designed for measurements with flow conditions representative of those in an automotive system. Data are presented for plates with square-edged circular orifices of varied dimensions. The results are reduced in terms of scaling parameters suitable for use in empirical perforate models.