Effect of orifice shape on acoustic impedance

Abstract

The effect on normal incidence acoustic impedance of a non-circular orifice shape is examined relative to a circular orifice. The impedance of an adjustable porosity perforate, formed from two identical perforates sliding over each other, is measured. As the orifice shape becomes more noncircular, the measured impedance is found to deviate from the predicted results for a circular orifice of the same area. Several isolated orifices of the same open area but different shapes are tested and compared with a circular orifice. Both low incident sound pressure levels using broadband noise and high incident sound pressure levels using sinusoidal tones are used to evaluate the impedance performance of these isolated orifices. One orifice mimics the unique shape produced by the adjustable perforate and results in a smaller attached mass (or mass end correction) compared with a round orifice. This is consistent with the perforate impedance results. The unique orifice shape does not appear to have measurable differences in acoustic normal resistance at high incident sound pressure levels. However, since the attached mass plays a key role in determining the peak absorption frequency of resonant liners, the reduction in attached mass relative to a circular orifice has implications where these types of orifices are used.