Akustische Impedanz einer luftdurchströmten Öffnung

The reflection coefficient and the acoustic impedance of tube orifices with flow discharge are measured by the method of the impedance tube. Flow velocities up to 150 m/s are used. With a flow discharge the reflection coefficient of the orifice is strongly increased, the orifice becoming more pressure release, whereas with suction the reflection coefficient decreases. The real part of the orifice impedance even becomes negative at low frequencies. The effects of the flow are reduced by grids and screens inserted into the orifice. At the low frequency limit the static pressure drop acts like an increase of the resistance part of the impedance.