Perforated walls are encountered in many acoustic dampers. For most of these perforations, the main cause of damping is the interaction of flow with the acoustic field at the perforation. We discuss the effect of changing the angle that the perforation makes with the wall on the acoustical performance. Experimental data are acquired by impedance tube measurements and a multi-microphone method Hereby we obtain the acoustic response of the perforation and flow to excitation by an external sound source. The accuracy in the measured normal incidence reflection coefficient is of the order of 1% in the frequency range 50Hz-800Hz. The perforations have a slit-shaped cross-section (10mmx50mm). The plate thickness is 15mm. We consider a flow that is either purely grazing along the perforation or a combination of grazing flow and flow through the perforation. Velocities below 20m/s are considered. We concluded that perforations for which the upstream edge (with respect to the grazing flow) makes an acute angle of 30degrees with the wall, have an increased sound absorption at low Strouhal numbers based on the perforation width in flow direction, compared to perforations with a right or an obtuse angle. We explain this qualitatively using the vortex sound theory.
|Title of host publication||Proceedings of the Acoustics 2012 Nantes Conference|
|Subtitle of host publication||23-27 April 2012, Nantes, France|
|Publication status||Published - 2012|
|Event||Acoustics 2012 Nantes Conference - Nantes, France|
Duration: 23 Apr 2012 → 27 Apr 2012
|Conference||Acoustics 2012 Nantes Conference|
|Period||23/04/12 → 27/04/12|