Are locally reacting acoustic liners always behaving as they should
Abstract
The properties of a Helmholtz resonator type acoustic lining material may be seriously affected by intercellular water drain holes. This was shown by experiments in the NLR flow duct facility on an actual aircraft engine liner where drain holes caused a considerable shift in the frequency for maximum attenuation. Furthermore the drain holes invalidate the assumption of local reaction which is usually made for this type of lining material in duct acoustics. This was confirmed by the phase differences measured between the signals of two microphones at the bottom wall of a cell of the material. In the absence of drain holes such phase differences were also found when the free space acoustic wavelength was smaller than about 3.5 times the cavity cross dimension. Some remarks are made concerning problems associated with the application of the two-microphone technique for in-situ impedance measurements on a rather large cell material.
- Publication:
-
American Institute of Aeronautics and Astronautics Conference
- Pub Date:
- March 1979
- Bibcode:
- 1979aiaa.conf.....Z
- Keywords:
-
- Acoustic Propagation;
- Aircraft Engines;
- Aircraft Equipment;
- Insulation;
- Linings;
- Acoustic Attenuation;
- Acoustic Impedance;
- Ducted Flow;
- Holes;
- Honeycomb Structures;
- Liquid-Solid Interfaces;
- Mach Number;
- Microphones;
- Noise Reduction;
- Perforated Plates;
- Phase Shift;
- Sound Waves;
- Water Flow;
- Acoustics