Producing a High Level Harmonic Acoustic Pressure Field with Harmonic Acoustic Pneumatic Source (HAPS): Experimental Validation of the Harmonic Distortion Reduction

Applications such as active control of fane noise, require high Sound Pressure Levels (SPL). Pneumatic acoustic sources are usually used for that purpose (e.g. the WAS 3000 and the Mark VI and VII). The harmonic acoustic pneumatic sources (HAPS) - which use the chopping of an airflow to generate an acoustic pressure field - are being studied for applications requiring high-level harmonic acoustic pressure fields. A recent study has shown that the amplitude, phase, and frequency generated by HAPS are accurately controllable, and can reach 120 dBSPL. However, higher harmonics are also generated at high SPL and this could be detrimental to the intended applications (e.g. active control of tonal noise). An experimental campaign dedicated to understanding the harmonic distortion of HAPS has been conducted on a single HAPS in a semi-anechoic chamber. Measurements with quarter-wave tube at the output of the HAPS were also carried out to reduce this distortion. Levels of 120 to 130 dBSPL for the fundamental were reached, with a difference of 10 dB between the fundamental and the second harmonic (H1/H2 distorsion). Adding a quarter-wave tube increased the acoustic pressure level from 4 to 6 dBSPL, and the H1/H2 distorsion from 2 to 8 dBSPL.