Harmonic acoustic pneumatic source (HAPS) to generate sound at very low-frequency

At very low frequency, the loudspeakers face technical issues (size, weight, energy consumption). An alternative to the electrodynamic loudspeaker is the Harmonic Acoustic Pneumatic Source (HAPS) demonstrated efficient in active tonal control. It comprises a high-pressure pneumatic air source, a rotating flow chopper, and an exhaust. The rotation of the flow chopper generates a pulsed flow which radiated noise out from the exhaust. An analytical model of the HAPS is presented to introduce the main challenge associated with its use at very low frequencies: having a high mean flow rate with a relatively small exhaust duct section. To overcome these challenge, a dedicated flexible tube (under light mean pressure) is added to the exhaust circuit to obtain a pulsating sphere activated by a HAPS. This configuration has been experimentally studied in a semi-anechoic room with two flexible tubes and without. The sound pressure level of the first harmonic at 1 meter range from 75 to 95 dB SPL (25 to 160 Hz, plenum pressure from 5 to 20 PSI). Thanks to fluid-structure interaction, the fundamental harmonic sound levels radiated by the compliant tube were free of jet noise. The drawback is the high harmonic distortion observed during the experiments.