Active noise control by use of an aeroacoustic instability

Abstract

Ducts that are lined with a series of equally spaced cavities exhibit strong amplification of sound waves under specific flow conditions. The amplification is caused by an aeroacoustic instability wave traveling in the boundary layer of the lining. Since the effect is coupled with a drastic variation of the static pressure drop that depends on the amplitude of incident sound, the possibility is implied to efficiently generate high intensity sound at low frequencies: An incident sound wave with harmonically varying amplitude will cause a periodic modulation of the pressure drop and thus lead to the generation of sound at the modulation frequency. Based on these observations, an aeroacoustic loudspeaker has been designed combining a lined duct section as part of a flow channel and a conventional loudspeaker. The output power of the device amounts to up to 95 dB re: 1 pW/Hz at Ma=0.25 for frequencies ranging between 0 and 200 Hz—an observation of some significance with regard to the fact that traditional loudspeakers tend to be rather inefficient at low frequencies. As a feasibility test, the aeroacoustic loudspeaker is used as an actuator in an active noise control setup. High attenuation of low-frequency noise is achieved.