Application of Embedded Thermophones Toward Tonal Noise Cancelation in Small-Scale Ducted Fan

Abstract

Modern civil aviation faces the critical task of reducing aircraft noise. Among different factors, tonal content is relatively more important due to regulatory definitions and its attenuation characteristics, with significant contributor being the fan aero-acoustics associated with rotor–stator interaction. Recognizing that existing passive mitigation methods are insufficient to meet the emission regulation, the present work focuses on powered noise cancelation to reduce a fan's acoustic signature. Identifying conventional actuator technology as the primary obstacle for the on-source creation of canceling soundwaves, thermo-acoustic transducers (thermophones) are implemented instead. These mechanically static, surface-deposited emitters encompass a periodically Joule-heated thin layer attached to electrically insulating substrate, and are directly situated on the fan stator, which is the source of tonal noise associated with blade passing frequency and its harmonics. Microphone measurements are performed on a small electric ducted fan. The thermophone sound pressure level is matched to the targeted blade pass frequency harmonic, and the acoustic spectra are collected at different speeds while thermophone relative phase is varied to form destructive interference. At conducive relative phase, the findings demonstrate consistent reduction of tonal fan noise irrespective of microphone position, highlighting the global nature of the noise cancelation. This corresponds to a 6–13 dB reduction in sound pressure level magnitude at the particular frequency of interest. In the following, the collective impact of this effect is assessed through evaluation of perceived noise level. In case of a single small stator, where the planar mode dominates the spectrum, a single thermophone is sufficient to reduce the dominating planar propagation mode.