Active Tonal Noise Cancellation in a Small Scale Ducted Fan Set-up using Thermo-acoustic Transducers

Modern civil aviation is faced with the critical task of reducing aircraft noise. Among different factors, the tonal content is relatively more important due to the regulatory definitions and its attenuation characteristics, with the largest contributor being the fan aero-acoustics associated with rotor-stator interaction. Recognizing that existing passive noise mitigation methods are insufficient to meet the emission regulations, this particular talk focuses on the use of active noise cancellation to reduce the fan’s acoustic signature. Identifying conventional actuator technology as the primary obstacle for on-source creation of equal magnitude opposite phase sound waves, thermo-acoustic transducers (thermophones) are implemented as mechanically static, surface-deposited sound emitters. This technology encompasses a periodically Joule heated electrically conductive thin layer attached to an electrically insulating substrate, directly situated on the fan stator or outlet guide vane, which is the source of the stationary tonal noise associated with blade passing frequency and its harmonics. The experimental demonstrations are performed in a small electric ducted fan with a single stator blade. The sound pressure level of the thermophone is matched to the targeted harmonic of the blade pass frequency, 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 6-13 dB in sound pressure level of fan tonal noise frequencies, irrespective of the microphone position, highlighting the global nature of the noise cancellation.

The work is towards M.Sc. Degree; under the supervision of Associate Professor Beni Cukurel, Faculty of Aerospace Engineering, Technion-Israel Institute of Technology