An acoustic comb filter by shunted electromechanical diaphragm

Abstract

A noise with multiple tonal components is prevalent in practice, and these tones, contributing to noise annoyance, may even span several octaves and the frequencies of them always vary with working condition. Conventional broadband noise control structures with limited spaces are hard to address multiple-tonal noise due to sum rules in acoustics. Resonator arrays is feasible but their dependency on the geometrical and mechanical parameters challenges matching variable frequencies of tonal components. This work introduces a Shunt-Electromechanical Diaphragm (SEMD) to construct sound absorption and transmission spectra with multiple peaks, thereby creating an acoustic comb filter in deep-sub-wavelength scale to address multiple-tonal noise. The SEMD consists of a moving-coil loudspeaker integrated with a circuit housing multiple resonance branches, whose acoustic impedance is determined by circuit parameters. This allows the frequencies and number of sound absorption and transmission peaks to be arbitrarily reconfigured by adjusting circuit parameters. A comprehensive lumped parameter model is devised to elaborate on the electromechanical coupling of the SEMD and predicts its comb filter effect. Experiments in an impedance tube show that comb spectra with three peaks for sound absorption and transmission are successfully constructed and reconfigured by collocating circuit parameters, verifying the theoretical model. The three peaks of the sound absorption spectrum have values above 0.9 and are electrically tunable over more than three octaves. The method in this work blends electromechanical dynamics with specialized analog circuits for efficient tonal noise reduction across a broad bandwidth, which removes the geometrical and mechanical constrains and paves the way for digital reconfigurable acoustic devices. Moreover, the multiple peaks matching between noise source and sound absorption/transmission spectra via the electromechanical coupling mechanism of the SEMD bypasses the sum rules in acoustics, providing an alternative method for controlling noise over a much broader bandwidth.