Acoustic impedance design using electrical method and its applications

Abstract

Low-frequency broadband noise is prevalent in our living and working spaces and is hard to be controlled by conventional passive methods such as microperforated panels, Helmholtz resonators and sound absorption materials. This thesis studies the use of a passive electrical method for acoustic impedance design and low-frequency broadband noise control. The electromechanical interaction between a moving-coil and a magnetic field is utilized to couple the electrical impedance of an analog circuit and the acoustic impedance of a suspended diaphragm. We call the coupled system “shunt-electro-mechanical-diaphragm” (SEMD). In this study, we use a moving-coil loudspeaker with a shunt circuit as the first realization of SEMD. The shunt circuit contains a negative impedance converter (NIC), which is an operational amplifier circuit, and it reduces the coil resistance, but it is shown also that the NIC may be removed when the coil is tailor-made with low resistance. Analysis reveals that the shunt circuit is able to reduce the reactance of the diaphragm through the electromechanical coupling at low frequencies and in a broadband, such as from 50 Hz to 1000 Hz, which greatly enhances the sound absorption of the moving-coil loudspeaker. Experimental data from SEMD is compared to that of the double-layer microperforated panel absorber as well as the more traditional sound absorption material, and the comparison shows that the SEMD can extend the lower limit of effective sound absorption by 1.5 octaves. Tuning of the sound absorption performance of the SEMD by adjusting the shunt circuit is also studied. First, based on the lumped-parameter model of the SEMD we prove that the SEMD can realize resonance and perfect sound absorption at any desired single frequency and it’s experimentally verified. Secondly, the parametrical investigation shows that the capacitance, resistance and inductance of the R-L-C shunt circuit can tune for the low-, moderate- and high- frequency sound absorption of the SEMD, respectively. By tuning them simultaneously a broadband tunable SEMD is achieved. Prediction based on the experimental data of acoustic impedance shows that the random-incident sound absorption can be tuned to 0.6, 0.7, 0.8 and 0.9 in the frequency range of 200 Hz- 800 Hz. The SEMD is further applied to the control the thermoacoustic instability in a Rijke tube. The thermoacoustic instability in an aero-engine is a high-risk factor for the low-emission aero-engine design. The frequency of the instability ranges from tens to thousands of Hertz, which hence needs an effective acoustic controller working over broadband frequencies. The experiments in the Rijke tube show that the SEMD can control the instability at least over an octave, for 95 Hz to 190 Hz with dimensionless size much smaller than those found in literature. Finally, a truly passive SEMD is designed and tested. Experiments show that the shunt circuit can reduce the original acoustic mass of the diaphragm to its 26.6% and the stiffness of the back cavity to 21.4%. Numerical comparison shows that the optimized sound absorption coefficient of the passive SEMD is slightly better than a parallel array of resonators.