Multi-tone Sound Absorption Based On Shunted Loudspeaker

The noise generated by large machines and equipment such as electrical power transformer and air compressor often generates low frequency harmonic components.The low-frequency components are difficult to control effectively with traditional sound absorbing materials and structures.The shunted loudspeaker sound absorbing system is a rapidly developing method.The shunted loudspeaker is a resonance sound absorption structure composed of a moving-coil loudspeaker and a shunt circuit.The loudspeaker converts the incident sound energy into electric energy,dissipating in the form of the internal energy through the shunt circuit.It has the advantages of simple structure,flexible adjustment,small space requirements,and suitable for low-frequency sound absorption.This paper investigates multi-tone sound absorption at low frequency based on shunted loudspeaker.First of all,a multi resonant shunt circuit is proposed to realize the shunted loudspeaker for multi-tone sound absorption.The form of the multi resonant shunt circuit is then simplified.To ensure the stability of the system,the sufficient stability requirement of the circuit is that the residual parasitic resistances and residual coil resistance should be positive.The procedure for implementing multi-tone sound absorption is proposed to ensure each resonance frequency is within the allowed frequency deviation requirements.Experiments are carried out in an acoustic impedance tube,where the sound absorption coef-ficients of the sample designed for transformer noise control at 100 Hz,200 Hz,300 Hz are 0.90,0.91 and 0.90.Secondly,it is proposed to combine the shunted loudspeaker units with different sound absorption characteristics to form a combined sound-absorbing unit for multi-frequency sound absorption.A model of the parallel shunted loudspeaker combination unit is proposed based on a modal solution,and then the acoustic properties of a shunted loudspeaker array under normal incidence are investigated using the finite element method.The simulation results show that the coupling between the shunted loudspeaker units is weak,when the bandwidth at each resonance frequency is smaller than the interval of the resonance frequencies.It is found each shunted loudspeaker works almost independently where each unit resonates.The experiment verified the feasibility of parallel shunted loudspeaker combination unit at four target frequencies.Unlike the parallel shunt speaker combination unit,each unit of the shunted loudspeaker array is identical.The sound absorption of periodically arranged multi-tone shunted loudspeakers is investigated at oblique incidence and in reverberation field,by the wave superposition method and parallel impedance method.The finite element method validates the correctness of the two theory analyses.The simulation results indicate the multi-tone shunted loudspeaker array has achieved excellent sound absorption both at oblique incidence and in reverberation field.Based on the measured parameters,the parallel impedance method is used to calculate the sound absorption coefficient of the multi-tone shunted loudspeaker array in reverberation field.On this basis,the effect of the area ratio on the sound absorption in reverberation room is investigated.The experimental results show that the inconsistency of the shunted loudspeaker units is necessary to be considered in practical applications.Finally,a shunted loudspeaker combination unit array is proposed,where the parallel impedance method is used to establish the correlation analysis model for calculating the sound absorption characteristics of the combined shunted loudspeaker unit array at oblique incidence and in reverberation field.Compared with the sound absorption at normal incidence,the resonance frequencies in reverberation field do not shift,where the sound absorption coefficients increase and the bandwidths decrease.The effect of the distributed area ratio on the sound absorption in reverberation room is investigated.In practice,the optimum distributed area ratios of all parallel shunted loudspeaker array units should be averaged to obtain the optimal distributed area ratio of the array.The shunted loudspeaker combination unit array could achieve multi-tone sound absorption under normal incidence,oblique incidence and reverberation conditions.