Investigation On Low-frequency Performance Measurement Of Acoustic Materials

With the increasing use of acoustic materials in the low frequency range,the corresponding research has been paid more and more attention,in which the acoustic reflection coefficient is one of the most important characteristics of low frequency performance of acoustic materials.At present,most of the methods are carried out in the free sound field for the study of low frequency performance of acoustic materials,but the acoustic reflectance coefficients of the materials cannot be accurately measured in this environment.Therefore,how to accurately measure the acoustic reflection coefficient of the material in the low frequency range in the semi-space sound field environment is of great significance to the application of the actual acoustic material.Due to the excellent reconstruction performance of acoustic holography in low frequency band measurement,this paper summarizes three kinds of low frequency performance measurement algorithms based on acoustic holography theory,which are planar near field acoustic holography(NAH),statistical optimal near-field acoustic holography(SONAH)and equivalent source method(ESM).According to the basic theoretical knowledge of the three algorithms,this paper discusses the main mechanism of error generation,and put forward the corresponding optimization scheme to reduce the error.For the NAH algorithm,the holographic data space domain zero-padding,the wavenumber domain plus filter window are adopted in the simulation process to reduce the window effect and winding error caused by the shortcomings of the two-dimensional Fourier transform itself,which greatly improves the reconstruction effect.For the SONAH algorithm,the effects of sound pressure method and vibration velocity method on reconstructing sound pressure parameters and vibration velocity parameters are compared.The results show that the measurement parameters and reconstruction parameters must be consistent.For the ESM algorithm,the effect of the number of equivalent sources on the precision is studied,which shows that the number of equivalent sources has little effect on the calculation accuracy.In order to apply the ESM algorithm to the mobile sound source,this paper solves the Doppler frequency shift problem by improving the traditional formula.For the inverse problem of acoustic,this paper uses the Tikhonov regularization criterion to compare the simulation results of L-curve and GCV methods at different frequencies and SNR configurations,which shows that L-curve and GCV are only suitable for low frequency,high signal-noise ratio environment,which provide a reference for the experiment that how to select the regularization parameters quickly and accurately.In order to accurately measure the low frequency performance of acoustic materials in the semi-space sound field,this paper improves the traditional measurement model,and establishes a measurement model of acoustic reflection coefficient based on double holographic surface.The calculation results of three algorithms at different frequency and sampling interval are simulated by MATLAB.The stability and calculation efficiency of the three algorithms are compared in the transverse direction,which provides a theoretical basis for the selection of measurement algorithms in different experimental environments.Finally,through the proof of the principle experiment,this paper proves the correctness and efficiency of three algorithms based on the double holographic surface measurement model.