Study Of The Acoustic Properties Measurements For Sound-absorbing Materials In The Free-field

Absorbing materials are widely used in many situations such as optimizing room acoustic properties,controlling the noise in a vehicle,or controlling the traffic noise in urban areas. When making use of them, it is necessary to know their absorbing properties generally characterized by the reflection factor, surface impedance or absorption coefficient. Many methods have been developed to measure these parameters. The two most well-known methods are reverberation chamber method and impedance method,which were proposed in the early time and have been standardized.They can give reliable results over a wide frequency range, however, the reverberation chamber method can not provide any information about the complex reflection coefficient or surface impedance, and it suffers from the edge effect which could result in an unreasonable value of absorption coefficient. Meanwhile, the tube method can usually give results at only normal incidence, and a small sample needs to be cut carefully from the original absorbing material and the mounting condition is usually different from those in practical places, like on the wall of a room. The free-field measurement methods can overcome the above problems.In this dissertation, the free-field methods for measuring the acoustic properties of the absorbing material are concerned. After a brief presentation of the state of the art of the free-field measurement methods, several new methods are proposed. Firstly, the impulse-echo method based on the particle velocity measurement is proposed to improve the impulse-echo method's immunity to the disturbing reflections. Secondly,in order to improve the computational efficiency of the acoustic fields method in the spherical wave group, a closed-form half-space Green's function for the material with infinite thickness is extended and proven to be also useful for the material with finite thickness, and then used to the acoustic fields method. Thirdly, the spatial Fourier transform methods based on the measuremennts of the particle velocity on two planes,and pressure and particle velocity on one plane are proposed to improve the results of the highly reflective material. In addition, the non-rectangular spatial windows are investigated for improving the results at small angles of incidence. Finally, the measurement method based on the equivalent source method is proposed. The main contents in the dissertation are summarized as follows:In chapter one, the history and the state of the art of the methods, especially for the free-field methods, for measuring the acoustic properties of the absorbing material have been proposed. And then the problems of the above method and the corresponding solutions are presented. Finally, the research topic in the dissertation is determined based on the literature review.In chapter two, the formula for the impulse-echo method based on the particle velocity measurement is developed and the shape of the impulse response of the particle velocity is given. And then the feasibility of the proposed method are examined by measuring the acoustic properties of three kinds of the absorbing material in the semi-anechoic room and its performance in a noisy environment,an office room,is also presented. Finally,the proposed method's better immunity to the disturbing reflection is verified by putting a disturbing object near to the measurement probe.In chapter three, a closed-form half-space Green's function for the material with infinite thickness is extended and proven to be also useful for the material with finite thickness, and then used to the acoustic fields method in the spherical wave group. The feasibility and the better computational efficiency of the proposed methods are examined by both numerical simulations and experiments.In chapter four, the spatial Fourier transform method is modified by allowing the input data to be measurements of particle velocity on two planes (VV-method) or particle velocity and pressure on one plane (PV-method). Numerical simulations and experiments are carried out to examine the feasibility and ability for reducing the effect of the finite measurement length of the PV- and VV-methods, and show their performance in comparison with the original spatial Fourier transform method based on the measurement of pressure on two planes.In chapter five, the theory of the equivalent source method is introduced. And then the method based on the equivalent source method for measuring the acoustic properties of the absorbing material is proposed. For the plane material, the feasibility of the proposed method is examined through numerical simulations and experiments; The influence of the sound source type is also investigated and the results show that the proposed method is valid whether the monopole or the dipole source is used. For the non-plane material, the feasibility of the proposed method is examined through numerical simulationsIn chapter six, the research in this dissertation is summarized, and some topics for further study are given.