System Division, Nittobo Acoustic Engineering Co., Ltd., 1-21-10 Midori, Sumida-ku, Tokyo 130-0021, Japan.
J Acoust Soc Am. 2009 Dec;126(6):3020-7. doi: 10.1121/1.3242355.
Three different techniques for evaluating the absorption coefficient of sound absorbing materials in free field conditions are discussed. One technique measures the acoustic impedance at one point nearby a specimen, the other two techniques evaluate the impedance from the transfer function of two sound pressures and two particle velocities at two points. These are called "PU-method," "PP-method," and "UU-method," respectively. An iterative algorithm to estimate the acoustic impedance of the locally reactive specimen in the spherical wave field is also applied. First, the effect of receiver positions, specimen areas, and source heights to the measured normal absorption coefficient is investigated by the boundary element method. According to these investigations, the PU-method is most stable against the effect of specimen area, and the UU-method is easily affected by that effect. Closer source to the specimen distance is advantageous for the signal to noise ratio of these measurement techniques, but correction for the effect of the spherical wave field has to be applied. As a finding, the iterative algorithm works for all of three techniques. Finally, the PU-method is applied experimentally with a pressure-velocity sensor and a loudspeaker in a hemi-anechoic room. As a result, the calculated results have been verified.
讨论了三种不同的方法来评估自由场条件下吸声材料的吸收系数。一种技术测量靠近试样的一个点的声阻抗，另外两种技术分别从两个声压和两个质点速度在两个点的传递函数来评估阻抗。这分别称为“PU 法”、“PP 法”和“UU 法”。还应用了一种迭代算法来估计球形波场中局部反应试样的声阻抗。首先，通过边界元法研究了接收器位置、试样面积和源高度对测量法向吸声系数的影响。根据这些研究，PU 法受试样面积影响最稳定，UU 法受其影响较大。声源离试样越近，这些测量技术的信噪比越有利，但必须对球面波场的影响进行修正。作为一个发现，迭代算法适用于所有三种技术。最后，在半消声室中使用压力-速度传感器和扬声器对 PU 法进行了实验。结果验证了计算结果。
