2021 年 42 巻 3 号 p. 125-133
The authors have previously proposed the normal-incidence absorption coefficient measurement method using multiple microphones which enables a measurement in the frequency range beyond the cut-on frequencies of the higher order acoustic modes in an impedance measurement tube. The maximum measurement frequency of the method is determined by the cut-on frequency of the first radial mode, (0,1) mode, in the cylindrical tube. In this study, in order to extent the measurement frequency range more, the method which can reduce the factor of the (0,1) mode in the sensing signals is investigated. The method consists of two techniques. The first one is a microphone placement way to make the (0,1) mode signal undetectable by placing the acoustic centers of the microphones at a node of the mode. The second one is related to a sound source structure not to excite the (0,1) mode. Using these techniques, the normal-incidence absorption coefficient can be evaluated under the cut-on frequency of the (4,0) mode, which is about 2.89 times higher than that of the conventional two-microphone transfer function method. The results of numerical simulations and experiments are shown to prove the validity and the feasibility of the proposed method.
