８マイクロホン法垂直入射吸音率測定におけるマイクロホンによる散乱の抑制法

抄録

The authors have previously proposed the measurement method of normal-incidence sound absorption coefficient using eight microphones which enables the measurement in the high frequency range beyond the cut-on frequencies of the higher order acoustic modes in an impedance measurement tube. The proposed method utilizes the symmetry of sound field in the tube to eliminate the influence of the higher order modes. The maximum measurement frequency of the proposed method is about three times higher than the conventional prevalent method. In the proposed method, in order to reduce the influence of the first radial mode ((0, 1) mode), microphones are inserted into the measurement tube so that the acoustic centers of the microphones can match the nodal line of the (0, 1) mode. However, because of this microphone configurations, the microphones may disturb the sound field in the tube and cause scattering. When the scattering occurs, the normal incidence sound wave is converted to the waves of higher order modes, which can affect the measured absorption coefficient. Hence, this study proposes the technique to suppress the effect of the scattering. This technique involves installing a cross-shaped spacers between two microphones facing each other. Since the phases at the outside and inside of the (0, 1) mode nodal line are opposite, it is considered that inserting the spacer can prevent the excitation of the (0, 1) mode. Numerical simulations using the finite element method are performed to discuss the validity of the proposed technique.