各種吸音構造の斜め入射吸音特性 : 相関法を用いた直接音消去法による測定

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A new method of measuring the oblique incidence absorption characteristics of various sound absorbers is described in the first half of the paper. The experimental arrangement is shown in Fig. 2. As shown in Fig. 3, the cross-correlation function between the source signal (1 octave band noise was employed) and the received signal which consists of the direct sound and the reflected sound by a hard board is measured first (a), and then, after the hard board is removed and the phase of the source signal into the loud speaker is reversed, the cross-correlation function between the source and direct sound is measured (b). By adding the latter to the former, the equivalent direct wave φ_<x0>(τ) is cancelled and the equivalent reflected wave φ_<xr>(τ) can be obtained. The reflected wave by a test material φ_<xs>(τ) is also measured in the same way. The complex reflection coefficient of the material H(f) is calculated from Φ_<xr>(f) and Φ_<xs>(f) which are the Fourier-transforms of φ_<xr>(τ) and φ_<xs>(τ) respectively (eq. (7)). Then, the oblique incidence absorption coefficient α(θ) and normal impedance ratio z_n can be calculated from H(f) by equations (8) and (9). As a check of the validity of this method, measurements of the normal incidence absorption coefficients α(0°) have been made by this method and by the tube method using polyurethane foam of 5 mm thick (U_5) and 10 mm thick (U_<10>). The results measured by these two methods have been found to be in good agreement as shown in Fig. 7. Subsequently, measurements of α(θ) and Z_n have been carried out in a 1/10 scale model experiment with various types of absorbing materials such as thin porous material (F_0 in Fig. 9, F_a in Fig. 10, F_b in Fig. 11), perforated facings (P_a in Fig. 12, P_b in Fig. 13), Rib wall (R in Fig. 14) and vibrating board (B in Fig. 15). Concurrently, the reverberant absorption coefficients α_<rev> have been measured by use of a scale model reverberant chamber in which air was replaced by N_2 gas, and compared with the statistical absorption coefficient α_stat (eq. (3) in appendix (1)) or α^*_stat (eq. (4) in appendix (1)) calculated from measured Z_n or α(0°).
社団法人日本音響学会の論文
1978-01-01

各種吸音構造の斜め入射吸音特性 : 相関法を用いた直接音消去法による測定

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