Estimation and experiment for sound absorption coefficient of three clearance types using a bundle of nested tubes.

Closely packed nested structure circular tubes form three types of clearances: circular clearances, concentric clearances, and longitudinal clearances. The cross-sectional shape of rice straw, including the above-mentioned three types of clearances, is similar to that of the samples of the present study. Herein, the propagation constant and characteristic impedance of the circular, concentric, and longitudinal clearances surrounded by three cylindrical surfaces are treated as a transfer matrix. Then, by connecting these transfer matrices in parallel, a transfer matrix integrating these clearances is determined, and the sound absorption coefficient is calculated. Test samples that have three types of clearances are also created, and measure their sound absorption coefficients. The contributions of the concentric and circular clearances are significant because of the dimensions of the used tubes. The results of the calculations that took eccentricity into account revealed no significant impact on the sound absorption coefficient in wide-frequency range. The results of the calculations that took flexure into account revealed that the theoretical value was close to the measured value in the low-frequency range.