Experiment and estimation of the sound absorption coefficient for clearance of corrugated honeycomb.

The sound absorption coefficient of thin tubes with a honeycomb-corrugated structure was estimated via theoretical analysis assuming the dimensions of the tube and the known physical properties of air. This analysis yields a propagation constant and characteristic impedance, which can be modeled as a one-dimensional transfer matrix. The sound absorption coefficient is then calculated by the transfer-matrix method and the results of comparison with the experiments are reported. The corrugated clearance was divided into elements for which approximations that assumed the clearance between two planes and took into account the perimeter and cross-sectional area of each element were considered. The theoretical value of the sound absorption coefficient obtained using this method was shown to be in good agreement with the experimental results. The experimental value of the sound absorption coefficient was larger than the theoretical value in the previous method in the analysis wherein each divided element was approximated by the distance between two planes taking into account the thickness and cross-sectional area of the clearance.

Theoretical and experiment analysis on the sound absorption characteristics of a layer of fine lightweight powder.

In this study, the sound absorption characteristics of lightweight powders with a particle size on the order of tens of micrometers have been investigated. Herein, to theoretically determine the sound absorption coefficient of a fine lightweight powder in the longitudinal vibration mode, it was assumed that the powder was continuum and could be treated as a multiple degree-of-freedom damping vibration system. In addition, the acoustic impedance was calculated by assuming proportional viscous damping to determine the sound absorption coefficient. Finally, the theoretical and experimental values were compared. With the presented theoretical analysis, values close to the experimental results were obtained. Among the three calculated values based on proportional viscous damping, i.e., Rayleigh damping, rigidity proportional damping, and mass proportional damping, the results for Rayleigh damping were the closest to the experimental values.

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.