Study for louver with sound attenuating function using rectangular flat tubes of cross-sectional area decreasing along with longitudinal direction (Estimation and experiment of transmission loss)

In this paper, we propose a louver with a simple structure and a high aperture ratio. A louver element with a sound attenuating structure composed of layered two flat rectangular tubes, each of which is wedge-shaped with thickness decreasing toward the closed end, was constructed and experimentally and theoretically analyzed. Samples having more basic structures were also constructed and compared. In the theoretical analysis, the sound attenuation characteristics of a louver element were clarified considering sound attenuation in the clearance between two planes, as is typically observed in the flat rectangular tubes furnished in the louver element. To consider the sound attenuation due to the viscosity of the air in the rectangular tube, the propagation constant and characteristic acoustic impedance were derived using the Navier-Stokes equations, etc. Further, the sound transmission loss was calculated by the transfer matrix method. The tendency of the theoretical values in consideration of the attenuation of sound wave roughly agreed with the experimental values. Note that the attenuation of sound wave is evident in a thin or wedge-shaped tube and has to be considered in the theoretical analysis. The number of flat rectangular tubes was doubled without increasing the thickness of a louver element. The structure is useful as a louver slat because sound attenuation is achieved in a wide frequency range while maintaining a high aperture ratio.