Acoustic measurements are not performed in any environment. They must have a specific acoustic environment. In addition to the generally quieter environment, which requires lower background noise, the test environment is required to meet the following three idealizations one of the conditions of the sound field. They are:
(1) Plane wave sound field;
(2) Free sound field;
(3) Diffusion sound field.
The relationship between the acoustic quantities in these three sound fields is very simple, so the measurement method can be greatly simplified and effective in principle. The plane wave sound field is generally easy to obtain. In a certain frequency range, when the sound wave propagates in the pipe of the rigid pipe wall, the wave shape of the sound wave mainly shows the plane wave mode. A high-frequency cutoff frequency for acoustic waves propagating in a plane wave mode in a pipe. For a rectangular section tube, the cutoff frequency fc = (c0/2lx), where lx is the width of one of the wider sides of the rectangular section. For a circular tube fc = 1.84 (c0 / π d), where d is the inner diameter of the pipe. The vertical incident sound absorption coefficient and the acoustic impedance ratio of the sound absorbing material and the structure are obtained by measuring in the standing wave tube.
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