The acoustic transmission line method is widely used for measurements of absorption coefficients and specific acoustic impedance of sound absorbing materials. As the diameter of the tube must not be greater than about half the wave length of sound, this method is not available at high frequency. This paper describes a new method for measuring acoustic impedance of the samples larger than the wave length. Firstly, the pressure amplitude and phase in the nearfield of a rectangular piston source are examined theoretically and experimentally. It was shown that the pressure amplitude along the axis of square piston source for k₁ b=20 (k₁ : wave number, 2b : width of piston) are nearly smooth at around k₁ x=12 (x : distance from the piston). Secondly, the square rubber samples 20cm in width with free backing plate are prepared. The standing wave ratio along the axis of rubber sample and phase shift at the surface of it are measured at frequency range 30-40kHz. The absorption coefficients of the samples are obtained by comparing measured impedance circles with those by calculation.

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