A New Tube Method for Measuring Complex Pressure Reflection Cofficients of Absorbers in Water

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  • Okushima Motoyoshi
    Research Laboratory of Precision Machinery and Electronics,Tokyo Institute of Technology.

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Other Title
  • 音響管による水中吸音材の複素音圧反射率の測定法
  • オンキョウカン ニ ヨル スイチュウキュウオンザイ ノ フクソオンアツ ハンシャリツ ノ ソクテイホウ

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Abstract

The author deviced a new tube method for measuring the complex pressure reflection coefficient in water, constructed the apparatus, and measured several samples. This method is a sort of the stationary wave method. In this set up, the bottom surface of a vertical steel tube with thick wall (length 400 mm, inner radius 17. 5mm, wall thickness 6 mm) is driven at one of the longitudinal resonant frequency (about 9, 18, 27 and 36kc)of a Langevin type transducer of barium titanate, and the stationary wave field is formed in the water column between the bottom surface and the sample inserted in the tube from the top. By this method, the principle that the mechanical impedance toward the sample at the bottom takes the minimum or the maximum values, when the sample is moved up and down and as the bottom surface coincides with the node or the loop plane of pressure in the stationary wave field is used. Namely, the amplitude of the complex reflection coefficient is derived from values of these two impedances, which are obtained by measuring the reaction to the transducer surface from the field in the tube, while the phase of the complex reflection coefficient is derived by measuring the distance between the sample and the pressure node plane in which the mechanical impedance takes the minimum value. Two methods, the motional admittance method and the vibrometer method, were devised for the purpose of measuringl the mechanical impedance. The transducer for the vibrometer method has two barium titanate elements for driving and for measuring the velocity amplitude, and it is so designed as to become a multiresonant transducer which vibrates with the same intensity for the most part and is capable of being employed for measuring with equal easiness at four resonant frequencies (9, 17, 27and 36kc). Wooden wedges is generally used for the lining of the unechoic water tank in this country, so the author measured the reflection coefficient of wooden(pine)wedges and plates by this apparatus.

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