実験水槽内の気泡発生による超音波の減衰異常とその防止法

抄録

In many experiments conducted in water tank such as the measurement of the sensitivity and the directivity of ultrasonic transducers or the measurement of sound field distribution, we experience sometimes severe attenuation anomaly. The anomaly is due to the bubbles which are generated naturally on the surface of the transducers or in the water. The authors discussed the condition of the bubble generation from the view point of supersaturation of dissolved air in the water due to temperature rise of the water. A chart was made representing air content in water and the solubility versus temperature and total pressure. The degassifying treatment of the water by heating, and the same by pressure reduction which are used in this country as the prevention of the attenuation anomaly are explained with this chart. The authors invented and tested an apparatus for the measurement of the air content in water or the degree of saturation. The accuracy is not so high, although the apparatus is sufficiently useful for the determination of whether the degassifying treatment is required or not. As the example of the attenuation anomaly, the transmission through the water in an anechoic tank was observed under heating condition by an electric heater and during natural cooling. The result showed clearly the fact that the considerable increase of attenuation takes place in the period of temperature rise and no increase in the cooling period for 50 kc. However, the abnormal attenuation was very little for 100 kc. Concerning the duration of the effectiveness of degassifying after the treatment, the author calculated the diffusion of air in the water, and concluded that the duration must be mainly due to the convection of the water in the tank. In addition, the causes of bubble generation other than the temperature rise such as the assimilation by plants in water or planktons and the decomposition of organic substances deposited on the bottom are discussed. And the reason that the water in the depths of a large tank has few chance to generate bubbles is also described.