抄録
With respect to ultrasonic fish finding, a theory for calculation of reflecting power of fish school is developed. The reflecting power is defined by us as the ratio of the sound pressure of the reflected wave from the fish school at the position of the source to that from an infinte rigid plane situated at the position of the fish school. The calculation shows that the reflecting power is independent of the distance, and is proportional to the beam angle, the square root of the pulse length and the square root of the fish density, as far as the school extend beyond the beam angle. The model experiment is performed in air, utilizing many soft tennis balls suspenced in random distribution as the reflecting obstacles. The frequency and the wave length are 14. 2kc and 2. 4cm, respectively. The theorectical results written above are proved approximately. The absolute value of the reflecting power of fish school is also given theoretically, assuming the cause of reflection from a fish to be the air-bladders and the density difference of the fish body and the water. This was not experimentally verified in this work, and the reflecting power of a single fish is now under measurement in other laboratory for many kinds of fish.
