抄録
We have had reported the results of our study on the propagation characteristics of ultrasound, from 100 kc to 400 kc. Lately, we have made experiments on the ultrasound of 1 Mc as described below. Taking the universality into consideration, this experiment was made simultaneously with the frequency of 28 kc, data of which had been collected for years. The 1 Mc fish-finder have been checked in comparison with the 50 kc fish-finder, applying them to fish school, seaweed, etc. The results of the above experiment have been summarized as follows: (1) The absorption constant in horizontal propagation is from 500 db/km to 560 db/km at the mouth of the Tokyo Bay. (2) The absorption constant in vertical propagation is 400 db/km there. (3) The reflection loss at the sea bottom is 30 db in case of mud and 15 db in case of coarse sand including pieces of shells. (4) The result of experiment of fish finding by 1 Mc made in comparison with that by 50 kc shows that the densities of fish schools were found more clearly by the former, while the absorption by bubbles in the wake was hardly noticeable.
