The propagation characteristics of ultrasound in sea‐water and its ultrasonic reflection loss on the sea‐bottom or on fish‐schools must be studied for both the design and the practical operation of echo‐sounders, fish‐finders, and SONARs. This paper shows a practical method for the above‐mentioned study, together with the results of measured propagation characteristics in both vertical and horizontal directions, as well as the ultrasonic reflection loss on the sea‐bottom and on fish‐schools, using 10, 14.5, 28, 50, 200, 300, and 400 kc. The propagation attenuation in a direction vertical to the sea‐surface is due mainly to spherical divergence, but the attenuation in a horizontal direction is partly caused by absorption as well. This absorption becomes larger at higher frequencies: in fact, the results of experiments show that the absorption is 10–20 db/km at 28 kc, 31–50 db/km at 200 kc, and about 120 db/km at 400 kc. The reflection loss of the ultrasound at a fish‐school decreases as frequency increases. On the contrary, the reflection loss of the sea‐bottom increases as frequency increases. At 200 kc, therefore, the echoes from ground‐fishes sometimes become stronger than that from the sea‐bottom.

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