For the development of echo-sounder which detect effciently the vertical density distribution of suspension layers, it is necessary for us to study into the relationships of ultrasonic frequency and acoustics to the grain size of suspension particles, suspension specific gravity and the behavior of settling suspension particles and to select the frequency suited to detection of suspension layers.
　So we studied in the laboratory on the acoustic character-istics of th 7 frequencies echo-sounder in the fly-ash suspension.
　Though the analysis of this edxperimental results, within the limits of the suspension concentration in valume (0.1～0.01%), the absorption coefficient of suspension α(dB/m) were experessed as follows;
　α=K・f0.5
where f: frequency.
　The other side, the echo-sounder which used comparatively high frequency may by possible to detect effeciently it from the detective records.