For the development of echo-sounder which detect efficiently the soft mud layer, it is necessary for us to study into the effect of ultrasonic reflection and propagation, transmission and absorption characteristics in soft mud layer.
　So we experimented in the laboratory on the acoustic characteristic of the 7 frequencies ultrasonic echo-sounder in the fly-ash mud soft sediment.
　Through the analysis of this experimetnal results, we found out that the specific gravity of fly-ash mud soft sediment was about 1.5
　The other side, reflection loss Lm (dB) and absoroption coefficient αm(dB/m) were expressed as follows:
　Lm=1.75f0.27
　αm=12.5f0.6
　where f:frequency
　From this empirical formula, we can take synthetic transmission loss and allowable distance of detection and allowable thickness of detecting mud layer.