ABSTRACT
The use of low-frequency audible sound has two inherent disadvantages: first, it is difficult to separate the signal from any nearby noise such as that coming from the ship’s engines and propellers, and secondly, it is difficult to screen the receiver from the transmitter. An important advance was made when audio-frequency transmitters were replaced by those giving an ultrasonic beam, that is, working with sound of a frequency just at or higher than can be detected by the human ear. The sound frequency used in modern machines varies from 15 to about 50 kilocycles per second.
