The ocean is largely opaque to radio frequency energy so communications underwater either needs to be transmitted by wire, or by sound. Increasingly as military and industrial processes take place in the ocean they will use various acoustical communication signals. Some of these signals are designed to transmit text, others to transmit control and condition data.
Because most of this data transmission is “digital” it uses a code of “ones” and “zeros.” These data streams do not sound pleasant at any volume, but at ear-shattering volumes they are even more problematic.
Mid frequency signals (between 1kHZ and 10kHz) are designed to work in settings up to 25km. High frequency (20kHz – 65 kHz) are used up to 10 km. Higher frequency (100kHz and up) are used in close proximity up to 2 km.
Mid frequency signals are in the range of human hearing, the higher frequencies are in the hearing range of dolphins, porpoises, and seals.