Modulated ultrasound is a technique for the transmission of audio information, comparable to radio.
It is usually used to carry messages underwater at ranges under five miles.
A modulated ultrasound receiver is a device that receives a modulated ultrasound signal and decodes it for use as sound, navigational-position information, etc.
Due to the absorption characteristics of seawater, ultrasound (sound at frequencies greater than human hearing, or approximately greater than 20,000 hertz) is not used for long-range underwater communications.
The higher the frequency, the faster the sound is absorbed by the seawater, and the more quickly the signal fades.