Ultrasound Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies ...

can be

modulated In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...

to carry an audio signal (like radio signals are modulated). This is often used to carry messages underwater, in

underwater diving Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment. It is also often referred to as diving, an ambiguous term with several possible meanings, depending on contex ...

communicators, and short-range (under five miles) communication with submarines; the received ultrasound signal is decoded into audible sound by a modulated-ultrasound receiver. A modulated ultrasound receiver is a device that receives a modulated ultrasound signal and

decode Decoding or decode may refer to: is the process of converting code into plain text or any format that is useful for subsequent processes. Science and technology * Decoding, the reverse of encoding * Parsing, in computer science * Digital-to-analog ...

s it for use as sound, navigational-position information, etc. Its function is somewhat like that of a radio receiver.

Range limitation

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. For this reason, most underwater "telephones" either operate in "baseband" mode (at the same frequency as the voice and is basically a loudspeaker), in a " UQC-1" mode (as defined in MIL-C-15240D) with a modulated carrier of 7,500 Hz, or in "UQC-2" mode (as defined in MIL-C-22509) from around 8,500 hertz to approximately 12,000 hertz, or in the later "WQC-2" mode from 8,500 hertz to approximately 100,000 hertz - with most use around 32,500 hertz) (Also see NATO STANAG-1074 ED.4 for descriptions of internationally used frequencies.)

Range limitation

See also