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Modulated ultrasound is a technique for the transmission of audio information, comparable to radio.Ultrasound can bemodulated to carry an audio signal (as radio signals are modulated). It is usually used to carry messages underwater at ranges under five miles.
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 anddecodes it for use as sound, navigational-position information, etc. Its function is somewhat like that of aradio receiver.
Applications include use inunderwater diving communicators as well as communication withsubmarines.
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 inMIL-C-15240D) with a modulated carrier of 7,500 Hz, or in "UQC-2" mode (as defined inMIL-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.)
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