Ultrasound

Ultrasound

Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

 can be modulated

Modulation

In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

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

Underwater diving

Underwater diving is the practice of going underwater, either with breathing apparatus or by breath-holding .Recreational diving is a popular activity...

 communicators, and short-range (under five miles) communication with submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

s; the received ultrasound signal is decoded into audible sound by a modulated-ultrasound receiver

Receiver (modulated ultrasound)

In modulated ultrasound terminology, a receiver is a device that receives a modulated ultrasound signal and decodes 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 definded 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.)

See Sound from ultrasound

Sound from ultrasound

Sound from ultrasound is the name given here to situations when modulated ultrasound can make its carried signal audible without needing a receiver set...

for modulated ultrasound that can make its carried signal audible without needing a receiver set.