Modulated ultrasound is a form ofunderwater acoustic communication in which information (such as voice or data) is transmitted through water by modulating an acoustic carrier signal. Early practical systems includedunderwater telephone equipment for ship-to-submarine communication, which used single-sideband suppressed-carrier amplitude modulation in the kilohertz frequency range.[1]
Some underwater telephone equipment (e.g., AN/UQC-1) specified an operational voice frequency band around 8–11 kHz and reported an approximate range on the order of miles under favorable conditions, reflecting the short-to-medium range typical for practical voice underwater acoustics in real environments.[2][1]
The received signal is demodulated (decoded) into audible sound or other information by a compatible receiver, in a manner analogous to demodulation in radio communications.[1]
Applications include use in underwater diver communication systems (“through-water” communications) and communication with submarines and other subsea assets.[3][4]
In seawater, acoustic absorption (attenuation) increases strongly with frequency, which limits the practical range of higher-frequency (including ultrasonic) underwater transmission. Higher frequencies generally suffer greater absorption and therefore fade more rapidly with distance than lower-frequency sound.[5][6]
Because of this, practical underwater voice systems historically used carrier frequencies in the low-kilohertz region (for example, around 8–11 kHz in early underwater telephone implementations) rather than ultrasonic frequencies, achieving communication distances on the order of several kilometers under suitable conditions.[1][2]
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