 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.