Lower frequencies tend to be able to travel around obstacles (and through media) more easily. If I am remembering correctly (and correct me if I'm wrong), "VLF" stands for "Very Low Frequency", "VHF" stands for "Very High Frequency", and "UHF" stands for "Ultra High frequency" (very imaginative names, right?
). This is (I would assume) why it's used underwater and over long distances. Audio frequency (20hz-20khz) waves actually travel faster through water than through air, and since VLF is between 3khz-30khz, it would actually travel faster through a medium such as water than through air.
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