In the ARRL Ham Radio License Manual (4th Edition) an Amateur Allocation chart labels VLF with "Audible Range":
Very Low Frequency (VLF) 0Hz 10Hz 10kHz 100kHz Activities [ Audible Range ] <- Infra-sonics -> <- Sonics -> <- Ultra-sonics ->
Is there a reason this is included? The other activities listed are referencing EM waves: Microwaves (in the photo), Infrared, and X-rays. I agree that if this were an acoustic wave these frequencies would be in the audible spectrum.
The wikipedia page for VLF amateur use mentions QRSS, MFSK, and coherent BPSK as the modes used. I was at first thinking the carrier wave with no modulation could be associated with sound somehow. Maybe submarines (not amateur!) interpret the carrier wave as a continuous tone as it relates to some existing system. Maybe it means the mathematics of digital audio can be applied here, so traditional sound cards can be used as receivers.
Or maybe it is just a reference to something more familiar, like comparing a wavelength to the size of the Eiffel Tower, and I'm reading into it too much. Are there some audio applications with VLF?