I see 15 Kilowatt audio amplifiers advertised, therefore there must be a speaker system, public address horns available for such amplifiers.

If one of these speaker horns was directed line of sight towards the horizon or a mountain top and a suitable dish positioned at the receive end, it must be possibel to intercept such a signal at audio frequencies. eg Transmit a tone which could be keyed at 9 - 20 Kcs and see what distance could be covered.

Keeping the signal just above the normal human audio range would avoid interference.

or just TX a signal at say 5 Kcs and see how far it could be heard by ear with this sort of power, maybe further than the same signal transmitted by radio means, except one had an exceptionally large antenna

Maybe a NOV is required for this experiment!!!!

de g3kev