If it's anything like the most common fog horn that the USCG uses across the country, it's a 273Hz tone (though I don't recall the waveform, most likely sinusoidal).
As of 8 years ago, they were using the FA-232 model fog horn, the circuitry was designed in the '70s and uses discrete BJTs on the boards to generate the tone and drive the "speaker".
As of 8 years ago, they were using the FA-232 model fog horn, the circuitry was designed in the '70s and uses discrete BJTs on the boards to generate the tone and drive the "speaker".