With regard to horns, what is flare rate frequency?

Examples, Steve Schell's "Big Ed" is a 15Hz flare rate, my pals Leviathan's are a 48Hz flare rate, not the LF cutoff, but flare rate in Hz.

I have scoured this and other sites, just bought the 2008 Handbook for Sound Engineers at the recomendation of a friend, and a week and $104.00 later, have not yet found it.

Its driving me crazy, the term is used like everyone learned it in grade school, I missed that somehow!

Thanks all.