Larry, I'm not an acoustical engineer, but the definition should still bore you to tears. The column heading of "dB referred to 2◊10−5N/m2" means standard sound pressure level referenced to human threshold of hearing (20 micro-pascals). The "1/3-octave band" refers to specified 450-800 hertz (first harmonic of the fundamental tone).
Taken all together, it means what I wrote earlier: a properly calibrated meter placed one meter in front of the horn of your compliant whistle should read 115 dB or higher. It doesn't tell you anything more, or whether dual tone carries farther or better. Personally, I think dual tone horns sound fuller and more melodic.
So, bottom line, the horn should be slightly higher in pitch than middle C (not lower, as I wrote earlier), and pretty dang loud, but doesn't need to be painfully loud.