This is a spectral graph of 2s worth of each whistle being blown (40mm,30mm,25mm,20mm). You can see clear bands (harmonics) in the spectral domain. Using this I found that the 25mm whistle produced beautiful bands in the range we want (clear harmonics at 20kHz, 28kHz, 35kHz, and 45kHz). The thickness of the bands depicts the magnitude of the frequencies in the band.
You can see the largest portion of our 25mm whistle's frequencies are in the 0-10kHz range. This area just sounds like white noise and without the spectral analyzer you might not know there's harmonics at all.
You might notice that the 20mm spectrum has more frequencies in higher ranges, but we want clearly defined bands to maximize the wolves' annoyance. Just by visual analysis you can tell that the bands in the 25mm whistle are the clearest and should, theoretically, have the most effect.
The build was constructed entirely from trial and error since the size was so small.