This is interesting, but there's data I would like to see. What does the audio output of each whistle look like? What does the background noise look like? I would think it's not the absolute sound that matters, but the signal to noise ratio of the sound of the whistle at the frequency of the whistle.

Also, the data from the distance/SPL chart doesn't make sense to me, except maybe to indicate attenuation. The distance scale is in linear units, but SPL is on a log scale. Without having the actual numbers, the difference in SPL from high to low looks like about 48dB, or 63000:1. Since audio power should diminish with the square of the distance (I think it works like light), that should mean a distance difference of 251:1, but it's not even close to that.

So what's the spectrum of the whistles, spectrum of the background noise, and the loss in the path.

I don't want to discount anyone's hard work, but I don't know how to make any use of this, except to point out that further testing would be interesting.