My everyday carry is the Acme 560. To round out my options on a hike, I might add the Acme 649 or the Tops, which have relatively deep tones. I have the Storm. It is my overall "loudest" whistle, but it never gets carried because it is so big. I own about 20 other whistle models.

Although people want to simplify a whistle down to decibels, the perceived loudness of a whistle is very subjective. Loudness is often vastly different to the person blowing the whistle compared to people listening from a distance. Whatever you hear from zero feet away is irrelevant. The deeper tones will sound soft up close, but will carry farther in the wilderness. Do you care more about being heard from 20 meters away or from 1,000 meters away? Also, many older folks cannot hear the higher pitches.

I did my own testing with whistles of various pitches at distances of 400 meters and more. I was surprised by the results. By the way, I tested ease-of-blow by allowing my 2-year-older to try. She could not make a sound with some whistles, a few of them being popular models. I find myself not carrying those whistles. I cannot get the image out of my head of her stuggling with those whistles, while blowing other whistles easily.

This is a practical, simple, cheap experiment for a student to do for science day or whatever they call it nowadays. The student could get rather technical in the theory if need be.