SilverstrikeQuoting from Kickstarter:
"The ‘three flute’ design provides three specific frequencies designed to increase sound levels over distance, cut through background noise caused by weather, rushing water, air and ground traffic, and other distracting noises. The primary central frequency of 3150 Hz is per government regulations as providing the most piercing sound over the widest range of detrimental conditions. The higher and lower frequencies are designed to fill the overall acoustical profile at frequencies between the primary 3150 Hz output and its various harmonics both above and below the primary."
https://www.kickstarter.com/projects/1838656053/tactical-whistle
Maybe I'll join this for that bug-out-bag I'll hopefully never need but can't quite resist setting up. Though I would prefer titanium...