The Fox 40 whistle has a patented 3 chamber design and no "pea" to stick and impede sound. The harder you blow, the louder the sound. Used in professional sports and by the Olympics, this whistle is the perfect safety item to have on hand at the job site.
Carry it with you while in the tree and be able to signal for help that can be easily heard over the noise of chippers and chainsaws. Also good for ground men to signal climbers of dangers or to communicate over the job site sounds.