Nitecore NWS10 TC4 Titanium Outdoor Emergency Whistle 120 Decibeles (NWS10 Emergency Whistle)
This is made from a solid block of titanium alloy, so there's no moving parts, and nothing to lose or replace. The small, streamlined air pipe is easy to blow and will produce a much louder sound with less effort. This means you can blow this whistle for longer periods without exhaustion.