Whistle

A whistle is audible over a longer distance than a human voice (even when shouting) and makes it much more possible to be heard. A pealess whistle is recommended because the whistle is much louder.

Dive Alert ®
(air horn)

This type of acoustic device makes more noise than a whistle but it needs the air from a dive cylinder in order to work. There are several types of air horns on the market and they are connected between the inflator hose and inflator of a BCD.