This is considered an important safety feature, as the surface team can monitor the diver's breathing sounds, which can give early warning of problems developing, and confirms that the diver is alive.
Through water communications systems are more suitable for scuba as the diver is not encumbered by a communications cable, but they can be fitted to surface supplied equipment if desired.
Underwater voice communication protocol is like radio communication protocol.
Dating hand signals
Hand signals are generally used when visibility allows, and there are a range of commonly used signals, with some variations.
A hard-wired diver communications unit mounted in a waterproof box for convenience of transport and protection.
Hand signals are a form of sign system used by divers to communicate when underwater.
Hand signals are useful whenever divers can see each other, and some can also be used in poor visibility if in close proximity, when the recipient can feel the shape of the signaller's hand and thereby identify the signal being given.
At night the signal can be illuminated by the diver's light.
Hand signals are the primary method of underwater communication for recreational scuba divers, and are also in general use by professional divers, usually as a secondary method.
Most through water systems have a Push To Talk (PTT) system, so that high power is only used to transmit the signal when the diver has something to say.
For commercial diving applications this is a disadvantage, in that the supervisor can not monitor the condition of the divers by hearing them breathe.
Two wire systems use the same wires for surface to diver and diver to surface messages, whereas four wire systems allow the diver's messages and the surface operator's messages to use separate wire pairs.