To state the obvious: water, being a denser medium than air, is a good conductor of sound. This property has both advantages and disadvantages to the diver. On the plus side, sound is magnified and can be heard over vast distances. The downside is that because sound travels more quickly through water, the source and direction may be difficult to determine.
A system that lends itself to basic communications, regardless of visibility and distance between the divers, sound signals are the simplest form of underwater communications. Many dive knives still feature a metal boss that can be used to tap against the sides of a cylinder; elastic tubing threaded with hardwood beads can be placed around the cylinder and used as a tank-banger; or attention-getting devices such as an air activated gadget that attaches between the H.P. hose and the B.C. inflator hose. All of these are effective in diver-to-diver communications, either as a stand-alone system based on a pre-arranged code, or to attract the attention of a buddy as a prelude to using, say, hand-signals.
Applying the K.I.S.S. (Keep It Simple, Stupid) principle to as much of diving as possible, sound codes are something that should be agreed upon in advance of the dive and reduced to an elementary minimum. Just as with Sign language, knowledge of the Morse code has little practical relevance for most divers. In that regard, basic line signals (see previous blog article) translate well into sound communications.
Sound can also be used effectively in surface-to-diver communication and is especially useful in recalling divers’ back to the surface. Because of environmental considerations – as well as restrictions on their availability – explosive devices like ‘thunderflashes’ that can be weighted to detonate at depth are now seldom used.
Metal diving ladders – or, at a pinch, the metal hulls of vessels – can be used as sound conductors by hitting them with a large hammer, wrench, or spare dive weight. More expensive to justify and maintain, underwater speakers and purpose-designed horns perform a similar function.
Attracting the attention of the shore or boat – or that of other divers – while on the surface, is made easier when a diver carries a whistle for just such purposes. More easily heard than a shout, whistles have the added advantage of being more efficient in terms of the energy that a diver expends in attracting attention. Their use also reduces the risk of distress through inadvertently swallowing water while the mouth is wide open.
Primarily used by occupational divers spending extended periods of time at depth, or those – like military, police and rescue divers – with other special needs, voice communication systems fall into two broad categories, hard-wire, (those in which sender and receiver are linked by a carrier cable) and wireless.
While each lends itself to particular diving applications, both suffer from the fact that water is an unsympathetic medium when it comes to electronics and the transmission of intelligible sound.
The prime requirement in voice communication systems is the need for an air pocket in front of the diver’s mouth. This is seldom an issue in surface supplied diving where helmets and full-face masks are used, and where the hard-wire cable forms part of the umbilical system. But even then communications may be hampered by ambient noise from the inlet valve, by exhaust bubbles, or even by the density of the gas being breathed. (In helium rich atmospheres, for example, voice distortions are corrected through the use of a compensating “de-scrambler”.)
Although better suited to the needs of un-tethered SCUBA divers, wireless systems pose their own problems in terms of through water communications. Most incorporate an oral-nasal pocket and require the diver to carry a transducer, a power pack, microphone and a receiver, items that all have their own failure points. Although the introduction of digital technology has improved the quality of reception, many have limited range and effectiveness. (Thermoclines, for example, may act as a barrier to receiving or transmitting a message.)
Usually expensive and high maintenance items that – in their present state of development – seem to offer few cost/benefit advantages for most recreational diving situations, voice systems depend on distinct, unhurried and clearly enunciated speech. Neither are they really suited to providing a running commentary on a dive, particularly in a SCUBA application where talking will quickly deplete the available gas supply. Ideally voice messages should be brief and relevant – and repeated back by the receiver to indicate that they have been properly understood. In that regard voice communications only complement the need that divers have to know a broad range of underwater ‘languages’.
Hardly a method that’s to be recommended – and one that best lends itself to divers with short hair and not wearing a hood – SCUBA divers with conventional regulator mouthpieces can ‘speak’ to each other by placing their heads together – side by side while facing one another – and then, while talking into the mouthpiece of the regulator, allowing the sound to be conducted through the skull bone above the ear. While this technique can be very effective, it’s one that should be used judiciously. Its use may communicate the wrong impression to other divers who witness two of their companions apparently locked together in an underwater embrace.