>Today — nearly 40 years later — communication systems have become standard kit for commercial, military, law enforcement, public safety, aquarium and scientific divers and for videographers, but they remain a niche product for recreational and technical divers. For many divers the added complexity and cost of what is essentially an underwater walkie-talkie outweighs the benefits in the absence of a mission-specific need. Others say they prefer to commune in silence.
>If you're wondering whether your next diving project might benefit from vox (voice) communication, it's worth learning a little about the technology, its implementation and use.
>There are two kinds of underwater communication systems: hardwire and wireless — or "through-water" — communications (comms). Hardwire systems are essentially waterproof intercoms that connect the diver to the surface; they're used primarily by divers on umbilical cables and in some public safety applications such as very low visibility. They are not designed for sport use.
>In contrast, through-water systems use ultrasound (typically 25-33 kHZ) to transmit and receive signals through water in the same way that walkie-talkies or mobile phones use radio waves. The density of water, which is 784 times greater than air, makes it an excellent conductor of sound energy, a fact not lost on nature — many marine animals, especially cetaceans, use ultrasound to communicate and navigate. Conversely, with the exception of extremely low frequencies, radio waves do not propagate through water.
>Originally developed for the U.S. Navy in the late 1960s, early through-water comms and their spinoffs (such as the Wet Phone) used the circuitry found in AM radios to encode voice conversations (0.3-4 kHZ) on an ultrasound carrier. However, amplitude modulation (AM), which transmits three simultaneous signals (a carrier and two sidebands), suffered from poor intelligibility because the signals would get out of sync as they traveled through the water and were reflected off various surfaces. Also divers had to remain nearly motionless in the water while talking, otherwise voices sounded garbled. These issues did not help Wet Phone sales.
>It wasn't until the mid-1980s that increasingly miniaturized electronics made it possible to use sophisticated single-sideband modulation, which transmits a single ultrasonic carrier signal to eliminate the reflection and multipath problems of AM. Single-sideband quickly became the de facto standard due to its near 100 percent intelligibility under good conditions.
>Today there are two vendors that build through-water comm systems for sport divers. Products range from all-in-one, half-watt, cigarette-pack-sized sport transceivers powered by a nine-volt battery that mount on the mask strap, to more powerful five-watt, walkie-talkie-sized units that can be attached to a diver's harness and include cabling for the mic and earpiece. Most units feature dual channels and are push-to-talk like a walkie-talkie, though some offer voice-activation as an option.
>Operational range varies from 150-1,500 feet for sport units to more than half a mile for professional comms, depending on sea conditions and noise levels. (Special military units can transmit more than three miles.) Vendors also sell receive-only units and a variety of surface stations for surface-to-diver communication.
>If you could add comms without changing anything else, it's likely that many more sport divers would be talking the talk. Unfortunately it's not that simple. To talk underwater you need an air-filled space to speak into (and place a mic), and the mouth must be unencumbered.
>Who Was That Masked Man?
>Early sport units like the Wet Phone utilized a "mouth mask," essentially a rubber pocket attached to the second-stage regulator and strapped over the mouth, but these proved to be largely unworkable. Instead, the community has moved to a full-face mask standard with comms, enabling divers to breathe through their nose and mouth.
>Today there are a variety of full-face masks on the market along with requisite training courses. Most masks feature a built-in oronasal pocket to reduce carbon dioxide (CO2) buildup, a built-in regulator and a port to attach a comm system. In addition, the majority of masks circulate breathing gas across the faceplate to keep it from fogging.
>Full-face masks offer divers a number of obvious advantages over scuba masks. They have a wider field of vision, they don't fog up, and they are warmer and more comfortable than a bite mouthpiece. They also protect the diver's airways in the event he or she goes unconscious underwater, which is a considerable safety benefit.
>The disadvantages are subtler. Mask fit and obtaining a good seal are critical. Equalization and mask clearing can be more difficult. With practice, gas consumption tends to be about the same with a full-face mask, but it increases considerably when you are talking, so good gas-management skills are required. In addition, bailout is a little trickier, particularly in cold water. The diver has to remove his mask and regulator simultaneously to use an alternative breathing source.
>Full-face masks present even more challenges for tech divers. First, they complicate open-circuit gas switching: The diver must use quick disconnects and/or a gas block to switch to his decompression gas or bailout without removing his mask. Second, despite added airway safety, the use of a full-face mask with rebreathers is at best problematic for several reasons, including fogging and an increased risk of CO2 buildup.
>Finally, comm gear is expensive: A full-face mask with a comm system can cost up to US $1,000-$2,000 per diver. As a result, recreational and tech divers tend to use full-face masks and comms for mission-specific applications. Some say the uses are growing. At present, underwater comm gear may be best suited for diving in cold or dirty water, outfitting support divers who need to be able to speak with the surface, performing long decompressions or remote in-water recompression and the occasional underwater wedding. Gadget geeks also enjoy these devices.
>Being able to talk underwater adds a layer of safety, coordination and control that may not be achieved otherwise, making underwater communications systems an indispensible tool for working divers. But given the added complexity and cost of the systems, most sport divers remain content to rely on hand and light signals to convey essential information and save their talking for after the dive.
>© Alert Diver — Q2 Spring 2015