As part of an SBIR Phase II project to develop novel applications for advanced transducer materials, Acentech developed an in-ear earphone for underwater communications for scuba divers. The goal was to replace standard diver communications receivers consisting of about a cubic inch of piezoelectric material embedded in a plastic casing and worn on the top of a dive mask. These are somewhat bulky, and they also radiate sound away from the diver which can limit their use during operations requiring diver stealth.
Acentech’s approach began with a previous design we had developed for a high output driver employing highly innovative single-crystal PMN-PT bimorphs for an active noise reduction earplug for Air Force and Navy personnel on aircraft carrier decks – possibly the most hazardous acoustic work environment anywhere. However, a key requirement for that device – the need for a tight seal in the ear canal to keep much of the sound out – was the opposite of what was needed for scuba which was a very open design that allows water to flow past the earphone to allow for easy pressure equalization as the diver moves down or up in the water column. This created a very strict volume limitation for an in-ear device; not only did the earphone have to fit in the ear canal, but it had to be even smaller to allow for pressure equalization. The need to operate at pressure also meant that the earphone had to be waterproof, pressure tolerant and generally more rugged than the in-air device. Fortunately, we were able to identify a class of cylindrical piezoelectric drivers with the right size, and with acoustic output comparable to that of the PMN-PT designs. These drivers consist of many extremely thin layers of standard PZT, each separately driven, with the net output proportional to the number of layers instead of to the total volume of piezoelectric material. Once encapsulated into waterproof assemblies, the resulting prototype devices provided clear communications when tested underwater with snorkel gear and a full face scuba mask. Because they are inside the ear canal, their radiated noise during communications is expected to be no greater than the respiration noise made by scuba gear or self-contained rebreathers.