Acentech, a nationally recognized multi-disciplinary acoustics, audiovisual system design, and vibration consulting firm, announced that it has received final funding on a three-year, $706,000 project for the U.S. Naval Sea Systems Command to develop technologies that reduce noise experienced by divers in dive helmets.
The undersea environment is full of biological and man- made sound. Inside dive helmets, noise levels, due to diver breathing or the use of power tools underwater, are high enough to rank among the Navy’s top ten noisiest work environments, posing a threat to diver hearing and well being. To minimize the danger, the “bottom times” allowed for undersea missions are limited to avoid a standard maximum allowable daily “noise dose.” Nevertheless, there is evidence that Navy divers can develop hearing damage over their careers, which contributes to the $2.5 billion annual costs borne by the Veterans’ Administration for treatment of hearing loss and tinnitus. Commercial divers face the same danger.
Acentech’s program seeks to reduce the sources of helmet noise and to interrupt the paths this noise takes to divers’ ears. The primary source of respiration noise is the dive regulator, which delivers breathable air at ambient water (depth) pressure to the diver and allows him or her to exhale, forming the familiar bubbles. While it may seem that the bubble noise is the loudest part of the breathing cycle, as the diver descends the critical “A-weighted” measure of the inhalation noise becomes louder than the bubble noise, making it the primary concern.
“Means for quieting the dive regulator are being developed and tested. However, the challenge of ensuring that any specified changes do not in any way compromise the operation of the helmets or regulators remains ahead of us,” said Acentech Program Manager, Dr. Steven Africk. “Our priority is the continued comfort and safety of the divers who rely on these helmets every day for underwater life support.”
Acentech’s mitigation approach has been to determine the sources of this noise – mostly high velocity airflow through the regulator – and to quiet the airways through which air is delivered to the diver. This includes the addition of metallic screens to, and changes of the geometry of, the airways. Means to attenuate the paths that carry the noise from the regulator to the ear are also under development. These include expansion chambers similar to mufflers and heavier structures through which the noise must travel once inside the helmet. Acoustic treatments on the outside of the helmets have also been developed to exclude tool noise from entering the interior. Experimental work to study the noise and demonstrate the effectiveness of modifications has been carried out in Acentech’s laboratory in Cambridge, Mass. and at Dive Lab, the Panama City, Fla. research facility of the helmet manufacturer, Kirby Morgan Inc.