Underwater noise levels inside dive helmets can be high enough to rank helmets among the Navy’s top ten noisiest work environments, and can pose a threat to a diver’s hearing and well being. This might sound unexpected as popular media often describes the undersea world as silent. However, this environment is full of biological and man-made sound. Acentech performed a three year project for the U.S. Navy to develop technologies to reduce noise experienced by divers inside dive helmets.
To minimize the danger of hearing loss, the “bottom times” allowed for missions are limited to avoid the standard maximum allowable daily “noise dose.” Commercial divers face the same hazard. Acentech’s program sought to reduce the sources of diver helmet noise and to interrupt the paths the noise travels to the diver’s ears, while working closely with the manufacturer to ensure that noise reduction techniques developed did not affect helmet respiration functions or compromise diver safety in any way.
The primary component of dive helmet noise is simply that of respiration generated by the dive regulator, which delivers breathable air for inhalation at ambient water (depth) pressure and allows the diver 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 –which determines hearing damage risk — becomes greater than that of the bubbles, making the former the primary concern. By measuring noise in helmets at Kirby Morgan’s Dive Lab R&D facility and by laboratory testing in Acentech’s reverberant room test facility it was determined that the major source of inhalation noise is high velocity airflow through the regulator and that of exhalation noise was bubble formation at the regulator exhaust “whiskers.“ Paths from these sources to the diver’s ear were identified. Means to reduce airflow sources by redesign of the geometry through which the air travels in the regulator and to attenuate the paths that carry the noise from the regulator to the ear with porous screens and acoustic expansion chambers similar to mufflers were identified. Acoustic treatments on the outside of the helmets were also developed to exclude tool noise from entering the interior.