An evaluation of three helium speech unscramblers to a depth of 1000 feet

This paper presents the results of speech intelligibility tests conducted during two simulated dives in a helium-oxygen environment and in a tri-mixture environment of helium, nitrogen, and oxygen at depths from 800 to 1000 feet of sea water. In order to avoid the narcotic effect of nitrogen at these depths, a large percentage of the nitrogen is replaced by helium. The introduction of helium into the breathing mixture seriously distorts the speech signal (primarily due to the greatly increased sound velocity in this mixture). To correct this problem, a number of devices (helium speech “unscramblers”) have been designed to return this distorted speech to a more natural and intelligible form. In the present study, it was found that several of these units are effective at depths near 1000 feet of sea water; however, their effectiveness is strongly influenced by the electroacoustic characteristics of the microphone. In addition, a significant four-way interaction among the diver/talkers, listeners, microphones, and unscramblers was discovered. This would seem to indicate that communications training of diving teams, consisting of both divers and surface personnel, would be desirable.