Submarine Base, Groton, Conn. REPORT NUMBER 602 AN EVALUATION OF THREE METHODS FOR UNSCRAMBLING HELIUM SPEECH PRODUCED AT DEPTHS OF 800 AND 1000 FEET

Three systems for "unscrambling" helium speech produced at simulated depths of 800 and 1000 feet were evaluated. Recordings of the Modified Rhyme test read by an experienced male diver in a pressurized helium atmosphere were processed through three commerical frequency-shifting devices. The intelligibility of words passed through these systems was compared to scores obtained when the same helium recording was presented directly without alteration and when presented at one-half playback speed to groups of listeners. A within-subjects' design was employed with suitable counterbalancing so that no word was heard twice by the same listener. Half speed playback (58.5% intelligibility) was significantly superior to all other conditions. The Kay Electric Company "Varivox" (43.2% intelligibility) was significantly inferior to the other two commercial systems. The Industrial Electronics Corp. and the Gotham Audio Corp. units were equal in performance (50.7^ and 51.0% intelligibility respectively) and superior in intelligibility to unaltered helium speech (45.7%). The increase in the degree of intelligibility imparted by the three commercial systems is limited at the depths under study. Methods for adapting these units for improved operation at greater depths are suggested. AN EVALUATION OF THREE METHODS FOR UNSCRAMBLING HELIUM SPEECH PRODUCED AT DEPTHS OF 800 AND 1000 FEET INTRODUCTION Efforts to "unscramble" helium-speech have led to the development of commercially available systems which lower the frequencies of helium speech to a more natural range. At present, these mechanical, electronic or electromechanical systems have a limited range through which frequencies can be translated. Since with increased depth the introduction of helium breathing mixtures causes a very considerable elevation in the frequency components of speech, the working capability of these units may well be exceeded. The present study evaluated three such systems for unscrambling helium speech, on the basis of the intelligibility of the speech at depths of 800 and 1000 ft. For comparison, a half-speed tape playback and an unaltered version of helium speech at these depths were included. The units studied were the Kay Electric Company "Varivox," the Eltro Company Rate-Changer (sold in the U.S.A. by Gotham Audio Corp. as the "Tempophon"), and the Integrated Electronics Corp. Model 701 Helium Voice Unscrambles Both the Varivox and Eltro units use a segmented rotating head which electromechanically splices tape so as to lower the frequency positions of recorded signals by reducing the relative speed of short sections of tape which contact energized segments of the playback head. Except for the short delay in speech reception caused by the distance between record and playback heads, real time is preserved. The all-electronic IEC System, on the other hand, achieves real time by using heterodyning techniques which lower frequency. The signal is divided into two separate frequency bands, heterodyned and finally reconstituted for real-time reception. *Normal speech produced by loudspeaker and picked up by this microphone at pressures equivalent to 1000 ft submergence was easily understood and instantly recognized as a reproduction of normal speech without distortions due to depth or helium. PROCEDURE The voice of an experienced male diver was recorded from inside a pressurized test chamber containing the gas mixtures shown in Table I, at pressures equivalent to those found at 800 and 1000 ft of water. Recordings were made of the diver's voice when he read the same 50-word list from the Modified Rhyme Test (MRT) twice at both depths, once with the microphone connected directly to a General Radio Data Recorder located outside the chamber, and then again with the IEC unit between the microphone output and the recorder. The microphone was especially constructed by IEC for use with their unscrambler under conditions of high ambient pressure.* For each of the two depths a working master tape was produced, which consisted of three identical re-recordings of the originally recorded unaltered helium-speech transmission and one re-recording of the originally recorded IEC alteration. An additional rerecording of the unaltered helium transmission, played back at half original speed, was also included on each tape. Ten stimulus tapes for presentation to groups of listeners were finally prepared, each consisting of the same word list presented at either of two depths, under one of five test conditions: (1) unaltered helium speech, (2) half-speed playback, (3) Eltro alteration, (4) Varivox alteration, and (5) IEC alteration. Field operation of the IEC system in this study was by IEC technicians. The Varivox and Eltro units were adjusted for optimum performance and operated in this laboratory by the authors. The intelligibility scores reported here are by no means representative of the performance of any of the three systems at the much shallower depths for which they were designed. Since all ten conditions (five at each depth) TABLE I