Correction of Helium Speech Distortions by Real-Time Electronic Processing

The use of helium in breathing gas mixtures for deep diving operations causes marked changes in the speech sounds of the divers. The most prominent effect is an elevation of the formant frequencies which, in conjunction with other phenomena, progressively reduces speech intelligibility as depth increases. Our analyses and investigations have shown that to correct these distortions, the following measures are required: 1) provide a passband extending to at least 10 kHz; 2) equalize high frequency losses in vocal output and in microphone response; 3) reduce voice spectrum envelope frequencies to their expected values for 1.0 atm of air; and 4) preserve the glottal rate of the talker. Recent advances in commercially available electronic signal analyzers have made possible the recording of voice spectra averaged over a wide range of time intervals (50 ms-50 s), over a wide range of frequencies (to 20 kHz) and over a total amplitude range exceeding 100 dB. These capabilities are essential in exploring the total extent of distortions that helium introduces. Analyses of helium speech by means of long term average spectra have revealed phenomena not previously reported. An electronic helium speech processor (unscrambler) incorporating appropriate corrective measures has been developed and evaluated. Its performance culminated in successful retrieval of voice intelligence from divers at 1000-ft pressure depth. Objective word intelligibility test results for corrected speech of one talker were 88 percent at 500-ft and 78 percent at 800-ft depth.