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Voice communication becomes extremely difficult, if not impossible, when helium-oxygen breathing mixtures are substituted for air in deep-sea diving to overcome physiological problems. The effects of helium and pressure on speech intelligibility are discussed. With the helium voice unscrambler described, effective speech conversation was conducted with various helium-oxygen mixtures at depths to 300 feet. The unscrambler is useful even when high ambient acoustic noise is present, as in "hard-hat" diver applications.