Hyperbaric is a term that refers to an environment of high pressure. Its antonyms, then, are terms that describe environments of low pressure. One antonym could be hypobaric, which refers to an environment of low atmospheric pressure. Another antonym is subatmospheric, which means "below atmospheric pressure," and describes environments with a lower pressure than normal. Other antonyms include vacuous, which means "empty" or "void of matter," and anechoic, which refers to environments with no echo or sound reflection. Ultimately, the antonyms for hyperbaric all refer to environments that are the opposite of high-pressure environments, whether they are empty, low in pressure, or lacking in sound reflection.