From Longman Dictionary of Contemporary Englishsubsonicsub‧son‧ic /ˌsʌbˈsɒnɪk◂ $ -ˈsɑː-/ adjective TTTMslower than the speed of sound → supersonic
Examples from the Corpus
subsonic• subsonic aircraft• A lot of dance music is using sub bass, subsonic sounds.• Sure enough, he could make out the same almost subsonic throbbing as he had heard earlier. 