subsonic

Definitions of subsonic
  1. adjective
    (of speed) less than that of sound in a designated medium
    “aircraft flying at subsonic speeds”
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    Antonyms:
    sonic, transonic
    (of speed) having or caused by speed approximately equal to that of sound in air at sea level
    supersonic
    (of speed) greater than the speed of sound in a given medium (especially air)
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