subsonic

From sub- +‎ sonic.

• Rhymes: -ɒnɪk

subsonic (not comparable)

1. (of a sound, scientific) having a frequency too low to be audible
2. (of a sound, literary) very quiet, almost inaudible

   The noise of the ship was all around her. The faint subsonic rumble of the reactor and drive. (Leviathan Wakes, James S. A. Corey)

3. (of a speed) (aviation) less than the speed of sound (in the same medium, and at the same temperature and pressure)
4. (fluid mechanics, of a flow) with a Mach number such that 0.3 < Ma < 0.8 (approximately - the exact limits vary between sources)

subsonic (plural subsonics)

1. an aircraft whose maximum speed is less than the speed of sound

Borrowed from French subsonique.

• IPA^(key): /subˈso.nik/

subsonic m or n (feminine singular subsonică, masculine plural subsonici, feminine and neuter plural subsonice)

1. subsonic

   Synonym: subacustic

   Antonym: hipersonic