subsonic

adj

1. (not-comparable)having a frequency too low to be audible
2. (literary, not-comparable)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. (not-comparable)(aviation) less than the speed of sound (in the same medium, and at the same temperature and pressure)

   “But British Airways Flight 112 made the trek in under five hours, setting a new record for the fastest subsonic commercial flight across the Atlantic Ocean.”

4. (not-comparable)with a Mach number such that 0.3 < Ma < 0.8 (approximately - the exact limits vary between sources)

noun

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