subsonic

having a frequency too low to be audible

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)

(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.

with a Mach number such that 0.3 < Ma < 0.8 (approximately - the exact limits vary between sources)

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