subsonic

1. (of a sound, scientific) having a frequency too low to be audiblenot-comparable

2. (of a sound, literary) very quiet, almost inaudibleExamples: "The noise of the ship was all around her. The faint subsonic rumble of the reactor and drive. (Leviathan Wakes, James S. A. Corey)"literarynot-comparable

3. (of a speed) (aviation) less than the speed of sound (in the same medium, and at the same temperature and pressure)Examples: "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."not-comparable

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