having a frequency too low to be audible
with a Mach number such that 0.3 < Ma < 0.8 (approximately - the exact limits vary between sources)
very quiet, almost inaudible
(aviation) less than the speed of sound (in the same medium, and at the same temperature and pressure)
an aircraft whose maximum speed is less than the speed of sound
Beyond (higher in frequency than) the range of sound perceptible to the human ear; with a frequency of 20 kilohertz or higher.