As adjectives the difference between subsonic and infrasonic
is that subsonic
is (of a sound) having a frequency too low to be audible while infrasonic
is (of sound waves|acoustics) having frequencies below the human audible range.
(of a sound) having a frequency too low to be audible
(of a speed) less than the speed of sound (in the same medium, and at the same temperature and pressure)
(fluid mechanics, of a flow) with a Mach number such that 0.3 < Ma < 0.8 (approximately - the exact limits vary between sources).
(of sound waves, acoustics) Having frequencies below the human audible range
(of a device) Generating or employing such sound waves