Answer: subsonic

AnswerCrossword Clue
SUBSONICBelow Mach 1
SUBSONICNot so fast?
SUBSONICSlower than Mach 1
SUBSONICAt turbo-prop speed
SUBSONICNot like an SST
subsonicnoting or pertaining to a speed less than that of sound in air at the same height above sea level
subsonicmoving at a speed less than that of sound
subsonicnoting or pertaining to a sound wave with a frequency below the audio-frequency range