Subsonic
/ˈsʌbˈsɑnɪk/
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Definition
(adj) having a frequency too low to be audible
Sentence Examples
The plane flew at a mostly subsonic speed during the trip.
Subsonic means relating to a speed that is less than sound.
CEFR Practice Quiz
The aircraft traveled at a ____ speed, which was much slower than the speed of sound and very quiet.