Synonyms of the word 
INFRASONIC → INAUDIBLE - UNHEARABLE
infrasonic
- adj. (of sound waves, acoustics) Having frequencies below the human audible range.
- adj. (of a device) Generating or employing such sound waves.
inaudible
- adj. Unable to be heard or not loud enough to be heard.
unhearable
- adj. That cannot be heard.
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