subsonic

[ suhb-son-ik ]

Phonetic (Standard)IPA

adjective

noting or pertaining to a speed less than that of sound in air at the same height above sea level.
infrasonic.

subsonic

/ sʌbˈsɒnɪk /

being, having, or travelling at a velocity below that of sound
a subsonic aircraft

subsonic

/ sŭb-sŏn′ĭk /

Having a speed less than that of sound in a designated medium, usually air; having a velocity less than Mach 1.
Compare hypersonic

From below came a vibrational rushing noise, nearly subsonic, which told him of the fueling operation.

A wave of some sort—probably subsonic or supersonic—continuously filled the Vininese atmosphere.

Most of their conversation is carried on in tones of subsonic frequency, so your ears cannot hear it.

And its booming notes descended to subsonic throbbings that gripped and wrung the nerves to anguish.

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