subsonic

Atoms going around 62 miles per second come from the inner heliosheath, where the solar wind becomes subsonic as it hits against the interstellar medium.

These airplanes are expected to seat 65 to 88 passengers and will travel at subsonic speeds over land and supersonic speeds over water—more than twice as fast as current commercial aircraft.

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.