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

1940–45; sub- + -sonic

subsonically, adverb Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
subsonic (sʌbˈsɒnɪk)
being, having, or travelling at a velocity below that of sound: a subsonic aircraft

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
subsonic  [%PREMIUM_LINK%]     (sŭb-sŏn'ĭk)  Pronunciation Key 
Having a speed less than that of sound in a designated medium, usually air; having a velocity less than Mach 1. Compare hypersonic, supersonic, transonic.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Flight engineers define three categories of speed: subsonic, supersonic, and hypersonic.
So the region where the flow changes from subsonic to supersonic is an event horizon.
Also, supersonic speeds produce shock that is not a part of subsonic velocities.
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