noun, noun which may take the genitive case particle 'no'
subsonic speed
Technical term used in physics and engineering to describe speeds below the speed of sound. Often used attributively with の.
この飛行機は亜音速で巡航する。
This aircraft cruises at subsonic speed.
亜音速の流れを解析する。
Analyze subsonic flow.
Means the speed of sound itself; 亜音速 is the range below that speed.
Compound of 亜 (a, 'sub-') + 音速 (onsoku, 'speed of sound'). A modern technical coinage.