This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. Of less than audible frequency.

adj. Having a speed less than that of sound in a designated medium.

adj. having a frequency too low to be audible

adj. less than the speed of sound (in the same medium, and at the same temperature and pressure)

adj. (of speed) less than that of sound in a designated medium

"There are standard pressure loads, so-called "subsonic" loads, and ultra fast, high pressure, +P and +P+ loads."

plasmasonic    hypersonic    transonic    supersonic   

supersonic    ultrasonic    caseless    low-frequency    infrasonic