SUBSONIC • DICCIONARIO INGLES.COM • Definition of Subsonic

... He listened to the innumerable noises around and below him. The clicks and hums near him meant that Ruiz, having given up following him, was checking out the flight controls, with power on only in the top stage. From below came a vibrational rushing noise, nearly subsonic, which told him of the fueling operation. He thought of the electrical relays governing the fuel input and shuddered. He violently disliked the idea of having hot wires near fuel of any kind, and rocket fuel ...

— Tight Squeeze • Dean Charles Ing