Denoting speed greater than speed of sound; pertaining to vibrations above the audible limit.
1. Having frequencies above those of audible sound; SYN. ultrasonic.
2. (Of speed) Greater than that of sound in a given medium especially air

Close to the speed of sound

Denoting air-waves or vibrations with frequencies higher than those audible to the human ear, i.e. more than about 20,000 cycles per second.