What is the X in X-mas?
1923, "having frequency beyond the audible range," from ultra- + sonic. For sense, see supersonic.
ultrasonic ul·tra·son·ic (ŭl'trə-sŏn'ĭk)adj.
Of or relating to acoustic frequencies above the range audible to the human ear, or above approximately 20,000 hertz.
Of, relating to, or involving ultrasound.