The sounds having frequency more than 20,000 hertz (Hz) are called ultrasonic or ultrasound and the sounds having frequency less than 20 Hz are called infrasonic or infrasound. Both of these sounds cannot be heard by human beings. The audible range for of hearing by human beings is 20 Hz to 20000 Hz.
The speed of sound is same for all frequencies in a particular medium. So, speed of ultrasonic sound and infrasonic sound are same in a particular medium.