What are supersonic and infrasonic?

Supersonic means "faster than the speed of sound" and is usually applied to atmospheric sound, since sound vibrations travel more quickly through solids.

Infrasonic means "at a lower frequency than human hearing typically detects" and the connotation is that the sounds are inaudible. The opposite adjective is ultrasonic (at too high a frequency to hear) and likewise the sounds would be inaudible to humans.

Supersonic and ultrasonic are not true synonyms because they are applied to different phenomena. For example, ultrasonic is used for rapidly vibrating toothbrush heads, primarily because they are vibrating at high speed, not traveling at high speed.