Question

What are infrasonic and ultrasonic waves?

Answer 1

the sound waves with frequency less than 20Hz is called infrasonic.
the sound waves with frequency more than 20000 Hz is ultrasonic.

Answer 2

Human beings can listen to only those sound waves which have frequency lying between 20Hz to 20000Hz. 
The sound with frequency less than 20Hz is called infrasonic waves.
And the sound with frequency greater than 20000Hz is called ultrasonic waves.

Both infrasonic and ultrasonic waves are not audible to humans.