Answer-The waves which are less than 20Hz are known as

The sounds of frequency less than 20Hz are called infrasonics. The infrasonic sounds have below the lower limit of human hearing. The audible range of human ear is 20 Hz to 20000 Hz. These are called Audible waves. The sound of frequency more than 20,000Hz are called ultrasonics. Ultrasonic sounds have a frequency above the upper limit of human hearing.