The sound of frequency less than 20 Hz is called :
(a) supersonic
(b) infrasonic
(c) ultrasonic
(d) ultrasound

(b) infrasonic

Reason — The sound of frequency in the range 20 Hz to 20 kHz is called the sonic or audible sound; the sound of frequency less than 20 Hz is called infrasonic and greater than 20 kHz is called ultrasonic.