According to their frequencies, waves are separated into three categories:
(1) Audible or sound waves : Range 20 Hz to 20 KHz. These are formed by vibrating bodies like as vocal cords, membrane or stretched strings.
(2) Infrasonic waves : Frequency lie below 20 Hz.
Example : waves formed during earth quake, ocean waves etc.
(3) Ultrasonic waves : Frequency greater than 20 KHz. Human ear cannot listen these rays, certain creatures such as dog, bat and mosquito show result to these.