(i) (a) Infrasonic wave The sound of frequency lower
than 20 Hz are known as infrasonic sound (or wave).
(b) Ultrasonic wave The sound of frequency higher than 20000 Hz are called ultrasonic wave.
(ii) (a) Animals like whales, elephants and rhinoceroses produce infrasonic sound of frequency 5 Hz.
(b) Dogs can hear and detect ultrasonic wave of frequency up to 50 Hz.