The energy to which the human ears are sensitive is known as sound. In general all types of waves are produced in an elastic material medium, Irrespective of whether these are heard or not are known as sound.
According to their frequencies, waves are divided into three categories :
(1) Audible or sound waves : Range 20 Hz to 20 KHz. These are generated by vibrating bodies such as
vocal cords, stretched strings or membrane.
(2) Infrasonic waves : Frequency lie below 20 Hz.
Example : waves produced during earth quake, ocean waves etc.
(3) Ultrasonic waves : Frequency greater than 20 KHz. Human ear cannot detect these waves, certain
creatures such as mosquito, dog and bat show response to these. As velocity of sound in air is 332 m/sec so the wavelength of ultrasonics λ< 1.66 cm and for infrasonics λ>16.6 m.