# What is Sound Waves , Infrasonic and Ultrasonic Waves

Sound waves : Sound waves are mechanical waves. They require a medium for their propagation i.e. they cannot propagate in vacuum.
As we have discussed earlier, in gases and liquids, mechanical waves are always longitudinal. Sound waves too are longitudinal in gases and liquids.

In solids however they can be transverse or longitudinal depending on the mode of excitation. The speeds of the two waves in the same solid are different (longitudinal waves travel faster than transverse waves).

Longitudinal mechanical waves can be divided into following three categories:

(a) Infrasonic waves : The longitudinal mechanical waves whose frequency lie below 20 HZ are called infrasonics . Earth quake (ρ-waves) or periodic motion of a pendulum at frequency lesser than 20 Hz produces these waves.

(b) Audible or sound waves : These are longitudinal mechanical waves which lie between 20 Hz and 20 kHz. This range lies within the range of sensitivity of the human ear. These are generated by vocal cords, stretched strings or membranes.

(c) Ultrasonic waves : Longitudinal mechanical waves having frequency greater than 20 kHz are called ultrasonic waves.
Some animals like the mosquito, fish, dog and bat can detect these waves.
These waves can be produced by the high frequency vibrations of a quartz crystal under an alternating electric field (piezo-electric effect) or by the vibrations of a ferromagnetic rod under an alternating magnetic field (magnetostriction effect).

Ultrasonics are used for navigation under water (SONAR) in ultrasonography or, to repel mosquitoes or, attract fishes.

Note : Velocity of all three waves (sound waves, infrasonic and ultrasonic waves) in air at NTP is 332 m/s. From V = nλ we find that:

(a) wavelength of sound waves lies in the range 1.66 cm to 16.6 m,

(b) for ultrasonics λ < 1.66 cm and

(c) for infrasonics λ > 16.6 m.