NATURE OF SOUND WAVES

Sound waves are longitudinal mechanical waves. They can be propagated in solids, liquids, and gases. There a is a large range of frequencies with in which longitudinal mechanical waves cab be generated sound waves being confined to the frequency range which can stimulate the human ear and brain to the sensation of hearing. This range is from 20 cycles/second (20Hz) to 20000 cycles/second (20 KHz), is called the audible range. A longitudinal wage whose frequency is below audible is called infrasonic wave and one above the audible range is called ultrasonic waves. Langevine used an ultrasonic transmitter and receiver made up of quartz during the First World War (1914 -18). Due to small wavelength, noiseless performance and directional property, this wave is preferred over audible sound

CLASSIFICATION OF ULTRASONIC WAVES
Ultrasonic waves are classified into four categories based on mode of vibration of the particles in the medium with respect to the direction of propagation of the waves
1. Longitudinal waves
2. Transverse or shear wave
3. Surface of Rayleigh waves
4. Plate or Lamb wave

LONGITUDINAL WAVE
The nature of longitudinal ultrasonic wave is similar to the longitudinal sound waves. Here motion of particles is parallel to the direction of propagation of incident wave. Due to the vibration of particles, an alternate compression and rarefaction zone are produced and is shown in fig. Both density and pressure of the medium fluctuate periodically They are propagated in solids, liquids and gases. It is easy to generate detect and also to convert them into other modes of vibrations.

TRANSVERSE OR SHEAR WAVES
The nature of transmission of transverse ultrasonic wave is same as that of shear sound waves. Here the vibration of particle is at right angles to the direction of the propagation of the wave motion. Velocity of shear wave is nearly half as that of longitudinal waves in the same material. Shear waves can pass only thorough solids. They cannot propagate in liquids or gases, because the shear modulus of fluids is zero.

SURFACE OR RAYLEIGH WAVES
In 1885, Lord Rayleigh, demonstrated that surface wave can be propagated over the plane boundary between an elastic half space and a vacuum or sufficiently rarefied medium. The amplitude of the waves decay rapidly with the depth of the wave in the medium. These waves are called Rayleigh waves. Surface waves are analogous to water waves; it can travel only on the surface layer of solids. The particle in the medium can vibrate up to about one wavelength from the surface of the medium.

PLATE WAVES OR LAMB WAVES
When a surface wave is introduced into a material having thickness equal to the three times the wavelength or less, a different kind of wave known as plate waves is produced. During the existence of plate wave, the material begins to vibrate as a plate, or the wave encompasses the entire thickness of the material. This phenomenon was described by Lamb in 1916 so it is called Lamb wave