ULTRASONIC WAVE

  1). They have a high energy content. 2). Just like ordinary sound waves, ultrasonic waves get reflected, refracted and absorbed. 3). They can be transmitted over large distances with no appreciable loss of energy. 4). If an arrangement is made to form stationary waves of ultrasonic in a liquid, it serves as diffraction rating . It is called an acoustic grating. 5). They produce intense heating effect when passed through a substance.

  Ultrasonic waves are produced by the following methods. 1). Magnetostriction generator or oscillator. Principle: Magnetostriction Effect  When a ferromagnetic rod like iron or nickel is placed in a magnetic field parallel to its length, the rod experiences a small change in its length. This is called magnetostriction effect. The change in length (increase or decrease) produced in the rod depends upon the strength of the magnetic field, the nature of the materials and is independent of the direction of the magnetic field applied. 2). Piezo-electric generator or oscillator. Principle: Inverse piezo electric effect If mechanical pressure is applied to one pair of opposite faces of certain crystals like quartz, equal and opposite electrical charges appear across its  other faces. This is called as piezo-electric effect.   The converse of piezo electric effect is also true. If an electric field is applied to one pair of faces, the corresponding changes in the dimensions of the other pair

Construction The Experiment arrangement in shown in Fig. XY is a rod of ferromagnetic materials like iron or nickel. The rod is clamped in the middle. The alternating magnetic field is generated by electronic oscillator. The coil L wound on the right-hand portion of the rod along with a variable capacitor C. This forms the resonant circuit of the collector tuned oscillator. The frequency of oscillator is controlled by the variable capacitor. The coil L2 wound on the left-hand portion of the rod is connected to the base circuit. The coil L2 acts as feed-back loop. Working When High Tension (H.T) battery is switched on, the collector circuit oscillates with a frequency, This alternating current flowing through the coil Li produces an alternating magnetic field along the length of the rod. The result is that the rod starts vibrating due to magneto strictive effect. The frequency of vibration of the rod is given by where, l = length of the rod, Y = Young's modulus of the rod material a

  Advantages 1). The design of this oscillator is very simple and its production cost is low. 2). At low ultrasonic frequencies, the large power output can be produced without the risk of damage of the oscillatory circuit. Disadvantages 1). It has low upper frequency limit and cannot generate ultrasonic frequency above 3000 kHz (ie. 3MHz). 2). The frequency of oscillations depends on temperature. 3). There will be losses of energy due to hysteresis and eddy current.

  Construction The circuit diagram is shown in figure The quartz crystal is placed between two metal plates A and B. The plates are connected to the primary (L3) of a transformer which is inductively coupled to the electronics oscillator. The electronic oscillator circuit is a base tuned oscillator circuit. The coils L, and L, of oscillator circuit are taken from the secondary of a transformer T. The collector coil L2 is inductively coupled to base coil Lj. The coil L, and variable capacitor C, form the tank circuit of the oscillator. Working When H.T. battery is switched on, the oscillator produces high frequency alternating voltages with a frequency. Due to the transformer action, an oscillatory e.m.f. is induced in the coil Lz. This high frequency alternating voltages are fed on the plates A and B. Inverse piezo-electric effect takes place and the crystal contracts and expands alternatively. The crystal is set into mechanical vibrations. The frequency of the vibration is given by T

Advantages 1). Ultrasonic frequencies as high as 5 x 108Hz or 500 MHz can be obtained with this arrangement. 2). The output of this oscillator is very high. 3). It is not affected by temperature and humidity. Disadvantages 1). The cost of piezo electric quartz is very high. 2). The cutting and shaping of quartz crystal are very complex.

  Ultrasonic Drilling Ultrasonics are used for making holes in very hard materials like glass, diamond etc. For this purpose, a suitable drilling tool bit is fixed at the end of a powerful ultrasonic generator. Some slurry (a thin paste of carborundum powder and water) is made to flow between the bit and the plate in which the hole is to be made Ultrasonic generator causes the tool bit to move up and down very quickly and the slurry particles below the bit just remove some material from the plate. This process continues and a hole is drilled in the plate. Ultrasonic welding The properties of some metals change on heating and therefore, such metals cannot be welded by electric or gas welding. In such cases, the metallic sheets are welded together at room temperature by using ultrasonic waves. Ultrasonic soldering Metals like aluminum cannot be directly soldered. However, it is possible to solder such metals by ultrasonic waves. An ultrasonic soldering iron consists of an ultrasonic gen