Ultrasonic Waves PDF

Ultrasonics Dr. Prabal Pratap Singh Bhadauria Assistant Professor, TIET, Patiala  Sound Wave: Longitudinal wave Direction of propagation Direction of oscillation Travels : Longitudinal Wave Motion Form of Energy : Emitted by a vib...

Ultrasonics Dr. Prabal Pratap Singh Bhadauria Assistant Professor, TIET, Patiala  Sound Wave: Longitudinal wave Direction of propagation Direction of oscillation Travels : Longitudinal Wave Motion Form of Energy : Emitted by a vibrating body Propagation : In all directions 2  Classification of Sound Waves: Depends upon Frequency Divided into 3 groups. Description Frequency range Hz Example Infrasound 0 - 20 Earth quake Audible sound 20 – 20,000 Speech, music Ultrasound > 20,000 to 5M Bat, Quartz crystal The word ultrasonic combines the Latin roots ultra, meaning ‘beyond’ and sonic, or sound. Ultrasonic refers to any study or application of sound waves that are higher frequency than the human audible range. Generally these waves are called as high frequency waves. 3  Ultrasonic waves Rhinoceroses use infrasonic frequencies as low as 5 Hz to call one another. Humans 20-20K Hz Cats 100-32K Hz Dogs 40-46K Hz Horses 31-40K Hz Bats use US frequencies up to 100 kHz for locating food sources and navigating. Elephants 16-12K Hz Cattle 16-40K Hz Bats 1K-150K Hz Grasshoppers 100-50K Hz Rodents 1K-100K Hz Whales, Dolphins 70-150K Hz 4  Properties of ultrasonic waves They have a high energy content. Just like ordinary sound waves, ultrasonic waves get reflected, refracted and absorbed. They show negligible diffraction due to their small wavelength. Hence can be transmitted over large distances without any appreciable loss of energy. Intense ultrasonic radiation has a disruptive effect in liquids by causing bubbles to be formed. If an arrangement is made to form stationary waves of ultrasonic in a liquid, it serves as a diffraction grating (called acoustic grating). 5  Applications of ultrasonic waves The broad sectors of society that regularly apply ultrasonic technology are the medical community, industry, the military and private citizens. The field of ultrasonic have applications for – imaging – Detection – Navigation 6  Ultrasonics Production Ultrasonic waves are produced by the following methods. (1) Magnetostriction method (2) Piezoelectric method 7  Magnetostriction method Principle: Magnetostriction effect When a magnetic field is applied parallel to the length of a ferromagnetic rod made of ferromagnetic materials such as iron or nickel, a small elongation or contraction occurs in its length. The change in length of rod depends upon i) the strength of the magnetic field, ii) the nature of the ferromagnetic materials iii) does not depend of the direction of the field. 8  Magnetostriction method AB is a rod of ferromagnetic materials Fe or Ni. The alternating magnetic field is generated by electronic oscillator. The coil L1 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. Used to produce low frequency Ultrasonic waves f  1 / 2 L1C1 9  Magnetostriction method The fundamental frequency of vibrating rod is given by 1 Y f = 2l  where l = length of the rod Y = Young’s modulus of the rod material and  = density of rod material The capacitor C1 is so adjusted such that the frequency of the oscillatory circuit is equal to natural frequency of the rod and thus resonance takes place. Now the rod vibrates longitudinally with maximum amplitude and generates ultrasonic waves from its ends. 10  Magnetostriction method 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 (i.e. 3MHz). 2. The frequency of oscillations depends on temperature. 3. There will be losses of energy due to hysteresis and eddy current. 11  PIEZOELECTRIC 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 of faces of the crystal are produced. This is known as inverse piezo electric effect or electrostriction Piezoelectric materials: titanates of barium and lead, lead zirconate (PbZrO3), ammonium dihydrogen phosphate (NH4H2PO4), and quartz (natural crystal). 12 Quartz crystals Quartz crystals The piezoelectric effect can only be achieved when small plates perpendicular either to x- axis or y-axis are cut out of the crystal. These are called x-cut and y-cut quartz crystals, respectively. x-cut crystals are used for the production and detection of longitudinal ultrasonic waves, y-cut crystals are used for the production and reception of transverse ultrasonic waves. These are seldom used in industry  Piezo-Electric method The coils L1 and L2 of base tuned electronic oscillator circuit form the primary of a transformer T. The secondary (L3) of a transformer which is inductively coupled to the electronics oscillator is connected to two metal plates where the quartz crystal is placed. The coil L1 and variable capacitor C1 form the tank circuit of the oscillator. 15  Piezo-Electric method When H.T. battery is switched on, the oscillator produces high frequency alternating voltages with a frequency. 1 f  2 L1C1 Due to the transformer action, an oscillatory e.m.f. is induced in the coil L3. This high frequency alternating voltages are fed on the metal plates. 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 1 Y Where l = thickness of the crystal f = Y = Young’s modulus of the crystal axis chosen 2l  ρ = density of the crystal. 16  Piezo-Electric method Advantages Ultrasonic frequencies as high as 500 MHz can be obtained with this arrangement. The output of this oscillator is very high. It is not affected by temperature and humidity. Disadvantages The cost of piezo electric quartz is very high The cutting and shaping of quartz crystal are very complex. 17  Numerical A quartz crystal of thickness 1 mm is vibrating at resonance. Calculate the fundamental frequency. Given Y for quartz = 7.9 x 1010 Nm-2 and ρ for quartz = 2650 kg m-3. What should be the minimum length of an iron rod to generate ultrasonic wave of frequency 0.03 MHz? Given Y for iron = 1.15 x 1011 Nm-2 and ρ of iron = 7.23 x 103 kg m-3. 18  Detection of Ultrasonic 1. Waves Piezoelectric Detector Piezoelectric effect can also be used to detect ultrasonic waves. If ultrasonic waves comprising of compressions and rarefactions are allowed to fall upon a quartz crystal a certain potential difference is developed across the faces which after amplification by a value amplifier can be used to detect ultrasonic waves. 19  Detection of Ultrasonic Waves 2. Kundt’s Tube Method Kundt’s tube is a long glass tube supported horizontally with a air column in it when the ultrasonic waves are passed the Kundt’s tube, the lycopodium powder (flash powder) sprinkled in the tube collects in the form of heaps at the nodal points and is blown off at the antinodal points. 20 3. Sensitive flame Method When a narrow sensitive flame is moved in a medium where ultrasonic waves are present, the flame remains stationary at nodes. At the positions of antinodes, the flame flickers because there is a change in pressure. In this way, positions of nodes and antinodes can be found out in the medium. The average distance between the two adjacent nodes is equal to half the wavelength. 4. Thermal Detector This is the most commonly used method of detection of ultrasonic waves. In this method, a fine platinum wire is used. This wire is moved through the medium. At the position of antinodes, due to alternate compressions and rarefactions, adiabatic changes in temperature takes place. The resistance of the platinum wire changes with respect to time. The change in resistance can be detected with the help of metre bridge arrangement. At the position of the nodes, the temperature remains constant. This will be indicated by the undisturbed balanced position of the metre bridge. 22 Applications of Ultrasonic Waves 23  1. Detection of flaws in metals (Non Destructive Testing –NDT) Ultrasonic waves are used to detect the presence of flaws or defects in the form of cracks, blowholes porosity etc., in the internal structure of a material. By sending out ultrasonic beam and by measuring the time interval of the reflected beam, flaws in the metal block can be determined. 24  1. Detection of flaws in metals (Non Destructive Testing –NDT) Ultrasonic waves are used to detect the presence of flaws or defects in the form of cracks, blowholes porosity etc., in the internal structure of a material. By sending out ultrasonic beam and by measuring the time interval of the reflected beam, flaws in the metal block can be determined. 25  2. 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. 26  3. Ultrasonic soldering Metals like aluminium cannot be directly soldered. However, it is possible to solder such metals by ultrasonic waves. An ultrasonic soldering iron consists of an ultrasonic generator having a tip fixed at its end which can be heated by an electrical heating element. The tip of the soldering iron melts solder on the aluminium and the ultrasonic vibrator removes the aluminium oxide layer. The solder thus gets fastened to clear metal without any difficulty. 4. Ultrasonic cleaning It is the most cheap technique employed for cleaning various parts of the machine, electronic assembles, armatures, watches etc, which cannot be easily cleaned by other methods. 27  5. SONAR SONAR is a technique which stands for SOund Navigation and Ranging. It uses ultrasonic waves for the detection and identification of under water objects. The method consists of sending a powerful beam of ultrasonic waves in the suspected direction in water. By noting the time interval between the emission and receipt of beam after reflection, the distance of the object can be easily calculated. Measuring the time interval (t) between the transmitted pulses and the received pulse, the distance between the transmitter and the remote object is determined using the formula., d = vt/2 where v is the velocity of sound in sea water. 28  6. Ultrasonics in Medicine Diagnostic sonography Medical sonography (ultrasonography) is an ultrasound-based diagnostic medical imaging technique used to visualize muscles and many internal organs, their size, structure and any pathological lesions. 30  7. Car airbag sensor Powerful US waves detect intensity of the shock and send an electrical signal to trigger the airbag. 8. Dispersion of fog Fog is defined as a mass of water vapour condensed into small water droplets on powery materials made from smoke, tyres etc. When Ultra Sonic waves travel through fog, they being longitudinal in nature, get reflected from its constituents and form stationary waves. The stationary waves, comprising of high and low density points, help coagulate the liquid and solid particles in the fog turning them into big particles. These particles fall on ground due to the gravity resulting in dispersal of fog 31  9. Green Energy Piezoelectricity being non-polluting is called green energy. It can be an alternative green energy source in providing piezoelectric flooring at places such as floors, airports, shopping malls, train stations or places where heavy foot traffic is available. It adopts a method which accumulates the mechanical energy from people’s pressure on the piezoelectric board during the day in order to light up the street at night. One can convert the pressure from cars passing by the streets into electric energy through the piezoelectric element under the street. This way all streetlights on highway and traffic lights can run on semi-permanent source of energy. As piezoelectric technology uses vibrations to attain energy, even the slightest vibration can be stored as electricity. 32