Engineering Physics - Ultrasonic - Application of Ultrasonic - SONAR

Engineering Physics - Ultrasonic , Application of Ultrasonic and SONAR 

Learning Objectives :

On completion of this topic you will be able to understand :

1. Application of ultrasonic waves in various fields e.g., engineering, medical, metallurgical, physical, chemical, etc 

2. SONAR 

Ultrasonic waves have a wide range of applications in various fields e.g., engineering, medical, metallurgical, physical, chemical, etc. Some of their uses are discussed in below

Ultrasonic Drilling and Cutting

Ultrasonic are used for making holes in very hard materials such as glass, diamond etc., When ultrasonic are passed through these materials it creates air bubbles. This air bubbles collapses within short span of time, thereby a larger amount of pressure and temperature which are used for cutting and drilling.

Ultrasonic Cleaning

Ultrasonic cleaning is an environmentally friendly alternative for the cleaning of continuous materials, such as wire and cable, tape or tubes. The effect of the cavitations generated by the ultrasonic power removes lubrication residues like oil or grease, soaps, stearates or dust. In addition, the pollution particles are dispersed into the cleaning liquid. By that, a new adhesion to the material to be cleaned is avoided and the particles are flushed away. 

By the use of an innovative proprietary ultrasonic technology, very strong cavitations fields are generated, so that very good cleaning results at high line speeds can be accomplished. As the cleaning effect is based on the physical cleaning effects of the ultrasound, it can be used for any ferrous and non-ferrous material, e.g. stainless steel, copper, aluminum, but also plastic or glass. Most commonly ultrasonic cleaning machines are used for drawn wire, e.g. before cladding or extrusion. By the concentration of the ultrasonic power to a low liquid volume, a very compact design can be realized. This can be easily integrated into existing or new production lines, e.g. directly after drawing or reel payoff. 

Cavitation is an effect that is generated in liquids by intensive ultrasonic waves. The resulting pressure waves create vacuum bubbles that implode subsequently. As a result of these implosions, very high pressures and temperatures occur in combination with liquid jets of up to 1000km/h. At surfaces, these mechanical forces loosen impurities, so they can be flushed away with the cleaning liquid. For an intensive cavitation - and by that for an intensive cleaning - high amplitudes and a low ultrasonic frequency (approx. 20 kHz) are needed. Ultrasonic Cleaners are used in various industries for a number of applications. 

Some of the Ultrasonic Cleaning applications are: 

1. Ultrasonic Cleaners in Scientific Labs 

Lab Glassware, Test Tubes, Pipettes, Optical & Contact Lenses, Eyeglass Frames, Scientific Instruments, Components 

2.Ultrasonic Cleaners in Industrial Manufacturing 

Switches, Relays & Motors, Gears, Precision Bearings, Metal & Plastic Parts, Assemblies 

3. Ultrasonic Cleaners in Electronics Manufacturing 

PC Boards, SMDs, Ceramic Substrates, Capacitors, Lapping Heads, Packaging Components, Quartz Crystals, High-resolution Glass Plates 

4. Ultrasonic Cleaners in Medical & Dental Labs 

Cannulae, Syringe Parts, Surgical Instruments, Blood Oxygenators, Dental Instruments, Burs, Dentures, Caps, Plates 

5. Ultrasonic Cleaners in Jewelry Manufacturing 

Watches, Clock Movements, Precious Metals & Gemstones, Intricate Settings, Chains, Charms, Coins 

Ultrasonic Welding Applications

Ultrasonic metal-welding is an advanced technical process for combining nonferrous metals, stranded wire and many metal-alloys. It is a cold-phase friction welding technique; there is no melting, no high-temperature buildup. The surfaces being joined are subjected to high- frequency mechanical oscillations while being rubbed together under pressure. The molecules of the surfaces begin to swirl and intermingle with one another, creating a firm and lasting bond. Improvements in quality and efficiency, reduced energy requirements and positive environmental factors are the decisive advantages of this new technology.

SONAR 

The word Sonar is an American term first used in World War II, it is an acronym for SOund, NAvigation and Ranging. The British also call Sonar, ASDICS, which stands for Anti- Submarine Detection Investigation Committee. Later developments of Sonar included the echo sounder, or depth detector, rapid-scanning Sonar, side-scan Sonar, and WPESS (within- pulseectronic-sector-scanning) Sonar.

Sonar is a system that uses transmitted and reflected underwater sound waves to detect and locate submerged objects or measure the distances underwater. It has been used for submarine and mine detection, depth detection, commercial fishing, diving safety and communication at sea. The Sonar device will send out a subsurface sound wave and then listens for returning echoes, the sound data is relayed to the human operators by a loudspeaker or by being displayed on a monitor.

As early as 1822, Daniel Colloden used an underwater bell to calculate the speed of sound underwater in Lake Geneva, Switzerland. This early research led to the invention of dedicated sonar devices by other inventors. Lewis Nixon invented the very first Sonar type listening device in 1906, as a way of detecting icebergs. Interest in Sonar was increased during World War I when there was a need to be able to detect submarines.

In 1915, Paul Langévin invented the first sonar type device for detecting submarines called an "echo location to detect submarines" using the piezoelectric properties of the quartz. He was too late to help very much with the war effort; however, Langévin's work heavily influenced future sonar designs.

The first Sonar devices were passive listening devices - no signals were sent out. By 1918, both Britain and the U.S had built active systems, in active Sonar signals are both sent out and then received back. Acoustic communication systems are Sonar devices where there is both a sound wave projector and receiver on both sides of the signal path. The invention of the acoustic transducer and efficient acoustic projectors made more advanced forms of Sonar possible. There are two major kinds of sonar, active and passive.

Active sonar creates a pulse of sound, often called a "ping", and then listens for reflections of the pulse. The pulse may be at constant frequency or a chirp of changing frequency. If a chirp, the receiver correlates the frequency of the reflections to the known chirp. The resultant processing gain allows the receiver to derive the same information as if a much shorter pulse of the same total power were emitted. In general, long-distance active sonars use lower frequencies. The lowest have a bass "BAH-WONG" sound. To measure the distance to an object, one measures the time from emission of a pulse to reception.

Passive sonars listen without transmitting. They are usually military (although a few are scientific). Passive sonar systems usually have large sonic databases. A computer system frequently uses these databases to identify classes of ships, actions (i.e. the speed of a ship, or the type of weapon released), and even particular ships.

Basic concept of SONAR

Sonar is based on the echo-sounding technique of ultrasound. When an ultrasonic wave is transmitted through water, it is reflected by the objects in the water and will produce an echo signal. By noting the time interval between the generation of the ultrasonic pulse and the reception of the echo signal (t), the depth of the object can be easily calculated. Since the ultrasonic velocity “v’ in sea water is known, the depth of sea is calculated as follows Depth of sea (distance between surface and bottom of the sea) = vt/2

The same procedure is also used to find the distance of submarine or iceberg from the surface of the sea and the distance between two ships in the sea.

Check your understanding :

1. What are the applications of ultrasonic in industry?

Ans: Ultrasonics are used in cutting, drilling, welding, soldering etc 

2. What is meant by cavitation?

Ans : Cavitation is the processes of creation and collapse of bubbles, due to the principle of negative local pressure created inside the bubble. 

3. What is SONAR 

Ans : SONAR is an acronym for “Sound Navigation and Ranging”. 

4. What are the applications of SONAR?

Ans : SONAR is used to (i) find the depth of sea (ii) guide the submarine or ships in seas, and (iii) locate the shoal of fish

Summary :

On completion of this topic you have learned

1. Ultrasonic waves have a wide range of applications in various fields e.g., engineering, medical, metallurgical, physical, chemical, etc.

2. SONAR is an acronym for “Sound Navigation and Ranging”. The principle of SONAR is based on the echo sounding technique of ultrasonic. It is the acoustical technique for locating the objects like submarine or iceberg in sea, by transmitting a high frequency sound pulse and receiving it after reflection from that object.

Suggested Reading
1. “Engineering Physics” by Dr.P.K.Palanisamy, Scitech Publications (India) pvt, Ltd, Chennai
2. “Engineering Physics” by Dr.G.SenthilKumar, VRB Publishers Pvt Ltd, Chennai.

Engineering Physics 1 - Ultrasonics – Detection, Properties, Acoustic Grating

Introduction :

Can you believe that the ultrasonic waves are used for cleaning purpose…???

Learning Objectives :

On completion of this chapter you will be able to:

1. Describe the different methods of detecting ultrasonic waves.
2. List out the properties of ultrasonic waves.
3. Define cavitation.
4. Explain the method of determination of velocity of ultrasonic waves in liquid using acoustic grating.

DETECTION OF ULTRASONIC WAVES 

Ultrasonic waves propagated through a medium can be detected in a number of ways. Some of the methods employed are as follows:

(1) Kundt’s tube method:
Ultrasonic waves can be detected with the help of Kundt’s tube. At the nodes, lycopodium powder collects in the form of heaps. The average distance between two adjacent heaps is equal to half the wavelength. This method cannot be used if the wavelength of ultrasonic waves is very small i.e., less than few mm. In the case of a liquid medium, instead of lycopodium powder, powdered coke is used to detect the position of nodes.

(2) Sensitive flame method:
A narrow sensitive flame is moved along the medium. At the positions of antinodes, the flame is steady. At the positions of nodes, 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. If the value of the frequency of ultrasonic wave is known, the velocity of ultrasonic wave propagated through the medium can be calculated. 

(3) Thermal detectors:
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 nodes, due to alternate compressions ad rarefactions, adiabatic changes in temperature takes place. The resistance of the platinum wire changes with respect to time. This can be detected with the help of Callendar and Garrifith’s bridge arrangement. At the position of the antinodes, the temperature remains constant. This will be indicated by the undisturbed balanced position of the bridge.

(4) Quartz crystal method:
This method is based on the principle of Piezo-electric effect. When one pair of the opposite faces of a quartz crystal is exposed to the ultrasonic waves, the other pairs of opposite faces developed opposite charges. These charges are amplified and detected using an electronic circuit.

PROPERTIES OF ULTRASONIC WAVES 

(1) Ultrasonic waves are having frequencies higher than 20 KHz and hence they are highly energetic and their wavelengths are small.

(2) Due to their small wavelengths, the diffraction is negligible. Hence, they can be transmitted over a long distances without any appreciable loss of energy.

(3) When they are passing through a medium, at discontinuities, they are partially reflected and this property is used in Non-Destructive Technique (NDT).

(4) When the ultrasonic wave is absorbed by a medium, it generates heat. (5) They are able to drill and cut thin metals.

(6) At room temperature, ultrasonic welding is possible.

(7) They mix molten metals of widely different densities to produce alloys of uniform composition.

(8) Using ultrasonic wave, acoustic grating can be formed in a liquid.

ACOUSTIC CAVITATION 

In general, cavitation is the phenomenon where small and largely empty cavities are generated in a fluid, which expand to large size and then rapidly collapse. When the cavitation bubbles collapse, they focus liquid energy to very small volumes.

Thereby, they create spots of high temperature and emit shock waves. The collapse of cavities involves very high energies.

Power ultrasound enhances chemical and physical changes in a liquid medium through the generation and subsequent destruction of cavitation bubbles. Like any sound wave ultrasound is propagated via a series of compression and rarefaction waves induced in the molecules of the medium through which it passes. At sufficiently high power the rarefaction cycle may exceed the attractive forces of the molecules of the liquid and cavitation bubbles will form. Such bubbles grow by a process known as rectified diffusion i.e. small amounts of vapour (or gas) from the medium enters the bubble during its expansion phase and is not fully expelled during compression. The bubbles grow over the period of a few cycles to an equilibrium size for the particular frequency applied. It is the fate of these bubbles when they collapse in succeeding compression cycles which generates the energy for chemical and mechanical effects.

Cavitation bubble collapse is a remarkable phenomenon induced throughout the liquid by the power of sound. In aqueous systems at an ultrasonic frequency of 20 KHz each cavitation bubble collapse acts as a localised "hotspot" generating temperatures of about 4,000 K and pressures in excess of 1000 atmospheres.

Application:

The acoustic cavitation is useful in the cleaning process. This type of cleaning has proven to be the most effortless, quick and efficient method of cleaning known today. The applications are virtually limitless. Ultrasonic cleaning is state-of-the-art technology. It utilizes a digital generator powering transducers submerged in a tank of hot water. The transducers vibrate at a frequency of 40 KHz creating millions of tiny bubbles that form and implode. This repeated formation and implosion creates a gentle cleaning action known as Cavitation. Cavitation has the ability to not only clean the surfaces of items, but also penetrate into the difficult to clean internal and crevice areas. It is the safest and most gentle form of cleaning. Ultrasonics will not scratch, pit or damage items the way that conventional cleaning methods can.

Ultrasonic cleaners accomplish the cleaning task within seconds. It can remove build-up of dirt, grime, grease or soot. Hard-to-clean areas such as the grooves of club heads, the lettering on the heel, and the grips also become clean, adding longevity of use and luster to the club.

ACOUSTING GRATING 

Principle: 

When ultrasonic waves are passed through a liquid, the density of the liquid varies layer by layer due to the variation in pressure and hence the liquid will act as a diffraction grating, so called acoustic grating. Under this condition, when a monochromatic source of light is passed through the acoustical grating, the light gets diffracted. Then, by using the condition for diffraction, the velocity of ultrasonic waves can be determined.

Construction & Working: 

The liquid is taken in a glass cell. The Piezo-electric crystal is fixed at one side of the wall inside the cell and ultrasonic waves are generated. The waves travelling from the crystal get reflected by the reflector placed at the opposite wall. The reflected waves get superimposed with the incident waves producing longitudinal standing wave pattern called acoustic grating.

If light from a laser source such as He-Ne or diode laser is allowed to pass through the liquid in a direction perpendicular to the grating, diffraction takes place and one can observe the higher order diffraction patterns on the screen. The angle between the direct ray and the diffracted rays of different orders (θn) can be calculated easily.

According to the theory of diffraction, 

d sin θn = n λ                                   -----(1) 

where n = 0, 1, 2, 3, … is the order of diffraction, λ is the wavelength of light used and d is the distance between two adjacent nodal or anti-nodal planes.

Knowing n, θn and λ, the value of d can be calculated from eqn. (1). If λa is the wavelength of the ultrasonic waves through the medium, then 

                                                        d = λa/2

      or 

 λa = 2d                            -------(2) 

If the resonant frequency of the Piezo-electric oscillator is N, then the velocity of ultrasonic wave is given by

        v = N λa = 2Nd                              .......(3) 

This method is useful in measuring the velocity of ultrasonic waves through liquids and gases at various temperatures. From these measurements, many parameters of the liquid such as free volume, compressibility, etc., can be calculated.

Check Your Understanding:

An ultrasonic beam is used to determine the thickness of a steel plate. It was noticed that the difference in two adjacent harmonic frequencies is 50 KHz. The velocity of sound in steel is 5000 m/s. Calculate the thickness of the steel plate.

Answer : d = 0.05 mm.

Summary :

On completion of this chapter you have learned the following:

The different methods for the detection of ultrasonic waves are

i) Kundt’s tube method,

ii) Sensitive flame method, 

iii) Thermal detector 

 and 

 iv) Piezo-electric detector. 

When ultrasonic waves propagate through a liquid, alternate compressions and rarefactions are generated at any point. Rarefaction results in sudden drop in pressure causing growth and collapse of cavitation (gas) bubbles. This phenomenon is known as cavitation. 

Due to the periodic vibration of the ultrasonic transducer inside a liquid, compressions and rarefactions are produced. This variation in density and consequently the refractive index act as a diffraction grating for any light passing through it and hence such an arrangement is called acoustic grating. 

Activity :

Try to collect some materials by searching through internet. 

Suggested Reading :
1. P.K. Palanisamy, Engineering Physics, Scitech Publications Pvt Ltd, Chennai.
2. N. Subrahmanyam and Brij Lal, A Text Book of Sound, Vikas Publishing House Pvt Ltd., New Delhi.
(And some other open resources from internet)

Engineering Physics 1 - Ultrasonics- Piezo-Electric Effect- Piezo-Electric Generator

Introduction 

Can all the crystals exhibit piezoelectric effect? What is special about the piezoelectric crystal?
Is the piezoelectric effect direction dependent? 

Learning Objectives
On completion of this chapter you will be able to:
1. define piezoelectric effect
2. define inverse piezoelectric effect
3. know what type of crystals will exhibit piezoelectric effect
4. Understand the working of piezoelectric generator 

Piezoelectric effect: 

When crystals like quartz or tourmaline are stressed along any pair of opposite faces, electric charges of opposite polarity are induced in the opposite faces perpendicular to the stress. This is known as Piezoelectric effect.

Piezoelectric effect- Mechanism:

Piezoelectric and inverse piezoelectric effects are only exhibited by certain crystals which lack centre of symmetry. In a piezoelectric crystal, the positive and negative electrical charges are separated, but symmetrically distributed, so that the crystal overall is electrically neutral. Each of these sides forms an electric dipole and dipoles near each other tend to be aligned in regions called Weiss domains. The domains are usually randomly oriented, but can be aligned during poling , a process by which a strong electric field is applied across the material, usually at elevated temperatures.

When a mechanical stress is applied, this symmetry is disturbed, and the charge asymmetry generates a voltage across the material. For example, a 1 cm cube of quartz with 2 kN (500 lbf) of correctly applied force can produce a voltage of 12,500 V.

Piezoelectric materials also show the opposite effect, called converse (inverse) piezoelectric effect, where the application of an electrical field creates mechanical deformation in the crystal.

Inverse piezoelectric effect: 

When an alternating e.m.f is applied to the opposite faces of a quartz or tourmaline crystal it undergoes contraction and expansion alternatively in the perpendicular direction. This is known as inverse piezoelectric effect. This is made use of in the piezoelectric generator.

Piezoelectric generator:

A slab of piezoelectric crystal is taken and using this a parallel plate capacitor is made. Then with other electronic components an electronic oscillator is designed to produce electrical oscillations >

20 kHz. Generally one can generate ultrasonic waves of the order of MHz using piezoelectric generators. Quartz slabs are preferred because it possesses rare physical and chemical properties. A typical circuit diagram is given below.

The tank circuit has a variable capacitor 'C' and an inductor 'L' which decides the frequency of the electrical oscillations. When the circuit is closed current rushes through the tank circuit and the capacitor is charged, after fully charged no current passes through the same. Then the capacitor starts discharging through the inductor and hence the electric energy is in the form of electric and magnetic fields associated with the capacitor and the inductor respectively. Thus we get electrical oscillations in the tank circuit and with the help of the other electronic components including a transistor, electrical oscillations are produced continuously. This is fed to the secondary circuit and the piezoelectric crystal (in our case a slab of suitably cut quartz crystal) vibrates, as it is continuously subjected to varying (alternating) electric field, and produces sound waves. When the frequency of electrical oscillations is in the ultrasonic range then ultrasonic waves are generated. When the frequency of oscillation is matched with the natural frequency of the piezoelectric slab then it will vibrate with maximum amplitude. The frequency generated is given as follows:

E- the Young's modulus of the piezoelectric material
and ρ- the density of the piezoelectric material

Check your understanding

1. Choose the right answer from the options given below: The following crystals show piezoelectric effect:
a) NaCl
b) Barium Titanate

c) Diamond
d) Quartz

Answer : b,d

2. State if the following statement is true or false?
One can generate ultrasonic waves of frequency f, 2f, ..., where f is the fundamental freqency 

a) True

b) False

Answer :a

3. State if the following statement is true or false?
The magnitude of the Piezoelectric effect of a crystalline material does not depend on direction
a) True

b) False

Answer :b

4. State if the following statement is true or false?
The Piezoelectric effect of a material is something to do with the crystal symmetry

a) True
b) False

Answer :a

5. State if the following statement is true or false?
The Piezoelectric effect of a material depends on its crystal structure

a) True
b) False 

Answer :a

Summary
On completion of this chapter you have learned :
1. piezoelectric effect and inverse (converse) piezoelectric effect.
2. the type of crystals showing piezoelectric effect.
3. principle of piezoelectric oscillator.
4. Working of piezoelectric oscillator.

Activity
Students can read standard texts or browse the net and read the relevant material before coming to the class. They can discuss extra points not covered during the lectures. They can find the actual working circuit for the piezoelectric oscillator (generator) to generate ultrasonic waves.

Suggested Reading
1. Engineering Physics by P.K.Palanisamy (Scitech publications)
2. http://en.wikipedia.org/wiki/piezoelectric

Engineering Physics 1 - Ultrasonic and Magnetostriction Method

“Dolphins, whales, dogs and some fishes can respond to sound that human ear cannot hear.” 

Introduction :
The term ultrasonics applies to sound waves that vibrate at a frequency higher than the frequency that can be heard by the human ear (or higher than about 20,000 hertz). 

Sound is transmitted from one place to another by means of waves. The character of any wave can be described by identifying two related properties: its wavelength (lambda, λ) or its frequency (f). The unit used to measure the frequency of any wave is hertz. One hertz is defined as the passage of a single wave per second.

Ultrasonics, then, deals with sound waves that pass a given point at least 20,000 times per second. Since ultrasonic waves vibrate very rapidly, additional units also are used to indicate their frequency. The kilohertz (kHz), for example, can be used to measure sound waves vibrating at the rate of 1,000 times per second, and the unit megahertz (MHz) stands for a million vibrations per second. Some ultrasonic devices have been constructed that produce waves with frequencies of more than a billion hertz.

Learning Objectives :

On completion of this session you will be able to:

1. Understand the term ultrasonics.

2. Explain the principle, construction and working of Magnetostriction oscillator.

3. Enunciate the merits and demerits of Magnetostriction method.

Ultrasonic Production :

There are three methods for producing Ultrasonic waves. They are: 

(i) Mechanical generator or Galton’s whistle.
(ii) Magnetostriction generator.

(iii) Piezo-electric generator. 

In this session, you are going to study the method of producing ultrasonic waves using Magnetostriction method. 

Magnetostriction method:

Principle: 

The general principle involved in generating ultrasonic waves is to cause some dense material to vibrate very rapidly. The vibrations produced by this material than cause air surrounding the material to begin vibrating with the same frequency. These vibrations then spread out in the form of ultrasonic waves.

When a magnetic field is applied parallel to the length of a ferromagnetic rod made of material such as iron or nickel, a small elongation or contraction occurs in its length. This is known as magnetostriction. The change in length depends on the intensity of the applied magnetic field and nature of the ferromagnetic material. The change in length is independent of the direction of the field.

When the rod is placed inside a magnetic coil carrying alternating current, the rod suffers a change in length for each half cycle of alternating current. That is, the rod vibrates with a frequency twice that of the frequency of A.C. The amplitude of vibration is usually small, but if the frequency of the A.C. coincides with the natural frequency of the rod, the amplitude of vibration increases due to resonance. 

Construction:

The ends of the ferromagnetic rod A and B is wound by the coils L1 and L. The coil L is connected to the collector of the transistor and the coil L1 is connected to the base of the transistor as shown in the figure. The frequency of the oscillatory circuit (LC) can be adjusted by the condenser C and the current can be noted by the milliammeter connected across the coil L. The battery connected between emitter and collector provides necessary biasing i.e., emitter is forward biased and collector is reverse biased for the NPN transistor. Hence, current can be produced by applying necessary biasing to the transistor with the help of the battery.

Working: 

The rod is permanently magnetized in the beginning by passing direct current. The battery is switched on and hence current is produced by the transistor. This current is passed through the coil L, which causes a corresponding change in the magnetization of the rod. Now, the rod starts vibrating due to magnetostriction effect.

When a coil is wounded over a vibrating rod, then e.m.f. will be induced in the coil called as converse magnetostriction effect. Due to this effect an e.m.f. is induced in the coil L1. The induced e.m.f. is fed to the base of the transistor, which act as a feed back continuously. In this way the current in the transistor is built up and the vibrations of the rod is maintained.

The frequency of the oscillatory circuit is adjusted by the condenser C and when this frequency is equal to the frequency of the vibrating rod, resonance occurs. At resonance, the rod vibrates longitudinally with larger amplitude producing ultrasonic waves of high frequency along both ends of the rod. 

Condition for resonance: 

Frequency of the oscillatory circuit = Frequency of the vibrating rod 

 (i.e)   

where, 

l is the length of the rod.

E is the young’s modulus of the material of the rod.

ρ is the density of material of the rod.

Merits:
1. Magnetostrictive materials are easily available and inexpensive.
2. Oscillatory circuit is simple to construct.
3. Large output power can be generated. 

Limitations
1. It can produce frequencies upto 3 MHz only.
2. It is not possible to get a constant single frequency, because rod depends on temperature and the degree of magnetization.
3. As the frequency is inversely proportional to the length of the vibrating rod, to increase the frequency, the length of the rod should be decreased which is practically impossible.

Check your understanding :

1. State if the following statement is true or false?
Magnetostriction oscillator can generate ultrasonic waves of single frequency. 

a) True          b) False 

Answer : b

2. Choose the right answer from the options given below:
The frequency of vibration of the D.C. magnetized rod in the Magnetostriction generator is

a) Equal to the frequency of alternating current. 

b) Twice the frequency of alternating current.
c) Half the frequency of alternating current.

Answer : a

3. Why not ultrasonics be produced by passing high frequency alternating current through a loud speaker?

Answer : At such high frequencies, inductive reactance is so high that no current flows through the coil of the loud speaker and hence ultrasonic waves cannot be produced

Summary
On completion of this topic you have: 

Learned that ultrasonics is the science of sound concerned with frequencies greater than 20 KHz, i.e above the human ear’s audible range.
Learned the principle, construction, working, merits and demerits of
Magnetostriction oscillator.

Activity
1. Study the phenomenon of ferromagnetism in materials.
2. Why do we need a biasing using a npn transistor in magnetostriction method ? 

Suggested Reading :
1. ‘Engineering Physics’ by P.K. Palanisamy.
2. ‘Engineering Physics’ by Dr. Senthil Kumar.