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.