UNIT 3 Ultrasonics

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Ultrasonics: Introduction – properties ofultrasonic waves – generation ofultrasonic waves: Magnetostriction - piezoelectric methods – detection of ultrasonicwaves. Determination of velocity ofultrasonic waves (acoustic grating).Applications of ultrasonic waves: SONAR– measurement of velocity of blood flow –study of movement of internal organs.

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General Objective

To understand the principle ofmagnetostriction to produce

ultrasonic waves

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General Objective

1. Classify three types of sound wavesbased on frequency (S)

2. List the seven properties of ultrasonicwaves (S)

3. State magnetostriction effect (S)4. Explain the construction and working

of magnetostriction oscillator (S, E)5. Identify two advantages and

disadvantages of magnetostrictionmethod (E)

Rhinoceroses use infrasonic frequencies as low as 5 Hz tocommunicate.

Rhinoceroses use infrasonic frequencies as low as 5 Hz tocommunicate.

InfrasonicsInfrasonics

Bats use ultrasonic frequencies as 100 Khz tocommunicate

Bats use ultrasonic frequencies as 100 Khz tocommunicate

UltrasonicsUltrasonics

Properties of ultrasonic wavesProperties of ultrasonic waves

1. They have a high energy content.

2. Just like ordinary sound waves, ultrasonicwaves get reflected, refracted and absorbed.

3. They can be transmitted over large distanceswith no appreciable loss of energy.

4. If an arrangement is made to form stationarywaves of ultrasonics in a liquid, it serves as adiffraction grating. It is called an acousticgrating.

5. They produce intense heating effect whenpassed through a substance.

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Ultrasonic waves are produced by the following

methods.

(1) Magneto-striction generator or oscillator

(2) Piezo-electric generator or oscillator

Ultrasonic waves are produced by the following

methods.

(1) Magneto-striction generator or oscillator

(2) Piezo-electric generator or oscillator

Ultrasonics ProductionUltrasonics Production

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Magnetostriction GeneratorMagnetostriction Generator

Principle: Magnetostriction effect

When a ferromagnetic rod like iron ornickel is placed in a magnetic fieldparallel to its length, the rod experiencesa small change in its length. This is calledmagnetostricion effect.

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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.

ConstructionConstruction

The experimental arrangement ofMagnetostriction oscillator

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• XY is a rod of ferromagnetic materials likeiron or nickel. The rod is clamped in themiddle.

• The alternating magnetic field is generatedby electronic oscillator.

• The coil L1 wound on the right hand portionof the rod along with a variable capacitor C.

• This forms the resonant circuit of the collectortuned oscillator. The frequency of oscillatoris controlled by the variable capacitor.

• The coil L2 wound on the left hand portion ofthe rod is connected to the base circuit. Thecoil L2 acts as feed –back loop.

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WorkingWorking

• When High Tension (H.T) battery isswitched on, the collector circuit oscillateswith a frequency,

f =

• This alternating current flowing through thecoil L1 produces an alternating magneticfield along the length of the rod. The resultis that the rod starts vibrating due tomagnetostrictive effect.

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1

1

2 L C

The frequency of vibration of the rod is givenby

n =

where l = length of the rodY = Young’s modulus of the rod

material and =density of rod material

Y

l2

1

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The capacitor C is adjusted so that the frequency of theoscillatory circuit is equal to natural frequency of therod and thus resonance takes plate.

Now the rod vibrates longitudinally with maximumamplitude and generates ultrasonic waves of highfrequency from its ends.

AdvantagesAdvantages

1. The design of this oscillator is very simpleand its production cost is low

2. At low ultrasonic frequencies, the largepower output can be produced without therisk of damage of the oscillatory circuit.

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DisadvantagesDisadvantages1. It has low upper frequency limit and cannot

generate ultrasonic frequency above 3000kHz (ie. 3MHz).

2. The frequency of oscillations depends ontemperature.

3. There will be losses of energy due tohysteresis and eddy current.

Piezo Electric Generator or OscillatorPiezo Electric Generator or Oscillator

Principle : If mechanical pressure is

applied to one pair of opposite faces ofcertain crystals like quartz, equal andopposite electrical charges appear acrossits other faces. This is called as piezo-electric effect.

• If an electric field is applied to one pair offaces, the corresponding changes in thedimensions of the other pair of faces of thecrystal are produced. This is known asinverse piezo electric effect orelectrostriction.

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ConstructionConstruction

The circuit diagram of Piezo electricoscillator

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• The quartz crystal is placed between twometal plates A and B.

• The plates are connected to the primary(L3) of a transformer which is inductivelycoupled to the electronics oscillator.

• The electronic oscillator circuit is a basetuned oscillator circuit.

• The coils L1 and L2 of oscillator circuit aretaken from the secondary of a transformerT.

• The collector coil L2 is inductively coupledto base coil L1.

• The coil L1 and variable capacitor C1 formthe tank circuit of the oscillator.

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WorkingWorking

• When H.T. battery is switched on, the oscillator produces highfrequency alternating voltages with a frequency.

• Due to the transformer action, an oscillatory e.m.f. is induced in thecoil L3. This high frequency alternating voltages are fed on the platesA and B.

• Inverse piezo-electric effect takes place and the crystalcontracts and expands alternatively. The crystal is set intomechanical vibrations.

• The frequency of the vibration is given by

n =

112

1

CLf

Y

l2

1

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Y = Young’s modulus of the crystal andρ = density of the crystal.

• The variable condenser C1 is adjustedsuch that the frequency of the appliedAC voltage is equal to the naturalfrequency of the quartz crystal, andthus resonance takes place.

• The vibrating crystal produceslongitudinal ultrasonic waves oflarge amplitude.

• The variable condenser C1 is adjustedsuch that the frequency of the appliedAC voltage is equal to the naturalfrequency of the quartz crystal, andthus resonance takes place.

• The vibrating crystal produceslongitudinal ultrasonic waves oflarge amplitude.

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• Ultrasonic frequencies as high as 5 x 108Hzor 500 MHz can be obtained with thisarrangement.

• The output of this oscillator is very high.• It is not affected by temperature and

humidity.

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• The cost of piezo electric quartz is very high• The cutting and shaping of quartz crystal

are very complex.

AdvantagesAdvantages

DisadvantagesDisadvantages

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1. Piezoelectric Detector

Piezoelectric effect can also be used to detect ultrasonics. If

ultrasonics 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 ultrasonics.

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 sprinkled in the tube

collects in the form of heaps at the nodal points and is blown off

at the antinodal points. This method is used provided that the

wavelength is not very small.

Detection of Ultrasonic WavesDetection of Ultrasonic Waves

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3. 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 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’sbridge arrangement.

At the position of the antinodes, the temperature remains

constant. This will be indicated by the undisturbed balanced

position of the bridge.

Detection of Ultrasonic WavesDetection of Ultrasonic Waves

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4. 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.

Detection of Ultrasonic WavesDetection of Ultrasonic Waves

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1. Ultrasonic, infrasonic and audible waves

2. Phenomenon of Magnetostriction method

3. Construction, working and production of Ultrasonics byMagnetostriction method

4. Advantages and limitations

5. Properties of Ultrasonics

6. Construction, working and production of Ultrasonics bypizoelectric method

7. Advantages and limitations

SummarySummary

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1. Can we use copper rod in magnetostriction generator?

2. Why ultrasonic frequencies <3 MHz cannot be producedby magnetostriction method?

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

4. Why is tank circuit used in piezoelectric generator?

QuestionsQuestions