ENGINEERING PHYSICS -IUNIT – I

ULTRASONICS

1.1. INTRODUCTION:

Sound waves of frequencies ranging from 20 Hz to 20 KHz are known as ‘audio –

waves.

Waves of frequencies beyond the audible upper limit (>20 KHz) are called

‘ultrasonic waves’ Human ear can not hear these waves.

Sounds of frequencies below 20 KHz are called ‘infrasonics’. Human ear can

also not able to hear these waves.

Some animals and birds are able to generate and detect ultrasonic waves.

Grasshoppers can produce sounds of frequency 40,000 Hz. Mammals like cats, dogs, and

rates can produce and hear frequencies up to 30,000 Hz. Bats can produce and listen to

frequencies as high as 1,00,000 Hz.Bats are almost blind but can detect objects and

obstacles by producing ultrasonic waves and receiving echoes. Hence, they can fly about

avoiding the obstacles.

The wave length of ultrasonic waves is very small. This is responsible for many

applications.

Ultrasonic waves are widely used in marine applications, medical diagnostics, non

– destructive testing of finished products and so on.

1.2. PRODUCTION OF ULTRASONIC WAVES :

In general, there are three methods of producing the ultrasonic waves. They are,

i) Mechanical generator (or) Galton’s whistle method.

ii) Magnetostriction method.

iii) Piezo – electric method.

Let us discuss the generation of ultrasonic waves using the last two methods.

1.3. MAGNETOSTRICTION EFFECGT AND MAGNETOSTRICTION

GENERATOR :

Principle :

Magnetostriction effect is the principle of producing ultrasonic waves.

1.3.1. Magneto – Striction Effect :

When an alternating magnetic field is applied to a rod of ferromagnetic material

such as nickel, iron, cobalt etc., or alloys of it, then the rod is thrown into longitudinal

vibrations as shown in fig 1.1, thereby producing ultrasonic waves at resonance.

North Pole

FERROMAGNRTIC

South Pole fig 1.1 Magnetostriction Effect

1.3.2. MAGNETOSTRICTION GENERATOR (OR) OSCILLATOR

CONSTRUCTION:

Fig 1.1 shows the circuit diagram for magnetostriction oscillator. A permanently

magnetized ferromagnetic rod is clamped in the middle between two knife edges. Two

coils L1 and L2 are wound on the two ends of the rod as shown in fig. 1.2. and L1

connected to the base and L2 is connected to the collector of the transistor. C is a variable

capacitor. L2 and C form the resonant circuit of the tuned collector oscillator.

coil L1 coil L2

ultrasonic waves ultrasonic waves c c b mA + e

Fig 1.2 Magnetostriction oscillator circuit Working :

when the battery is switched ON, a current is produced by the transistor. The

current passing through the coil L2 causes a corresponding change in the magnetization

of the rod. Therefore, the rod starts vibrating due to magnetostriction effect.

‘When a coil is wounded over a vibrating ferromagnetic rod, an e,m.f is induced

in the coil’. This effect is called ‘converse magnetostriction effect’.

Due to the above effect, an e.m.f is induced in the coli L2. This induced e.m.f. is

fed to the base of the transistor as a feed back continuously. In this way, the current is

built up and the vibrations of the rod are maintained.

The frequency of the oscillatory circuit is adjusted by the condenser ( c ). 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 = Vibrating rod

i.e., 1 = 1 √ E 2Β√L2c 2Ρ √∆

where ‘Ρ’ is the length of the rod.

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

‘∆’ is the density of the rod.

Advantages :

i) The design of this oscillator is very simple and its production cost is low.

ii) At low frequencies, large power output is possible without the risk of damage

to the oscillatory circuit.

Disadvantages :

i) It can produce frequencies up to 3 MHz only.

ii) It can not withstand at higher temperatures

iii) There will be loss of energy due to hysteresis and eddy currents during the

oscillations.

Uses

It is used for high power applications such as drilling, grinding welding etc.,

1.4 PIEZO – ELECTRIC EFFECT AND PIEZO – ELECTRIC GENERATOR :

1.4.1. Piezoelectric Effect :When pressure (or) mechanical force is applied along one pair of opposite faces

of quartz crystal then equal and opposite charges are produced along the another pair of

opposite faces of the crystal as shown in fig. 1.3. this phenomenon is called ‘piexo –

electric effect’.

The electric charges developed by this method are proportional to the amount of

pressure.

Fig. 1.3 optic axis

optic axis Mechanical pressure

_ + Piezo – electric _ +ve charges crystal _ + _ + _ + _ve charges Mechanical pressure.

Fig 1.3 Piezoelectric Effect1.4.2 Inverse Piezo – electric Effect.

The piezo – electric effect is reversible. If an electric field is applied to one pair

of opposite faces of the quartz crystal, alternative mechanical expansions and

contractions are produced across the other pair of opposite faces of the crystal as shown

in fig. 1.4. This is known as ‘inverse piezo – electric effect’.

Ultrasonic waves + + + + + + Ultrasonic waves

_ _ _ _ _

optic axis _ _ _ _ _ _

Fig . 1.4. Inverse Piezo – electric effect.

1.4.3. Piezo – Electric crystals :

The crystals which produce piezo – effect and converse piezo – electric effect are

termed as ‘Piezo – electric crystals’.

Exanples : Quartz, Tourmaline, Rochelle salts etc.,

x axis x – axis y - cut

x-cut y axis y axis y axis z-axis Optic axis x axis

y axis y –axis y -axis (a) (b) x – axis y - axis

Fig . 1.5 piezo electric crystals

An example for a piezo – electric crystal made of quartz is shown in fig 1.5.a. It

has hexagonal shape with pyramids attached at both ends.

It consists of 3 axes as shown in fig 1.5.a . They are

(i) Optic axis (z – axis) which joins the edges of the pyramid.

(ii) Electrical axis (x – axis) which joins the corners of the hexagon and

(iii) Mechanical axis (y –axis) which joins the centers of the sides of the

hexagon.

x – Cut and y – Cut Crystals : When a quartz crystal (thin plate) is cut

perpendicular to the x – axis (electric axis) it is known as ‘X – cut crystal’. as shown in

fig. 1.5.b. X cut crystals are generally used to produce longitudinal ultrasonic waves.

when a quartz crystal (thin plate) is cut perpendicular to the y – axis (mechanical

axis), it is known as ‘Y – cut crystal’ as shown in fig 1.5.c. Y– cut crystals are generally

used to produce transverse ultrasonic waves.

1.4.4. Piexo – Electric Generator (or) Oscillator :

Principle:

Inverse piezo – electric effect is the principle behind the production of ultrasonic

using piezo – electric oscillator circuit. Here, ultrasonic waves are produced at resonance

i.e. when the frequencyof the oscillatory circuit is equal to the frequency of the vibrating

crystals.

Construction :

As shown in fig 1.6, the piezo – electric generator consists of primary and

secondary circuits. This is a tuned base oscillatory circuit.

The primary circuit consists of coils L1 and L2. The coil L1 is connected to the

base of the transistor. L2 is connected to the collector of the transistor. The capacitor C1

is used to vary the frequency of the oscillatory circuit (L2c )

The coil L2 is inductively coupled to the secondary circuit. The secondary

circuit comprises of the coil L3 and two metal plates A and B as shown in fig 1.6.

The crystal is kept in between the plates A and B for the production of ultrasonics.

Necessary biasing is given with the help of the battery.

quartz crystal T Primary S T

A L1 +

L3 shift e -emitter B b b - base Battery L2 c -collector _ Ultrasonic c e waves

L2 secondary

(Fig . 1.6) Piezo electric oscillator

Working :

when the battery is switched ON, the L1, L2 base oscillator produces an alternating

voltage with high frequency in the coils.

By the transformer action, an oscillatory e.m.f is induced in the coil L3 and is fed

to the plates A and B. Due to the inverse piezo – electric effect, the crystal is set into

mechanical vibrations.

The frequency of the oscillatory circuit is adjusted by the capacitor C. When this

frequency is equal to the frequency of the vibrating crystal, resonance occurs. At

resonance, the crystal vibrates vigorously and ultrasonic waves are produced along both

ends of the crystal.

Condition For Resonance

Frequency of the oscillatory circuit = Frequency of the vibrating crystal

i.e., 1 P √ E 2П√ L2C = 2t √ ρ

where P is a constant, P = 1, 2, 3 etc, for fundamental, 1st overtone, 2nd

overtone etc.,

E is the young’s modulus of the crystal.

∆ is the density of the crystal plate, and

4 is the thickness of the crystal plate.

Hence, the output frequency of the crystal is given as,

P √ E f = 2t √ρ

Advantages :

i) Using Piezo – electric method, we can generate frequencies upto 500 MHz

ii) We can get a stable and constant frequency of ultrasonic waves.

iii) Using different transducers, we can generate wide range of frequencies.

Disadvantages :

i) Piezo – electric crystals are very expensive.

ii) Cutting and shaping are not easy.

1.5. Detection of Ultrasonic Waves:

Ultrasonic waves can not be detected directly by human ears. Because they lie in

the audible region. But some animals like bats can hear it.

However, the presence of ultrasonics can be detected indirectly by the following

methods.

i) Kundt’s tube method.

ii) Sensitive flame method

iii) Thermal methods

iv) Piezo – electric methods.

(i) Kundt’s Tube Method of Detection:

A Kundt’s tube is used to detect ultrasonic waves. It consists of a long glass tube

fitted with an

Movable Piston Quartz tube A

cork Lycopodium powder B Quartz crystal

Fig.1.7. Kundt’s tube detector.

adjustable piston at one end as shown in fig 1.7. The quartz crystal is placed in between

the two metal plates A and B.

First lycopodium powder is sprinkled uniformly in the tube. Ultrasonic waves are

passed through the tube by the quartz crystal. The standing wave pattern is obtained by

adjusting the movable piston. the lycopodium powder is in the form of heaps at the nodes

and is blown off at the antinodes as shown in fig 1.7.

This confirms that they are ultrasonics. If the heaps are not formed, we can say

that they are not ultrasonic. Thus, a kundt’s tube can be used to detect ultrasonics.

ii) Sensitive Flame Method of Detection :

When a sensitive flame is placed in the path of ultrasonic, it flickers at the nodes

and it remains stationary at the antinodes. Thus, using sensitive flame, ultrasonics can be

detected.

iii) Thermal Detection Method :

During the ultrasonic wave propagation in a medium, alternate compression and

rarefactions are generated.

At any point in the medium, temperature increases

during compression and decreases during rarefaction. This

principle is used to detect the ultrasonic waves as follows.

Suppose ultrasonic waves are passed through platinum R1 R2

wire, connected to a metre bridge as shown in fig 1.8. The pt wire Bt

temperature of the platinum wire changes. Due to this, the R3

resistance of the platinum wire changes and hence the metre Ultrasonic waves

bridge goes to unbalanced condition. Thus, with respect to Fig 1.8 Thermal Detection

the balancing position of the metre bridge ultrasonic waves

are detected.

iv) Piezo – Electric Method of Detection :

This method of detecting ultrasonic waves is based on the piezo – electric effect

When one pair of opposite faces of quartz crystal is subjected to ultrasonic waves,

opposite electrical charges are developed on the other pair of opposite faces of the crystal

as shown in fig 1.9.

The charges developed are of small amplitude. Hence, these charges are amplified

and detected. Ultrasonic waves

+ _ +ve charge + + _ _ + + + _ _ _ ve charged

+ + _ _ + _

Fig 1.9 Piezo . electric elector

1.6. Properties of Ultrasonic waves :

i) The frequency of an ultrasonic wave is greater than 20,000 Hz

ii) They travel longer distance in the medium without any loss.

iii) They travel as well – defined sonic beam.

iv) Their velocity is constant for a homogeneous medium.

v) They have many modes of vibrations such as longitudinal, shear and different

modes of surface vibrations.

vi) Direct interaction of the ultrasonic waves with the materials takes place.

vii)At high frequencies, the wave length is shorter and hence produces higher

resolution in flaw detection.

viii) They undergo reflection and refraction at the interface, due to the change in

elastic and physical properties of the medium.

1.7. Cavitations:

When ultrasonics pass through a liquid medium a large number of low pressure

bubbles are produced. A high power compression and rarefaction of sound waves are

produced inside the liquid. This causes continuous formation and collapse of millions of

microscopic low pressure bubbles. This is called ‘cavitations’

The collapsing of these bubbles produces tiny explosions releasing tremendous

pressure of hundreds of atmospheres. The cavitation is effective at low frequencies

between 20 KHz to 40 KHz.

The size of the vapour bubbles formed by cavitations is inversely proportional to

the frequency of ultrasonic waves.

1.8 Determination of Velocity of Ultrasonic Waves in Liquid (Acoustic Grating)

Priciple :

When Ultrasonic waves travel through a transparent liquid, due to alternate

compressions and rarefactions, longitudinal waves are produced. If a monochromatic

light is passed through the liquid perpendicular to these waves, the liquid behaves as a

diffraction grating and the light gets diffracted. Such a grating is known as “Acoustic

Grating”. Here lines of compression act as opaque and rarefactions act as transparent for

light waves. This principle is used to find the wavelength (} ) velocity (v) of ultrasonic

waves in the liquid.

Construction:

The experimental arrangement consists of a glass tank filled with the liquid as

shown in fig 1.10.

piezo electric crystal

collimator piezo electric oscillator

_ _ _ _ Telescope _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Fig 1.10

A piezo – electric crystal (quartz) is fixed at the top of the glass tank. It is

connected to a piezo – electric oscillator.

An incandescent lamp is used as a monochromatic source of light ‘S’. A telescope

arrangement is made to view the diffraction pattern. A collimator consisting of two lenses

L1 and L2 is used to focus the light effectively onto the glass tank .

Working :

Case (i) When the piezo – electric crystal is kept at rest.

Initially the piezo – electric crystal is kept at rest. The monochromatic source of

light is switched ON. The light is focussed onto the glass tank filled with the liquid.

Then, a single image (or) a vertical peak is observed on the telescope. This shows that

there is no diffraction.

Case (ii) : When the piezo – electric crystal is set into vibration.

The piezo – electric crystal (quartz) is set into vibrations produced and are passed

through passed through the liquid using piezo – electric oscillator circuit. Ultrasonic

waves are produced at resonance and are. The waves are reflected by the walls of the

glass tank. The reflected waves form a standing wave pattern with nodes and antinodes in

the liquid.

At nodes, the density of the liquid becomes more and at the anti – nodes the

density becomes less. Thus the liquid behaves as a diffraction grating called ‘Acoustic

Grating”.

Now, the light from monochromatic source is passed through the acoustic grating.

the light gets diffracted and a diffraction pattern is viewed through a telescope. This

pattern consists of a central maxima and principal maxima on either side as shown in

fig 1.11.

Excited piezo – electric crystal

II order Ist orderLiquid ……………. maximo ……………. I order ……………. …………… 0 order. Central maximo ……………. I order …………….Stationary patte……………. II order Ist order maximo

Reflector

Expression for Ultrasonic wavelength and velocity :

The condition for diffracted maxima is given by

d Sin 2 = 08 (1)

Where, ‘d’ is the distance between successive nodes or antinodes ‘2’ is the angle of

spectrum.

8 is the wave length of the monochromatic source of light.

d d

u

If 8u is the wavelength of ultrasonics, then we can write,

8u = 2.d

(or) d = 8u / 2 (2)

Using equation (2) in equation (1), we have,

8u Sin 2 = 2n8

2n 8 (or) 8u = (3) sin 2

From equation (3), we can determine the wave length 8u of ultrasonic waves.

we know, Ultrasonic velocity = Frequency x wave length

i.e, v = Λ8u

2Λn8Using equation (3) , we have, v = (4)

sin2

From equation (4), we can determine the velocity v of ultrasonic waves.

This method is useful in measuring the wavelength and velocity of ultrasonic

waves in liquids and gases at various temperatures.

1.9. SONAR (SOund Navigation And Ranging)

SONAR is a device which stands for Sound Navigation And Ranging. It uses

highly directional ultrasonic waves for locating objects and determining their distance in

the sea. Sonar is based on the echo – sounding technique of ultrasound.

Principle :

When an ultrasonic wave is transmitted through water, it is reflected by the

objects in the water and an echo signal is produced.

By noting the time interval between the generation of the ultrasonic pulse and the

reception of the echo signal, the distance of the object can be easily calculated.

The change in frequency of the echo signal due to the ‘Doppler effect’ helps us to

determine the velocity of the object and its direction.

Description :

Fig 1.13 shows the block diagram of SONAR. It consists of a timing section

which triggers the electric pulse from the pulse generator. This pulse generator is

connected to the transducer (transmitter and receiver) so that ultrasonic can be produced.

The transducer is further connected with the CRO for display. The timing section

is also connected to the CRO display for reference of the tuning at which the pulse is

transmitted.

Timing Pulse CROSection Generator

Transducer Object Transducer (Transmitter) (Receiver)

Fig 1.13 Block diagram of SONAR

Working :

The transducer is mounded on the ship’s hull without any air gap between them as

shown in fig 1.14. The timing at which the pulse generated is recorded in the C.R.O. for

reference.

Ship

……………………………………………………….. …………………………………………Transducer

………………………………………(Transmitter Receiver)………………………………………………………..

……………Sea…………………………………………………………………(Object)……………………………………………………………………………………………………………………………………

1.14. SONARThis electrical pulse triggers the transducer which is kept in the hull of the ship.

These produce ultrasonic waves due to the principle of inverse - piezo – electric effect.

These ultrasonic waves are transmitted through the water in the sea. On striking the

object the ultrasonic waves (echo pulses) are reflected in all directions.

The echo pulses are picked by the receiver (Same transducer) as shown in fig

1.15. They are again converted in to electrical pulses due to piezo – electric effect. These

pulses are weak and hence amplified and are recorded in the C.R.O.

Transmitted 1st echo Pulse 2nd echo 3rd echo

Fig 1.15 SONAR Transmitted and Echo Pulse

The time interval (t) between the transmitted and received signals is noted from

the time base of the C.R.O.

By knowing the velocity of sound (v) in sea water, the distance of the object (d)

from the surface of water can be calculated using the relation,

2dvelocity v =

t v xt

i.e. d = 2

Uses of SONAR :

1. Sonar is used in the location of ship wrecks and submarines on the bottom of

the sea.

2. It is used for fish – finding application and the detection of the shoals.

3. It is useful in all merchant and military ships.

4. It is used for seismic survey.

1.10. Non – Destructive Testing (N.D.T)

NDT is a method of Testing the material without destruction (or) damaging the

material, by passing ultrasonic (or) x – rays (or) any other radiations through the material.

Therefore, in the NDT method, the product (or) specimen is examined without

impairing (or) changing its usefulness for future service.

NDT is used to examine the material to detect imperfections and to determine its

properties without damaging the material.

1.10.1. Objectives of NDT :

The objectives of NDT are

(i) To detect the internal (or) surface flaws.

(ii) To measure the dimensions of the materials,

(iii) To determine the materials structure and

(iv) To evaluate the physical and mechanical properties of the

materials.

If these factors are determined in the earlier stages of production process, the

quality of product, service life time, productivity, profits and safety factors can be

increased.

1.10.2 Various steps (or Aspects) Involved in NDT :

i) A probing (or) inspecting medium has to be applied to the specimen.

ii) The defects (or) material property (or) structure should modify this

probing medium.

iii) Proper detection of their modification should be sensed by a sensor.

iv) A device should be used to record the sensed output in a suitable form for

interpretation.

v) Now, from the interpretation, the information the specimen can be

evaluated.

1.10.3. Comparison Between Destructive And Non – Destructive Testing :

Sl.No. Destructive Testing Non – Restrictive Testing1 This method is applied only to a sample This method is applied directly on

production items.2 Different tests can not be performed on

the same item.

Different tests can be performed on

the same item.3 Tested parts are damaged during testing Tested parts are not damaged during

testing4 Preparation of testing specimen is often

required

Little (or) no specimen preparation is

required5 This test can not be performed even to

single part that are in service.

This test can be performed on various

parts that are in service.6 Time consumption is high Time consumption is low.7 Capital equipment and labour costs are

high

Labour costs are low.

1.10.4. Uses of NDT:

NDT is commonly employed in ship building manufacturing of aerospace

vehicles and automobiles, metals manufacturing, electric power plant construction and

maintenance.

1.10.4 Various Methods of NDT :

The defects (or) flows in a specimen can be detected by various NDT methods as

follows:

i) Visual inspection.

ii) Liquid Penetration method.

iii) Ultrasonic flow detection technique.

iv) Radiography methods.

a) x – ray radiography and Fluoroscopy.

b) r – ray radiography.

v) Eddy current testing.

vi) Magnetic particle testing.

1.11. Ultrasonic Flaw Detector – Pulse Echo System Through Transmission And Reflection Models :

Principle :

Whenever there is a change in medium, then the ultrasonic waves will be

reflected. This is the principle used in ultrasonic flow detector. Thus, from the intensity

of the reflected echoes, the flaws are detected without destroying the material. Hence, this

method is known as a ‘non – destructive testing’ method.

Description :

Fig 1.16 shows block diagram representation of ultrasonic flaw detector. It

consists of a piezo electric transducer coupled to the upper surface of the specimen

(metal) without any air gap between them. A pulse generator is connected to the

transducer (transmitter) to produce ultrasonics and the same is also connected to the

C.R.O. in order to record the transmitted pulse.

An amplifier is connected in between the transducer (Receiver) and the C.R.O. in

order to amplify the received signals. The timing section is used to note the time interval

between the transmitted and received signals in the C.R.O.

Timing Pulse / Freq C.R.O Amplifier Section Generator

Transducer Specimen Transducer (Transmitter) (Receiver)

Fig 1.16

Working

i) The pulse generator generates a high frequency wave and is applied to the

piezo electric transducer. The same is recorded in the C.R.O (pulse A as

shown in fig 1.17) for reference.

ii) The piezo electric crystals are resonated to produce ultrasonic waves.

iii) The ultrasonic waves are transmitted through the specimen.

iv) These ultrasonic waves travel through the specimen and is reflected back by

the other end.

v) The reflected ultrasonics are received by the transducer and is converted into

electrical signals. These reflected signals are amplified and is recorded in the

C.R.O (pulse B in fig 1.17)

vi) If the reflected pulse (pulse B) is the same as that of the transmitted pulse

(pulse A) as show in fig 1.17 then it indicates that there is no defect in the

specimen.

vii) On the other hand, if there is any defect on the specimen like a small hole (or)

pores, then the ultrasonics will be reflected by the holes (i.e.,) defects due to

the change in the medium.

viii) These defects give rise to another signal (pulse) in between pulses ‘A’ and

‘B’ as shown in fig 1.18.

A B A Z B

C.R.O

pulse generator Amplifier Pulse Amplifier Generator

Transducer O (Both Transmitter holeSpecimen and receiver) Specimen

Fig 1.17 Fig. 1.18

Similarly, if the specimen has many such holes, many z- pulses will be seen over

the screen of C.R.O (ix) from the time delay between the transmitted and received

pulses, the position of the hole can be found, (x)

From the height of the pulse received, the depth of the hole can also be

determined.

Advantages :

i) It can reveal internal defects.

ii) This method is highly sensitive to most of the cracks and flows.

iii) It gives immediate results at very low cost and at a very high speed.

iv) It indicates the size and location of the flaws exactly.

v) Since, there is no radiation in this process; it is a safest method among the

other methods.

Disadvantages :

i) It is difficult to find the defects of the specimen which has complex shapes.

ii) Trained, motivated technicians alone can perform this testing.

1.11. Ultrasonic Scanning Methods – a,b And c scan Displays :

Ultrasonic scanning method has the same principle construction and working as that

of the ultrasonic flaw detector.

Based on the position of the transducer and the output displayed in the C.R.O

screen, the scanning methods can be classified into three types.

i) A – Scan (or) Amplitude mode display

ii) B – Scan (or) Brightness mode display.

iii) T.M scan (or) Time Motion mode (or) C – Scan display.

i) A Scan (or) Amplitude Mode Display:

A – Scan mode display gives the one dimensional information. In this, a single

transducer is used to both transmit and receive the pulses from the specimen.

The received (or) reflected echo signals from the specimen is given to Y – p late

and time base is connected to X – plate of C.R.O. They are displayed as vertical spikes

along horizontal base line as shown in fig. 1.19.

A - Scan

Transmitted Pulse Defect pulse Reflected Pulse

TTTtt Voltage

Fig 1.19 Penetration depth

The height of the vertical spikes correspond to the strength of the echo from the

specimen. The position of the vertical spike from left to right along the x – axis

corresponds to the depth of penetration. This gives the total time taken by the ultrasonic

sound to travel from transmitter to the specimen and from the specimen to the receiver.

Hence, by passing the ultrasonic of known velocity and by noting the time delay,

the distance at which the defects or flaws are present can be found using the formula.

Distance = velocity x time

A – scan method is used to detect the position and size of the flaws.

ii) B – Scan (or) Brightness Mode Display :

B – mode means brightness modulation. The reflected echoes are shown as dots

on the screen. Every reflection produces a single dot. the brightness of the dot depends on

the intensity of the reflected echo. Fig 1.20 shows the B – mode display.

B – scan presents a two dimensional image of a stationary object. Here, the

transducer is moved with respect to the object.

B - Scan

Voltage

Penetration depth

Fig 1.20

iii) T.M. Scan (or) Time – Motion Mode (or) C – Scan Display :

This method is used to obtain the information about the moving object. It

combines certain features of A – scan and B – scan. In TM scan the transducer is held

stationary as in A – scan and echoes appear as dots as in the B – scan.

In fig.1.21, X – axis indicates the dots at relevant location (or) position of the

defect depending on the depth of the reflection.

Y – axis indicates the movement of the object. Therefore when the object moves,

the dots also move at a low speed. Thus, an object with an oscillatory movement will

appear as a trace as shown in fig 1.21.

Y Transmitted pulse Trace of a stationary Object

Movement

(or) Trace of a moving object

Motion

X

Location of position fig 1.21

Uses :

i) It is used to measure the velocity of the fluids through pipes and it gives the

three dimensional image of the specimen.

ii) It is also used to find the corrosion in pipes and pressure vessels.

1.12. SONOGRAMS – Recording of Movement of Hearts:

Heart :

Acoustic events of heart are divided into

i) Heart Sounds

ii) Murmers

i) Heart Sounds :

The sounds produced due to the opening and closings of heart valves are called

‘heart sounds’. They have low frequency and higher amplitudes.

Murmers :

Noisy characters which have long duration are called ‘Murmurs’. This is due to

turbulent flow of blood in the heart. They have high frequency and smaller amplitudes.

Phonocardiography (PCG) :

Electrocardiography (ECG) deals with the electrical activity of heart. Using ECG,

one can diagnose the rhythematic disturbance of the myocardial activity. But the valvular

defects can not be identified by ECG. Hence, PCG can be used for detecting these

defects.

‘Phonocardiography (PCG)’ deals with the graphic information of heart sounds.

PCG provides information on heart rate, rhythmicity, blood pumping, valve action etc.

Fetal Heart Movement Using Phonocardiography

Principle :

The principle used here is the ‘Doppler effect’. Doppler effect is defined as “

there is an apparent change in frequency between the incident sound waves on the fetus

and the sound waves reflected from the fetus”.Description:

As shown id fig 1.22, the block diagram representation of this method consists of

a radio frequency oscillator (RFO) to produce pulses of frequency 2 MHz. A radio

frequency amplifier (RFA) is used to mix the transmitted and received signals. A loud

speaker and the C.R.O. help to hear and view the output of the sound waves respectively.

RFO RFA Mixer Band CROPass

Filter

Mother’s Transducer APabdominal AMPwall

Fetus Fig 1.22

Working:

The transducer is fixed over the mother’s abdominal wall with the help of a gel

(or) oil. R.F.O is switched ON to drive the pulses. Hence the transducer produces waves

of 2 MHz. These ultrasonic waves are made to incident on the fetus.

The reflected ultrasonic waves from the fetus are received by the transducer and

are amplified by RFA. Both incident and received signals are mixed by the mixer. Then,

it is filtered to distinguish the various types of sound. Finally the Doppler shift (or)

change in frequency is measured. After necessary amplification by Audio frequency (AF)

amplifier, the movement of heart can be viewed visually by CV.R.O (or) can be heard

from the loud speaker.

Diagnosis :

It is found that when the heart of the fetus is moving towards the transducer, i.e.,

towards the source of sound, the shift in frequency is higher.

Otherwise, if the heart of the fetus is moving away from the transducer, i.e., away

from the source, the frequency shift is lower.

Thus, from the Doppler shift in frequency the movement of the fetal heart can be

found.

1.14Applications of Ultrasonics :

1.14.1 Industrial (or) Engineering Applications:

i) Sound Signaling :

The ultrasonic signals can be used for the identification for landing the ships. In

military field, the method of sound signaling can be used to identify our warships.

ii) Depth Sounding :

Echo sounding is the principle used to find the depth of the sea. The depth of the

sea can be directly calibrated using the instrument Fathometer or Echometer.

ii) Ultrasonic Welding and Soldering :

Some materials cannot be welded at high temperatures. In such cases the welding

can be done at room temperature using ultrasonic and is called cold welding. It is also

used for soldering aluminum foil condensers, aluminum wires etc., without any flux.

iv) Ultrasonic Drilling and Cutting:

Ultrasonic are used for making holes in very hard materials such as glass,

diamond etc.,

v) Ultrasonic cleaning and cutting:

Ultrasonic can also used in cleaning motors, aero planes, electronic assemblies

etc, Further, they are dried using acoustic drier.

vi) Coagulation ;

They are used in coagulation (changing from liquid phase to a semi – solid phase)

and crystallization. Hence, it can be used in the manufacturing of paints, polishers etc.,

vii) They are used to increase the sensitivity of colour in photographs by dispersion of

dye in the emulsion.

viii) They are used to remove air bubbles in the liquid metals and convert them into fused

metals.

ix) Low frequency ultrasonics are used in sorting paper fibers from the paper pulp.

x) They are used in sound Navigation purposes (SONAR).

1.14.2. Medical Applications :

i) Ultrasonic are used to release the contained enzymes.

ii) Ultrasonic are also used to find the velocity of blood flow and the movement of heart

in the human body.

iii) Meniere’s disease produces hearing loss in the ear. This can be cured by Ultrasonic

exposure in the ear through the destruction of diseased tissue in the middle ear.

iv) While treating the patient, it is used to gather the blood is columns separated by

plasma.

v) It is used to decrease the strength of the scar by affecting the directions of elastic

fibers, which are responsible for the scar.

vi) It is used to increase the fibroblasts by stimulation, producing myofibroblasts.

Recording of Heart Sounds Using Phonocardiography:

Fig. 1.23 shows the block diagram of the recording set up of phonocardiogram.

amplifier CRO Display

Microphone Filter FM Tape Recorder

Fig 1.23. Phonocardiogram

A condenser microphone is placed on the chest of the patient and the picked up

heart sounds are converted into electrical signals by the microphone.

The electrical signals from the microphone are amplified by an amplifier. The

output of the amplifier is given to the different band pass filters to separate the first,

second, third and fourth heart sounds. Each band pass filter output is given to

multichannel CRO display. Thus the different heart sounds are displayed separately on

the C.R.O.

For reference, the ECG method can be used. Electrodes are placed on the chest and limbs.

They pick up the electrical activity of the heart. The ECG signals are also displayed

simultaneously.

A tape recorder is used to store both PCG and ECG informations for future

reference.

Applications of PCG :

i) The damaged heart valves and valvular defects can be identified.

ii) By detecting the heart murmur, one can diagnose the neutral stenosis, mitral

regulation, aortic stenosis and arctic regurgitation. (Regurgitation means the

backward flow of blood through a defective heart valve).

iii) Fetus growth can be monitored by placing the microphone over the maternal

abdomen and picking up the fetal heart tones.

Differences Between ECG and PCG:

Sl.No Electro – cardiography (ECG) Phono – cardiography (PCG)1 The activity of heart such as rhythmic

disturbance can be found with the help

of electrical signals.

Recording the sound of pumping heart

is done with help of sound signals.

2 It is not possible to find valvular

defects.

Valvular defects can be identified by

using PCG.

Advantages of Ultrasonic over other Methods:

i) There is no mutation (or) residual effects

ii) There is no ionization

iii) There is no side effects

iv) By means of controlled focusing the normal tissues situated around diseased

tissues can be saved.

v) Here, the physiological effects depend on frequency and amplitude of

ultrasonics.

vi) It doesn’t affect the fetus in the mother’s womb during diagnosis.

vii) It is non – invasive compared to x –ray and laser radiations.