Introduction to the Physics of Ultrasound

Sound? Sound is a longitudinal wave that travels in a straight line Sound requires a medium for traveling Range of audible sound 20Hz -20kHz

When a longitudinal wave moves through a material, the particles of the material move backwards and forwards along the direction in which the wave is travelling. The wavelength of a longitudinal wave can be measured as the distance between the centre of two compressions

Below is a picture of a longitudinal wave travelling along a spring.

Ultrasonic wave Ultrasonic wave is a longitudinal wave with a frequency exceeding the upper limit of human hearing, which is 20 kHz.

Frequency greater than audible sound (f>20kHz). ULTRASONIC WAVE Frequency greater than audible sound (f>20kHz).

ULTRASONICS - Frequency greater than audible sound. INFRASONICS -Frequency lesser than audible sound SUPERSONIC – Speed greater than sound.

General Properties of Ultrasonics Acoustic waves Frequency > 20kHz High energy waves Speed in a thin rod v = (Y/ρ)1/2 ,in liquid (K/ρ)1/2 in gas (γP/ ρ)1/2 Speed depends on frequency. Greater the frequency, higher the velocity Modes of propagation- longitudinal & transverse Can be reflected, refracted & diffracted

Heating effect Stirring effect. Attenuation A=A0 e –αx Can be transmitted through large distances Stationary waves are produced

Piezo-electric effect When crystals like quartz, tourmaline etc. are subjected to stress along the mechanical axis, a P.D is developed across the perpendicular electrical axis. This is called Piezo electric effect Discovered by J & P. Curie Converse of this effect is also possible.

Converse piezo electric effect When a P.D is applied between the two opposite faces of a crystal, then stress or strain is induced along the perpendicular faces. If frequency of osculation is equal to the natural frequency of the crystal, resonance takes place and ultrasonics are produced.

Krautkramer NDT Ultrasonic Systems Piezoelectric Effect + Battery Piezoelectrical Crystal (Quartz) Krautkramer NDT Ultrasonic Systems

Krautkramer NDT Ultrasonic Systems Piezoelectric Effect + The crystal gets thicker, due to a distortion of the crystal lattice Krautkramer NDT Ultrasonic Systems

Krautkramer NDT Ultrasonic Systems Piezoelectric Effect + The effect inverses with polarity change Krautkramer NDT Ultrasonic Systems

U(f) Piezoelectric Effect Sound wave with frequency f An alternating voltage generates crystal oscillations at the frequency f Krautkramer NDT Ultrasonic Systems

Piezo electric crystals Piezoelectric crystal: a crystal that exhibits piezo electric effect is called piezo electric crystals Eg:- Quartz, tourmaline, Rochelle salt etc

Piezoelectric Crystals The thickness of the crystal determines the frequency of the scanhead Low Frequency 3 MHz High Frequency 10 MHz

Piezoelectric generator

The circuit is basically a Hartley oscillator. The tank circuit produces high frequency alternating potential which is fed to the quartz crystal.

Tank circuit To produce high frequency alternating potential. The frequency

Piezoelectric generator This is based on the converse Piezo electric effect . When the frequency of oscillation coincides with the natural frequency of the crystal, resonance will take place. This principle is used for the production of ultrasonics.

Frequency up to 15 MHz can be produced by this method.

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Magnetostriction Magnetostriction is a property of magnetic materials that causes them to change their shape when subjected to a magnetic field. The effect was first identified bby James Joule James Prescott Joule, (1818 – 1889)

Magnetostriction When a ferromagnetic rod is placed in an alternating M.F, with its length parallel to the field, the length of the rod increases and decreases rapidly (or the rod vibrates) This phenomena is known as Magnetostriction. This principle is used to produce ultrasonic waves

FREQUENCY The frequency generated is given by

APPLICATIONS

DEPTH OF SEA

Bats navigate using ultrasound

ULTRASONIC MIXING

SOUND SIGNALING

SOLDERING & METAL CUTTING

CLEANING

STERILIZING

BIOLOGICAL EFFECT

MEDICAL FIELD

DIAGNOSIS THERAPY

Ultrasound imaging: How does it work? Need to clarify difference between probe and element Mention coupling gel An ultrasound element acts like a bat. Emit ultrasound and detect echoes Map out boundary of object

More about how it works… Ultrasound probe The probe contains a transmitter and a receiver. A pulse of ultrasound is sent out by the transmitter. skin The pulse is reflected from a surface and returns to the receiver. The ultrasound machine measures how long it takes for the pulse to return Body tissue (muscle etc)

Ultrasound imaging: foetus feet Pictures from http://www.gehealthcare.com/uken/ This is a 2D ultrasound scan through the foot of a foetus. You can see some of the bones of the foot.

Ultrasound imaging: more surface rendering Pictures from http://www.gehealthcare.com/uken/

Ultrasound imaging: imaging the heart heart valves atrium ventricle Pictures from http://www.gehealthcare.com/uken/

Why Use Ultrasound? Ultrasound is very safe. There is no firm evidence that it does any harm to the body (or the baby in the case of pregnancy scans). X-rays are potentially dangerous, particularly to young children and pregnant women (they damage the unborn baby).

COAGULATION & CRYSTALLIZATION

FORMATION OF ALLOYS

TO FIND VELOCITY OF SOUND IN GASES & LIQUID

NON DESTRUCTIVE TESTING

Non destructive testing is a new method of testing of material Without destruction of the material This can be used to detect the imperfections like flaws , cracks , breakings, cavity, airpockets, discontinuities etc in material

Aircraft Inspection Nondestructive testing is used extensively during the manufacturing of aircraft.

Pulse echo system Pulse transmission system Pulse resonance system

Pulse echo system

back surface echo

Ultrasonic Inspection (Pulse-Echo) High frequency sound waves are introduced into a material and they are reflected back from surfaces or flaws. Reflected sound energy is displayed versus time, and inspector can visualize a cross section of the specimen showing the depth of features that reflect sound. f 2 4 6 8 10 initial pulse crack echo crack plate Oscilloscope, or flaw detector screen

Pulse transmission system

Advantages of Ultrasonic testing Simple and accurate Very minute flows can be detected Nature, size and location of defect can be accurately determined Cheap method Used as high speed testing Large specimen can be inspected in a short time.

Ultrasonic diffractometer To find the velocity and wavelength of ultrasonics a quartz crystal is set into vibrations in a liquid using an R.F oscillator. Ultasonics produced This makes the liquid an aquastic grating Collimated sodium light allowed to fall normally on the grating

Ultrasonic diffractometer.

Procedure

Diffraction takes place d sin θ= nλ--------(1) where d= the distance between 2 nodal or antinodal planes d= λa/2 or λa= 2d V=טλa