Ultrasonic SoundProperties

Examples of oscillation

ball ona spring

pendulum rotatingearth

The ball starts to oscillate as soon as it is pushed

Pulse

Oscillation

Movement of the ball over time

Time

One full oscillation T

Frequency

From the duration of one oscillation T the frequency f (number of oscillations per second) is calculated: 1/T = f

180 36090 270

Phase

Timea

0

The actual displacement a is termed as:

sinAa

Spectrum of sound

Frequency range Hz Description Example

0 - 20 Infrasound Earth quake

20 – 20,000 Audible sound Speech, music

> 20,000 Ultrasound Bat, Quartz crystal

gas liquid solid

Atomic structures

• low density• weak bonding forces

• medium density• medium bonding

forces

• high density • strong bonding forces• crystallographic

structure

Ultrasonic Sound Properties

• Sound waves used in most engineering applications are longitudinal waves.

• These mechanical waves can propagate in solids, liquids, and gases.

• The waves originate at a vibration source and move through a medium by material particles oscillating in the direction of the propagation of the wave.•Electromagnetic waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization and scattering.•Wave propagation is any of the ways in which waves travel.

Understanding wave propagation:

Spring = elastic bonding forceBall = atom

T

distance travelled

start of oscillation

T

Distance travelledFrom this we derive speed of wave through the medium (c):

or Wave equation

During one oscillation T the wave front propagates by the distance :

Tc

fc

• Longitudinal waves, also known as "l-waves", are waves that have the same direction of vibration as their direction of travel, which means that the movement of the medium is in the same direction as or the opposite direction to the motion of the wave. Mechanical longitudinal waves have been also referred to as compressional waves or compression waves.

Direction of oscillationDirection of propagationLongitudinal wave

Sound propagation

• A transverse wave is a moving wave that consists of oscillations occurring perpendicular (or right angled) to the direction of energy transfer. If a transverse wave is moving in the positive x-direction, its oscillations are in up and down directions that lie in the y–z plane.

Direction of propagationTransverse wave

Direction of oscillation

Sound propagation

• A light wave is an example of a transverse wave.

Plane pressure pulse wave.

transverse wave longitudinal waves

Wave propagation

AirWaterSteel, longSteel, trans

330 m/s1480 m/s

3250 m/s

5920 m/s

Longitudinal waves propagate in all kind of materials.Transverse waves only propagate in solid bodies. Due to the different type of oscillation, transverse wavestravel at lower speeds.Sound velocity mainly depends on the density and E-modulus of the material.

Reflection and Transmission

As soon as a sound wave comes to a change in material characteristics, e.g. the surface of a workpiece, or an internal inclusion, wave propagation will change too:

Behaviour at an interface

Medium 1 Medium 2

Interface

Incoming wave Transmitted wave

Reflected wave

Reflection + Transmission: Perspex - Steel

Incoming wave Transmitted wave

Reflected wave

Perspex Steel

1,87

1,00,87

Reflection + Transmission: Steel - Perspex

0,13

1,0

-0,87

Perspex Steel

Incoming wave Transmitted wave

Reflected wave

Amplitude of sound transmissions:

• Strong reflection• Double transmission

• No reflection• Single transmission

• Strong reflection with inverted phase

• No transmission

Water - Steel Copper - Steel Steel - Air

• The use of ultrasonic waves has primarily focused on two wave properties: speed of sound and attenuation.

• In addition, the Doppler frequency shift effect provides information in systems that involve fluid motion.

Application: speed of sound

• For some food systems, the speed of sound alone can be used to measure various physical properties.

• Speed of sound has been considered an accurate index for determining the concentration, composition and temperature of aqueous solutions.

• Contreras et al. studied the relation between speed of sound of sugar solutions with concentration and temperature and summarized the results with an empirical equation.

Application: attenuation

• In some food systems, ultrasonic attenuation is a more useful property than measurements of sound velocity.

• Attenuation indicates the energy loss of the sound waves during transmission in a medium.

• Gladwell et al. characterized the rheological properties of edible oil using ultrasonic attenuation behavior.

• The radius of the droplets in an emulsion can be measured by studying the attenuation based on ultrasonic scattering theories. This can be applied to evaluate the milk homogenization process.

Cavitation

• Cavitation is the formation and then immediate implosion of cavities in a liquid – i.e. small liquid-free zones ("bubbles") – that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low.

• Cavitation is a significant cause of wear in some engineering contexts. When entering high pressure areas, cavitation bubbles that implode on a metal surface cause cyclic stress.

• 2 types: Stable and Transient cavitation

Liquid driven transducer

Magnetostrictive transducer

• Electrical energy mechanical energy by magnetostriction from ferromagnetic materials (Ni, Fe) that occur dimension change in magnetic field (continuous change cause of oscillation produce low frequency of ultrasound).

Piezoelectric transducer

• Use ceramics that consist of piezoelectric materials (barium titanate, lead metaniobate)

Piezoelectric Effect

Piezoelectrical Crystal (Quartz)

Battery

+

+

The crystal gets thicker, due to a distortion of the crystal lattice

Piezoelectric Effect

+

The effect inverses with polarity change

Piezoelectric Effect

An alternating voltage generates crystal oscillations at the frequency f

U(f)

Sound wave with

frequency f

Piezoelectric Effect

A short voltage pulse generates an oscillation at the crystal‘s resonantfrequency f0

Short pulse ( < 1 µs )

Piezoelectric Effect

Reception of ultrasonic waves

A sound wave hitting a piezoelectric crystal, induces crystal vibration which then causes electrical voltages at the crystal surfaces.

Electrical energy

Piezoelectrical crystal Ultrasonic wave

Ultrasonic baths (40 kHz)

Cup horn type

Ultrasonic probe systems

• Transducer + horn

Horn

Ultrasonic probe systems

ultrasonic measurement system

Ultrasound Techniques

• Cause of direct oscillation : use in surface cleaning

• The pulse-echo ultrasound: A pulsed-echo system utilizes the echo of the transmitted sound waves to characterize the sample. A single transducer both transmits the sound waves and receives the echo in this system.

• c = speed of sound• d = distance that sound travels• t = time

• A1 = amplitude of the first echo

• Ai = amplitude of the incident wave• R = reflection coefficient

• Since R(air) ≈1 (i.e., no sample is in the sample holder),

• Z1 and Z2 = acoustic impedances of the material the wave is traveling through and the reflecting material, respectively.

• Acoustic impedance (Z) is defined as the wave pressure over the particle velocity.

• For a wave in a weakly absorbing medium the acoustic impedance can be written:

Ultrasound Techniques

• The Doppler effect : the echoes• The basis of ultrasonic Doppler velocimetry

(UDV) techniques is that reflected/scattered ultrasonic waves from a moving interface undergo a frequency shift.

• The measurement of a flow velocity is based on detecting the Doppler effect of the propagated ultrasound waves due to the fluid movement.

• There are two types of UDV: a continuous-wave Doppler system and a pulse-wave Doppler system. The pulse system is more commonly used in engineering applications.

Application in food industry

• Oxidation process: accelerate of aging process of wine using 1 MHz.

• Enzyme reactions: to reduce peroxidase activity by using 20 kHz at 371 W/cm2

• stimulation of living cells : use in fermentation• Sterilization process: use for surface

decontamination

• ultrasonic emulsification• Extraction• meat products: apply to extract protein by salt solution

(ultrasound can use to destroy myofibrils in tissue and connective tissues to obtain more soft tissues)

• Crystallization: accelerate nucleation rate and crystal growth• Degassing of liquids: cavitation• Acoustically aided filtration: agglomeration of fine particles• Acoustic drying: combination with drying to reduce drying

temp.• Defoaming

Sound reflection at a flaw

Probe

Flaw Sound travel path

Work piece

s

Plate testing

delaminationplate 0 2 4 6 8 10

IP

F

BE

IP = Initial pulseF = FlawBE = Backwall echo

0 2 4 6 8 10

ss

Wall thickness measurement

Corrosion

Through transmission testing

0 2 4 6 8 10

Through transmission signal

1

2

1

2

T

T

R

R

Flaw

Weld inspection

0 20 40 60 80 100

s

aa'

d

x

a = s sinßa = s sinß

a' = a - xa' = a - x

d' = s cosßd' = s cosß

d = 2T - t'd = 2T - t'

s

Lack of fusionWork piece with welding

Fß = probe angles = sound patha = surface distancea‘ = reduced surface distanced‘ = virtual depthd = actual depthT = material thickness

ß

Straight beam inspection techniques:Direct contact,

single element probeDirect contact,

dual element probeFixed delay

Immersion testingThrough transmission

surface = sound entry

backwall flaw

1 2

water delay

0 2 4 6 8 10 0 2 4 6 8 10

IE IEIP IP

BE BEF

1 2

Immersion testing