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High School Sound

ome - Education Resources - Science of NDT - Sound

SoundTable of Contents

1. Introduction2. Vibration3. The Speed of Sound in Air4. The Speed of Sound in Other

Materials5. Temperature and Speed of Sound6. The Human Ear7. The Components of Sound

8. Frequency and Pitch9. The Doppler Effect

10. Sound Wave Interference11. Refraction of Sound12. Reflection of Sound13. Pulse-echo Ultrasonic Test14. Ultrasound and Ultrasonic Testing15. Angle Beam Testing16. Immersion Ultrasonic Testing

NOTE: Many of the Flash animations havesound associated with them. Make sure youhave you computer volume turned up so that

you can hear them.

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Introduction to Sound

ome - Education Resources - Science of NDT - Ultrasound

INTRODUCTION TO SOUND

fter reading this section you will be able to do the following:

q Discuss why sound plays an important role in your life.

veryday your world is filled with a multitude of sounds. Sound can let youommunicate with others or let others communicate with you. It can be a warning ofanger or simply an enjoyable experience. Some sounds can be heard by dogs orther animals but cannot be heard by humans. Click on the buttons below to listen to

arious sounds.

t is hard to imagine a world without sound. The ability the hear is definitely anmportant sense. But people who are deaf are remarkable in the ways that they canompensate for their loss of hearing. You will learn more about sound in the followingages.

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Vibration


VIBRATION


q Summarize how sound travels and explain what the energy is thatallows it to occur.

q Describe the different components waves have.q List and discuss the different types of waves that exist.

Questions

1. In each case, what is the energy that makes the sound happen?

he discussion feature allows you to summarize the concepts that you learn in the sectionbove. Click the Discussion button below to start.

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The Speed of Sound in Air


THE SPEED OF SOUND IN AIR


q Discuss the relationship between the speed of sound and speed oflight.

q Describe what the sound barrier is.

Questions

1. What conclusion can you draw about the speed of sound relative tothe speed of light?

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The Speed of Sound in Other Materials


THE SPEED OF SOUND IN OTHER MATERIALS


q Explain whether or not the speed of sound is constant for allmaterials.

q Describe what elasticity and density are and what relationship theyhave to the speed of sound.

ou are in a long mining tunnel deep under the earth. You have a friend that iseveral thousands of feet away from you in the tunnel. You tell this person using aalkie talkie to yell and clang on the pipes on the tunnel floor at the same time. Presshe play button below to find out what happens.

Speeds of Sound

Material Speed of Sound

Rubber 60 m/s

Air at 40oC 355 m/s

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The Speed of Sound in Other Materials

Glass 4540 m/s

Lead 1210 m/s

Stone 5971 m/s

Copper 3100 m/s

Questions

1. What happens when you change the material through which thesound travels?

2. Through which material does sound move faster? Why do you thinkit is faster?

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Temperature and the Speed of Sound


TEMPERATURE AND THE SPEED OF SOUND


q Observe the demonstrations below and explain the differences inthe speed of sound when the temperature is changed.

Speed of Sound

358.0 m/s

343.6 m/s

330.4 m/s

Questions

1. What happens to the speed of sound when the temperaturechanges?

2. Does sound travel faster or slower as temperature increases?

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Temperature and the Speed of Sound

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The Human Ear


THE HUMAN EAR


q Explain the main parts of the human ear and how they contribute toour hearing.

he human ear has three main sections, which consist of the outer ear, the middlear, and the inner ear. Sound waves enter your outer ear and travel through your earanal to the middle ear. The ear canal channels the waves to your eardrum, a thin,ensitive membrane stretched tightly over the entrance to your middle ear. Theaves cause your eardrum to vibrate. It passes these vibrations on to the hammer,ne of three tiny bones in your ear. The hammer vibrating causes the anvil, the smallone touching the hammer, to vibrate. The anvil passes these vibrations to thetirrup, another small bone which touches the anvil. From the stirrup, the vibrationsass into the inner ear. The stirrup touches a liquid filled sack and the vibrations

ravel into the cochlea, which is shaped like a shell. Inside the cochlea, there areundreds of special cells attached to nerve fibers, which can transmit information tohe brain. The brain processes the information from the ear and lets us distinguishetween different types of sounds.

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The Components of Sound


THE COMPONENTS OF SOUND


q Explain what three things cause the differences in sounds.q Discuss why some sounds are pleasing and others are not.

Why are sounds different?

s you know, there are many different sounds. Fire alarms are loud, whispers areoft, sopranos sing high, tubas play low, every one of your friends has a differentoice. The differences between sounds are caused by intensity, pitch, and tone.

What is the difference between music and noise?

oth music and noise are sounds, but how can we tell the difference? Some sounds,ke construction work, are unpleasant. While others, such as your favorite band, arenjoyable to listen to. If this was the only way to tell the difference between noisend music, everyones opinion would be different. The sound of rain might be pleasantusic to you, while the sound of your little brother practicing piano might be an

npleasant noise. To help classify sounds, there are three properties which a soundust have to be musical.

sound must have an identifiable pitch, a good or pleasing quality of tone, andepeating pattern or rhythm to be music. Noise on the other hand has no identifiabletch, no pleasing tone, and no steady rhythm.

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Frequency and Pitch


FREQUENCY AND PITCH


q Explain how you can change pitch by altering sources.q Describe what resonance is.

Questions

1. What happens when you make the string shorter? Longer? Thicker?Thinner? Tighter? Looser?

2. What happens when you make the string out of different material?

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Frequency and Pitch

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The Doppler Effect


THE DOPPLER EFFECT


q Observe the experiment below and discuss why you hear adifference when an object is moving, but the sound itself is notchanging.

Questions

1. If the noise the object makes is not changing, why do you hear achange?

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Sound Wave Interference


SOUND WAVE INTERFERENCE


q Explain what can happen to the energy of sound waves when thewaves interact.

q Compare and contrast constructive interference and destructiveinterference.

q Explain what a critical angle is.

Questions

1. What is the difference in sound between the overlap area and thesingle color area?

2. What is the difference in sound in the white area?

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Sound Wave Interference

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Refraction of Sound


REFRACTION OF SOUND


q Define sound refraction and why it occurs.q Describe what occurs when a sound wave reaches the critical angle.

Click on the experiment button below to open a Java applet.Make sure that your browser is set to allow you to see Java Applets.

Questions

1. What happens to sound traveling in one material when it entersanother material at an angle normal to surface between the two

materials (90 degrees to the surface)?2. What happens to sound traveling in one material when it enters

another material at an angle other than normal to surface betweenthe two materials?

3. What happens to the sound as the incident angle approaches beingparallel to the surface?

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Reflection of Sound


REFLECTION OF SOUND


q Observe the experiment below and explain why the wave reactsdifferently depending on what surface it hits.

q Discuss how echoes are made.

he Multi- Material Room

Questions

1. What happens when a sound wave hits a concave shaped surface?2. Is the sound reflected back to the source from a concave shaped

surface more or less than that reflected from a flat surface?3. What happens when a sound wave hits the porous surface?4. What happens when a sound wave hits an irregular surface?

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Reflection of Sound

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Pulse-Echo Ultrasonic Test


PULSE-ECHO ULTRASONIC TEST


q Explain what a pulse-echo ultrasonic test is measuring.q In general terms, explain how a pulse-echo ultrasonic test is

completed.q Perform your own simulated pulse-echo ultrasonic test and be able

to communicate what is happening.

our Turn - Try this normal beam test

pulse-echo ultrasonic measurement can determine the location of a discontinuityith a part or structure by accurately measuring the time required for a shorttrasonic pulse generated by a transducer to travel through a thickness of theaterial. Then it reflects from the back or surface of a discontinuity and iseturned to the transducer.

he applet below allows you to move the transducer on the surface of a stainlessteel test block and see the reflected echoes as the would appear on an oscilloscope.

What the graphs tell us?

he ultrasonic tester graphs a peak of energy whenever the transducer receives aeflected wave. As you recall, sound is reflected any time a wave changes mediums.hus, there will be a peak anytime the waves change mediums. Right when the initialulse of energy is sent from the tester, some is reflected as the ultrasonic waves gorom the transducer into the couplant. The first peak is therefore said to recordhe energy of the initial pulse. The next peak in a material with no defects is theackwall peak. This is the reflection from waves changing between the bottom of theest material and the material behind it, such as air or the table it is on. Theackwall peak will not have as much energy as the first pulse, because some of thenergy is absorbed by the test object and some into the material behind it.

he amount of distance between peaks on the graph can be used to locate theefects. If the graph has 10 divisions and the test object is 2 inches thick, eachvision represents 0.2 inches. If a defect peak occurs at the 8th division, we know

he defect is located 1.6 (0.2 x 8) inches into the test object.

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Pulse-Echo Ultrasonic Test

What if the thickness is unknown?

f the thickness of the object is unknown, it can be calculated using the amount ofme it takes for the backwall peak to occur. The thickness of the object is traveledwice in that time, once to the backwall and once returning to the transducer. If wenow the speed of our sound, then we can calculate the distance it traveled, which ishe thickness of the object times two.

What happens when a defect is present?

f a defect is present, it will reflect energy sooner also. Another peak would thenppear from the defect. Since it reflected energy sooner than the back wall, theefect's energy would be received sooner. This causes the defect peak to appearefore the backwall peak. Since some of the energy is absorbed and reflected byhe defect, less will reach the backwall. Thus the peak of the backwall will be lowerhan if had there been no defect interrupting the sound wave.

When the wave returns to the transducer, some of its energy bounces back into theest object and heads towards the back wall again. This second reflection will

roduce peaks similar to the first set of backwall peaks. Some of the energy,owever, has been lost, so the height of all the peaks will be lower. Theseeflections, called multiples, will continue until all the sound energy has beenbsorbed or lost through transmission across the interfaces.

eview

1. A pulse-echo ultrasonic test can locate a discontinuity in a material.2. During a pulse-echo ultrasonic test the time is measured to see how

long it takes a short ultrasonic pulse generated by a transducer totravel through a material, and then it is reflected from the back orsurface of a discontinuity and is returned to the transducer.

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Ultrasound and Ultrasonic Testing


ULTRASOUND AND ULTRASONIC TESTING


q Define the acronym "NDT."q Explain how sound is used in NDT to find flaws.q Explain how sound is used in NDT to measure material thickness.

Why is it important to understand sound?

here are many uses for sound in the world today. We have already mentioned a few.Musicians can benefit from a greater understanding of sound, architects mustnderstand sound to design effective auditoriums, detectives can use sound toentify people, and many new types of technology apply sound recognition. Another

se of sound is in the area of science called Nondestructive testing, or NDT.

What is NDT?

ondestructive testing is a method of finding defects in an object without harminghe object. Often finding these defects is a very important task. In the aircraft

dustry, NDT is used to look for internal changes or signs of wear on airplanes.iscovering defects will increase the safety of the passengers. The railroad industryso uses nondestructive testing to examine railway rails for signs of damage.nternally cracked rails could fracture and derail a train carrying wheat, coal, orven people. If an airplane or a rail had to be cut into pieces to be examined, it wouldestroy their usefulness. With NDT, defects may be found before they becomeangerous.

ow is ultrasound used in NDT?

ound with high frequencies, or ultrasound, is one method used in NDT. Basically,

trasonic waves are emitted from a transducer into an object and the returningaves are analyzed. If an impurity or a crack is present, the sound will bounce off ofhem and be seen in the returned signal. In order to create ultrasonic waves, aransducer contains a thin disk made of a crystalline material with piezoelectricroperties, such as quartz. When electricity is applied to piezoelectric materials,hey begin to vibrate, using the electrical energy to create movement. Rememberhat waves travel in every direction from the source. To keep the waves from goingackwards into the transducer and interfering with its reception of returning waves,n absorptive material is layered behind the crystal. Thus, the ultrasound waves onlyravel outward.

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ne type of ultrasonic testing places the transducer inontact with the test object. If the transducer is placedat on a surface to locate defects, the waves will gotraight into the material, bounce off a flat back wallnd return straight to the transducer. The animation onhe right, developed by NDTA, Wellington, New Zealand,ustrates that sound waves propagate into a objecteing tested and reflected waves return fromscontinuities along the sonic path. Some of the energy

ill be absorbed by the material, but some of it willeturn to the transducer.

ltrasonic measurements can be used to determine thehickness of materials and determine the location of ascontinuity within a part or structure by accuratelyeasuring the time required for a ultrasonic pulse to travel through the material andeflect from the backsurface or the discontinuity.

When the mechanical sound energy comes back to the transducer, it is convertedto electrical energy. Just as the piezoelectric crystal converted electrical energyto sound energy, it can also do the reverse. The mechanical vibrations in theaterial couple to the piezoelectric crystal which, in turn, generates electricalurrent.

our Turn - Try this normal beam test

pulse-echo ultrasonic measurement can determine the location of a discontinuityith a part or structure by accurately measuring the time required for a shorttrasonic pulse generated by a transducer to travel through a thickness of theaterial. Then it reflects from the back or surface of a discontinuity and is

eturned to the transducer.

he applet below allows you to move the transducer on the surface of a stainlessteel test block and see the reflected echoes as the would appear on an oscilloscope.

What the graphs tell us?

he ultrasonic tester graphs a peak of energy whenever the transducer receives aeflected wave. As you recall, sound is reflected any time a wave changes mediums.hus, there will be a peak anytime the waves change mediums. Right when the initialulse of energy is sent from the tester, some is reflected as the ultrasonic waves gorom the transducer into the couplant. The first peak is therefore said to recordhe energy of the initial pulse. The next peak in a material with no defects is theackwall peak. This is the reflection from waves changing between the bottom of the

est material and the material behind it, such as air or the table it is on. Theackwall peak will not have as much energy as the first pulse, because some of thenergy is absorbed by the test object and some into the material behind it.

he amount of distance between peaks on the graph can be used to locate the


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efects. If the graph has 10 divisions and the test object is 2 inches thick, eachvision represents 0.2 inches. If a defect peak occurs at the 8th division, we know

he defect is located 1.6 (0.2 x 8) inches into the test object.

What if the thickness is unknown?

f the thickness of the object is unknown, it can be calculated using the amount ofme it takes for the back wall peak to occur. The thickness of the object is traveledwice in that time, once to the back wall and once returning to the transducer. If wenow the speed of our sound, then we can calculate the distance it traveled, which ishe thickness of the object times two.

What happens when a defect is present?

f a defect is present, it will reflect energy sooner also. Another peak would thenppear from the defect. Since it reflected energy sooner than the back wall, theefect's energy would be received sooner. This causes the defect peak to appearefore the backwall peak. Since some of the energy is absorbed and reflected byhe defect, less will reach the backwall. Thus the peak of the backwall will be lower

han had there been no defect interrupting the sound wave.

When the wave returns to the transducer, some of its energy bounces back into theest object and heads towards the back wall again. This second reflection willroduce peaks similar to the first set of backwall peaks. Some of the energy,owever, has been lost, so the height of all the peaks will be lower. Theseeflections, called multiples, will continue until all the sound energy has beenbsorbed or lost through transmission across the interfaces.


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Angle Beam Testing


ANGLE BEAM TESTING


q Explain why it is important to know about sound refraction andSnell's Law when performing an angle beam inspection.

q Explain what a shear wave is.

ften straight beam testing will not find a defect. For example, if the defect isertical and thin enough, it will not reflect enough sound back to the transducer tot the tester know that it exists. In cases like this, another method of ultrasound

esting must be used. The other method of ultrasound testing is angle beam testing.ngle beam testing uses an incidence of other than 90 degrees. In contact testing,n angled plastic block is place between the transducer and the object to create theesired angle. For angle beam testing in immersion systems, a plastic block is noteeded because the transducer can simply be angled in the water.

f the angle of incidence is changed to be anything other than 90 degrees,ngitudinal waves and a second type of sound wave are produced. These other wavesre called shear waves. Because the wave entered at an angle, it does not all travelrectly through the material. Molecules in the test object are attracted to each

ther because solids have strong molecular bonds. The molecules carrying the soundre attracted to their surrounding molecules. Because of the angle, those soundarrying molecules get pulled by attracting forces in a direction perpendicular to therection of the wave. This produces shear waves, or waves whose molecules travel

erpendicular to the direction of the wave.

ngle beam testing and a change in the angle of incidence also creates furtheromplications. Remember that when a wave hits a surface at an angle, it will beefracted, or bent, when it enters the new medium. Thus, the shear waves and thengitudinal waves will be refracted in the test object. The amount of refractionepends on the speed of sound in the two mediums between which the wave israveling. Since the speed of shear waves is slower than the speed of longitudinal

aves, their angles of refraction will be different. By using Snells law, we canalculate the angle of refraction if we know the speed of sound in our material.

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Angle Beam Testing

eview

1. An angle beam test cannot be performed unless the angle ofrefraction is calculted using Snell's law, and the speed of soundmust be known too.

2. Shear waves are produced when the angle of incidence is not 90degrees.

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Immersion Ultrasonic Testing


IMMERSION ULTRASONIC TESTING


q Explain what an immersion ultrasonic test is and why they areneeded in NDT.

nother way to couple the sound fromransducer to a test object is coupling theound with water. This can be done with

quirters where the sound travels through aet of water or by immersing the transducernd test object in a tank of water. Bothechniques are called immersion testing. Inmmersion testing, the transducer is placed inhe water, above the test object, and a beamf sound is projected.

he graph of peaks using the immersion method is slightly different. Between theitial pulse and the back wall peaks there will be an additional peak caused by the

ound wave going from the water to the test material. This additional peak is calledhe front wall peak. The ultrasonic tester can be adjusted to ignore the initial pulseeak, so the first peak it will show is the front wall peak. Some energy is lost whenhe waves hit the test material, so the front wall peak is slightly lower than the peakf the initial pulse.

ltrasonic testing is an NDT test technique that interrogates components andtructures to detect internal and surface breaking defects, and measures wallhickness on hard (typically metallic or ceramic) components and structures.

ow does ultrasonic testing work?

ltrasonic operates on the principle of injecting a very short pulse of ultrasoundypically between 0.1 MHz and 100 Mhz) into a component or structure, and then

eceiving and analyzing any reflected sound pulses.

onventionally, an operator scans a transducer over the surface of the component inuch a way that he inspects all the area that is required to be tested by means of acanning motion. The inspection relies on the training and integrity of the operatoro ensure that he has inspected all that is necessary.

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Immersion Ultrasonic Testing

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ound pulses reflected from features within the component or structure areonventionally displayed on a screen. The operator also has to interpret these signalsnd report if the component or structure is defective or acceptable according to theest specification that he is given.

ypical detection limits for fine grained steel structures or components (handcanning) are single millimeter sized defects. Smaller defects can be detected bymmersion testing and a programmed scan pattern with higher frequency ultrasound

lower testing). Detection limits are in the order of 0.1 to 0.2 mm, although smallerefects (typically 0.04mm diameter) can be detected under laboratory conditions.

eview

1. Immersion testing is completed with squirters where the soundtravels through a jet of water or by taking the transducer and testobject and immersing them in a tank of water.

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