INTRODUCTION TO SOUND

INTRODUCTION TO SOUNDAfter reading this section you will be able to do the following: Discuss why sound plays an important role in your life.

Everyday your world is filled with a multitude of sounds. Sound can let you communicate with others or let others communicate with you. It can be a warning of danger or simply an enjoyable experience. Some sounds can be heard by dogs or other animals but cannot be heard by humans. Click on the buttons below to listen to various sounds.It is hard to imagine a world without sound. The ability the hear is definitely an important sense. But people who are deaf are remarkable in the ways that they can compensate for their loss of hearing. You will learn more about sound in the following pages.VIBRATIONAfter reading this section you will be able to do the following: Summarize how sound travels and explain what the energy is that allows it to occur.

Describe the different components waves have.

List and discuss the different types of waves that exist.

Questions1. In each case, what is the energy that makes the sound happen?

DiscussionHow does sound travel?Sound comes from a series of vibrations, and all the sounds you heard in the experiment occurred because of vibrations and energy. Sound travels in waves. When a source, or something that produces sound, vibrates, it produces kinetic energy. Your vocal chords and the strings on a guitar are both sources which vibrate to produce sounds. Sound energy is in the form of waves, which travel outward in all directions from the source. Without energy, there would be no sound. Let's take a closer look at sound waves.What do waves consist of?Waves are made up of compressions and rarefactions. Compression happens when molecules are forced, or pressed, together. Rarefaction is just the opposite, it occurs when molecules are given extra space and allowed to expand. Remember that sound is a type of kinetic energy. As the molecules are pressed together, they pass the kinetic energy to each other. Thus sound energy travels outward from the source. These molecules are part of a medium, which is anything that carries sound. Sound travels through air, water, or even a block of steel, thus, all are mediums for sound. Without a medium there are no molecules to carry the sound waves. In places like space, where there is no atmosphere, there is no sound.Let's look at the example of a stereo speaker. To produce sound, a thin surfaced cone, called a diaphragm, vibrates back and forth and creates energy. When the diaphragm moves to the right, its energy pushes the air molecules on the right together, opening up space for the molecules on the left to move into. We call the molecules on the right compressed and the molecules on the left rarefied. When the diaphragm moves to the left, the opposite happens. Now, the molecules to the left become compressed and the molecules to the right are rarefied. These alternating compressions and rarefactions produce a wave. One compression and one rarefaction is called a wavelength. Different sounds have different wavelengths.What do sound waves look like?We cannot see the energy in sound waves, but a sound wave can be modeled in two ways. One way is to create a graph of the reeds position at different times. Think of a number line. We call the reeds rest position zero. As it travels to the right, it moves to an increasingly positive position along the number line. As is travels to the left, its position becomes more and more negative. The graph of the reeds position as it vibrates looks like the sine graph, with its highest point when the reed is the farthest right and its lowest point when the reed is farthest left.Another graph can be made using the amount of pressure versus time. The most pressure is applied when the reed is moving through its original position. This is similar to the way we feel the greatest force on a swing as we move through the center, where we started. As the reed moves to the right, it is exerting less and less force. At its rightmost position, it is exerting no pressure and begins its trip the opposite way. Similarly, the reed is exerting no pressure at its leftmost position. For our graph, we say the pressure is the least, or the most of a pull rather than a push, when the reed moves through its starting position heading the opposite way. When the pressure is exerting a pulling force, we assign negative values to it. A graph of the pressure versus time also resembles the sine graph.More about compression and rarefaction Compression and rarefaction are terms defining the molecules near the reed. Compression is the point when the most pressure is being applied to a molecule and rarefaction is the point when the least pressure is applied. It is important to note that when a molecule to the right of the reed is experiencing compression, a molecule to the reeds left is experiencing rarefaction. For right hand molecules, compression occurs when the reed is in its original position, moving towards the right. This is where the molecule experiences the most pressure. Rarefaction happens when the reed is once again in the center, this time moving towards the left. At this point, the molecule is experiencing the least pressure. Of course, this is the opposite for molecules to the reeds left.Longitude

Shear

Surface

Plate(Symmetric)

Plate(Asymmetric) Different types of wavesAs the reed vibrates back and forth, the sound waves produced move the same direction (left and right). Waves that move in the same direction, or are parallel to their source are called longitudinal waves. Longitudinal sound waves are the easiest to produce and have the highest speed, however, it is possible to produce other types. Waves which move perpendicular to the way their source does are called shear waves. Shear waves travel at slower speeds than longitudinal waves, and can only be made in solids. Another type of wave is the surface wave. Surface waves travel at the surface of a material and move in elliptical orbits. They are slightly slower than shear waves but difficult to make. A final type of sound wave is the plate wave. These waves also move in elliptical orbits but are much more complex. They can only be created in very thin pieces of material.THE SPEED OF SOUND IN AIRAfter reading this section you will be able to do the following: Discuss the relationship between the speed of sound and speed of light.

Describe what the sound barrier is.

Questions1. What conclusion can you draw about the speed of sound relative to the speed of light? DiscussionSound and speedIf you have ever been to a baseball game or sat far away from the stage during a concert, you may have noticed something odd. You saw the batter hit the ball, but did not hear the crack of the impact until a few seconds later. Or, you saw the drummer strike the drum, but it took an extra moment before you heard it. This is because the speed of sound is slower than the speed of light, which we are used to seeing. The same thing is at work during a thunderstorm. Lightning and thunder both happen at the same time. We see the lightning almost instantaneously, but it takes longer to hear the thunder. Based on how much longer it takes to hear thunder tells us how far away the storm is. The longer it takes to hear the thunder, the farther the distance its sound had to travel and the farther away the storm is.The sound barrierThe speed of sound through warm air has been measured at 346 meters per second or 0.346 km per second. That is the same as a car traveling about 780 miles per hour! Even jets do not travel nearly that fast. The speed of sound is called the sound barrier. If a plane does break the sound barrier, or go faster than the speed of sound, it will produce a sonic boom. On October 14, 1947, Chuck Yeager did just that. In a small plane called the X-1, he was the first person to fly faster than the speed of sound and the listeners on the ground were the first to witness the loud echo of a sonic boom.Why do we see lightning before the thunder?Lightning travels at 300,000 kilometers per second or 186,000 miles per second. This is why we see it so much sooner than we hear the thunder. If lightning occurs a kilometer away, it will take 1.5 seconds before you hear the thunder. If you prefer to think in terms of miles, it takes thunder 5 seconds to travel 1 mile. Next time you see lightning count the number of seconds before the thunder arrives, then divide this number by 5 to find out how far away the lightning is.

THE SPEED OF SOUND IN OTHER MATERIALSAfter reading this section you will be able to do the following: Explain whether or not the speed of sound is constant for all materials.

Describe what elasticity and density are and what relationship they have to the speed of sound.

You are in a long mining tunnel deep under the earth. You have a friend that is several thousands of feet away from you in the tunnel. You tell this person using a walkie talkie to yell and clang on the pipes on the tunnel floor at the same time. Press the play button below to find out what happens.Speeds of SoundMaterialSpeed of Sound

Rubber60 m/s

Air at 40oC355 m/s

Glass4540 m/s

Lead1210 m/s

Stone5971 m/s

Copper3100 m/s

Questions1. What happens when you change the material through which the sound travels? 2. Through which material does sound move faster? Why do you think it is faster?

DiscussionWhat happens when there is a change in the material through which the sound travels? The speed of sound is not always the same. Remember that sound is a vibration of kinetic energy passed from molecule to molecule. The closer the molecules are to each other, the less energy it takes for them to pass the sound to each other and the faster sound can travel. It is easier for sound waves to go through solids than through liquids because the molecules are closer together in solids. Similarly, it is harder for sound to pass through gases than through liquids, because gaseous molecules are farther apart. The speed of sound is faster in solid materials and slower in liquids or gases.ElasticityThe speed of sound is also different for different types of solids, liquids, and gases. Some materials, like nickel or iron, are more elastic than others. Elasticity describes how quickly the molecules return to their original positions. Molecules that return to their original shape quickly are ready to move again more quickly, thus they can vibrate at higher speeds. Sound can travel faster through mediums that vibrate faster. Sound travels faster through elastic solids like nickel or iron than through solids like lead, which is less elastic.DensityThe density of a medium also affects the speed of sound. Density describes the mass of a substance per volume. A substance that is denser per volume has more mass per volume. Usually, larger molecules have more mass. If a material is denser because its molecules are larger, it will transmit sound slower. Sound waves are made up of kinetic energy. It takes more energy to make large molecules vibrate than it does to make smaller molecules vibrate. Thus, sound will travel at a slower rate in the denser object. If sound waves of the same energy were passed through a block of wood and a block of steel, which is more dense than the wood, the molecules of the steel would vibrate at a slower rate. Thus, sound passes more quickly through the wood, which is less dense.Suppose, however, that two substances are made of different molecules which weigh the same amount. They have the same volume, but one substance is more dense. We know the denser substance must have more mass per volume. Since both substances have molecules of similar weight, this extra mass means the denser substance has more molecules per volume. More molecules are squeezed into the same volume, therefore the molecules are closer together. Since sound is more easily transmitted between close molecules, it travels faster in the denser substance. Sound moves faster through denser air because the molecules are closer together in dense air and sound can be more easily passed on.TEMPERATURE AND THE SPEED OF SOUNDAfter reading this section you will be able to do the following: Observe the demonstrations below and explain the differences in the speed of sound when the temperature is changed.

Questions1. What happens to the speed of sound when the temperature changes? 2. Does sound travel faster or slower as temperature increases?

DiscussionTemperature and the speed of soundTemperature is also a condition that affects the speed of sound. Heat, like sound, is a form of kinetic energy. Molecules at higher temperatures have more energy, thus they can vibrate faster. Since the molecules vibrate faster, sound waves can travel more quickly. The speed of sound in room temperature air is 346 meters per second. This is faster than 331 meters per second, which is the speed of sound in air at freezing temperatures.The formula to find the speed of sound in air is as follows:v = 331m/s + .6m/s/C * Tv is the speed of sound and T is the temperature of the air. One thing to keep in mind is that this formula finds the average speed of sound for any given temperature. The speed of sound is also affected by other factors such as humidity and air pressure.

THE HUMAN EARAfter reading this section you will be able to do the following: Explain the main parts of the human ear and how they contribute to our hearing.

The human ear has three main sections, which consist of the outer ear, the middle ear, and the inner ear. Sound waves enter your outer ear and travel through your ear canal to the middle ear. The ear canal channels the waves to your eardrum, a thin, sensitive membrane stretched tightly over the entrance to your middle ear. The waves cause your eardrum to vibrate. It passes these vibrations on to the hammer, one of three tiny bones in your ear. The hammer vibrating causes the anvil, the small bone touching the hammer, to vibrate. The anvil passes these vibrations to the stirrup, another small bone which touches the anvil. From the stirrup, the vibrations pass into the inner ear. The stirrup touches a liquid filled sack and the vibrations travel into the cochlea, which is shaped like a shell. Inside the cochlea, there are hundreds of special cells attached to nerve fibers, which can transmit information to the brain. The brain processes the information from the ear and lets us distinguish between different types of sounds. THE COMPONENTS OF SOUNDAfter reading this section you will be able to do the following: Explain what three things cause the differences in sounds.

Discuss why some sounds are pleasing and others are not.

Why are sounds different?As you know, there are many different sounds. Fire alarms are loud, whispers are soft, sopranos sing high, tubas play low, every one of your friends has a different voice. The differences between sounds are caused by intensity, pitch, and tone. What is the difference between music and noise? Both music and noise are sounds, but how can we tell the difference? Some sounds, like construction work, are unpleasant. While others, such as your favorite band, are enjoyable to listen to. If this was the only way to tell the difference between noise and music, everyones opinion would be different. The sound of rain might be pleasant music to you, while the sound of your little brother practicing piano might be an unpleasant noise. To help classify sounds, there are three properties which a sound must have to be musical. A sound must have an identifiable pitch, a good or pleasing quality of tone, and repeating pattern or rhythm to be music. Noise on the other hand has no identifiable pitch, no pleasing tone, and no steady rhythm.FREQUENCY AND PITCHAfter reading this section you will be able to do the following: Explain how you can change pitch by altering sources.

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? DiscussionResonanceSound waves traveling through the air or other mediums sometimes affect the objects that they encounter. Recall that sound is caused by the molecules of a medium vibrating. The molecules vibrate at a specific frequency for each source, called its natural frequency. Steel, brass, and wood all have different natural frequencies.Occasionally, objects vibrating at their natural frequencies will cause resonance. Resonance is when objects with the same natural frequency as the vibrating source also begin to vibrate. Resonance does not happen very often and only affects object close to the vibrating source. Sometimes, the effects of resonance can be powerful. A singer can make glass vibrate enough to shatter, just by singing a note with the glasss natural frequency!Changing PitchA string vibrates with a particular fundamental frequency. It is possible, however, to produce pitches with different frequencies from the same string. The four properties of the string that affect its frequency are length, diameter, tension, and density. These properties are described below:1. When the length of a string is changed, it will vibrate with a different frequency. Shorter strings have higher frequency and therefore higher pitch. When a musician presses her finger on a string, she shortens its length. The more fingers she adds to the string, the shorter she makes it, and the higher the pitch will be.2. Diameter is the thickness of the string. Thick strings with large diameters vibrate slower and have lower frequencies than thin ones. A thin string with a 10 millimeter diameter will have a frequency twice as high as one with a larger, 20 millimeter diameter. This means that the thin string will sound one octave above the thicker one.3. A string stretched between two points, such as on a stringed instrument, will have tension. Tension refers to how tightly the string is stretched. Tightening the string gives it a higher frequency while loosening it lowers the frequency. When string players tighten or loosen their strings, they are altering the pitches to make them in tune.4. The density of a string will also affect its frequency. Remember that dense molecules vibrate at slower speeds. The more dense the string is, the slower it will vibrate, and the lower its frequency will be. Instruments often have strings made of different materials. The strings used for low pitches will be made of denser material than the strings used for high pitches.

THE DOPPLER EFFECTAfter reading this section you will be able to do the following: Observe the experiment below and discuss why you hear a difference when an object is moving, but the sound itself is not changing.

Questions1. If the noise the object makes is not changing, why do you hear a change? Discussion

Sound and motionWhen we are moving, or a source producing a sound is moving, we hear things differently. You may have noticed that a train whistle gets lower as it passes you. The whistle is not changing pitch, but you are hearing a change. This principle is known as the Doppler effect. The Doppler effect is named after the Austrian physicist, Christian Johann Doppler, who discovered it.What did Christian Johann Doppler discover?Doppler claimed that if a sound is getting closer to you, either because its source is approaching you or because you are going towards the source, the sound will seem higher than it really is. If you are heading away from a source or it is going away from you, he believed the sound would seem lower than its actual pitch. To test his theory, scientists hired 15 trumpeters to play on a moving train. As the train passed by them, they heard a drop in pitch, just like Doppler predicted.The Doppler effect happens because distance affects the amount of time it takes you to hear the sound. Imagine you are playing in the park and your friend rolls a ball to you. The ball would reach you sooner if you walked towards it and later if you moved away from it. The same is true for sound. Remember that frequency is wavelengths per time. If you hear a frequency in a shorter amount of time, it seems like you are hearing a higher frequency. For example, say you heard a sound that had 50 wavelengths by the time it reached you, it would have taken it 5 seconds to reach you. The frequency of that sound is 50 divided by 5, or 10 Hertz. Imagine you heard the same sound, but this time you were moving towards its source and it only took 2 seconds for 50 wavelengths to reach you. Now the frequency you hear is 50 divided by 2, or 25 Hertz. The frequency seemed higher because you were moving. If you were not moving, after 2 seconds, only 20 wavelengths would have reached you and the frequency would still sound like 10 Hertz.The opposite happens when the distance between you and a source of sound widens. Now it takes longer for you to hear a certain amount of wavelengths. Therefore, the frequency seems lower. The Doppler effect makes a pitch appear to change when you, or the source, are in motion.SOUND WAVE INTERFERENCEAfter reading this section you will be able to do the following: Explain what can happen to the energy of sound waves when the waves interact.

Compare and contrast constructive interference and destructive interference.

Explain what a critical angle is.

Questions1. What is the difference in sound between the overlap area and the single color area? 2. What is the difference in sound in the white area? DiscussionWave InterferenceWhen two or more sound waves from different sources are present at the same time, they interact with each other to produce a new wave. The new wave is the sum of all the different waves. Wave interaction is called interference. If the compressions and the rarefactions of the two waves line up, they strengthen each other and create a wave with a higher intensity. This type of interference is known as constructive.

When the compressions and rarefactions are out of phase, their interaction creates a wave with a dampened or lower intensity. This is destructive interference. When waves are interfering with each other destructively, the sound is louder in some places and softer in others. As a result, we hear pulses or beats in the sound.Dead spotsWaves can interfere so destructively with one another that they produce dead spots, or places where no sound at all can be heard. Dead spots occur when the compressions of one wave line up with the rarefactions from another wave and cancel each other. Engineers who design theaters or auditoriums must take into account sound wave interference. The shape of the building or stage and the materials used to build it are chosen based on interference patterns. They want every member of the audience to hear loud, clear sounds.Sound Traveling Between MaterialsRemember that sound travels faster in some materials than others. Sound waves travel outward in straight lines from their source until something interferes with their path. When sound changes mediums, or enters a different material, it is bent from its original direction. This change in angle of direction is called refraction. Refraction is caused by sound entering the new medium at an angle. Because of the angle, part of the wave enters the new medium first and changes speed. The difference in speeds causes the wave to bend.Critical AngleThe angle of refraction depends on the angle that the waves has when it enters the new medium. As the angle from the wave to the barrier between the two mediums gets smaller, the angle of refraction also gets closer to the barrier. When the waves entering angle reaches a certain point, called the critical angle, the refraction is parallel to the dividing line between the mediums. The critical angle depends on the two mediums the sound is coming from and going to. The speed of sound is different in every medium. Because of this, even if the sound hits at the same angle, the angle of refraction will vary for different mediums. The greater the difference in speed between the two mediums, the greater the critical angle will be.If sound hits the new medium with any angle smaller than the critical angle, it will not be able to enter. Instead it will bounce off, or be reflected, from the dividing line. When a wave is reflected, it returns with an angle equal to the one with which it hit. Whenever sound hits a new medium, part of it is reflected back. The rest enters the new medium and is refracted. Imagine sound is traveling through the air and hits the wall of a brick building. Some of the wave is reflected, but much of it enters the brick. The part of the wave going through the brick is now going faster than the part in the air. This is because brick is a solid whose molecules are closer together and can transmit sound more quickly. This difference in speeds caused the wave to bend, or be refracted. Suppose that the wave hits the building with an angle that is smaller than its critical angle. This time, the wave cannot enter the brick and all of it is reflected. If the wave struck the wall with an angle of 15 degrees, it would reflect back with the same angle from the other side. Since there are 180 degrees total, the reflected angle would be 165 degrees, 15 degrees measured from the other direction.

REFRACTION OF SOUNDAfter reading this section you will be able to do the following: Define sound refraction and why it occurs.

Describe what occurs when a sound wave reaches the critical angle.

Questions1. What happens to sound traveling in one material when it enters another 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 between the two materials?

3. What happens to the sound as the incident angle approaches being parallel to the surface? Sound traveling between materials Remember that sound travels faster in some materials than others. Sound waves travel outward in straight lines from their source until something interferes with their path. When sound changes mediums, or enters a different material, it is bent from its original direction. This change in angle of direction is called refraction. Refraction is caused by sound entering the new medium at an angle. Because of the angle, part of the wave enters the new medium first and changes speed. The difference in speeds causes the wave to bend.The angle of refraction depends on the angle that the waves has when it enters the new medium. As the angle from the wave to the barrier between the two mediums gets smaller, the angle of refraction also gets closer to the barrier. When the waves entering angle reaches a certain point, called the critical angle, the refraction is parallel to the dividing line between the mediums. The critical angle depends on the two mediums the sound is coming from and going to. The speed of sound is different in every medium. Because of this, even if the sound hits at the same angle, the angle of refraction will vary for different mediums. The greater the difference in speed between the two mediums, the greater the critical angle will be.If sound hits the new medium with any angle smaller than the critical angle, it will not be able to enter. Instead it will bounce off, or be reflected, from the dividing line. When a wave is reflected, it returns with an angle equal to the one with which it hit. Whenever sound hits a new medium, part of it is reflected back. The rest enters the new medium and is refracted. Imagine sound is traveling through the air and hits the wall of a brick building. Some of the wave is reflected, but much of it enters the brick. The part of the wave going through the brick is now going faster than the part in the air. This is because brick is a solid whose molecules are closer together and can transmit sound more quickly. This difference in speeds caused the wave to bend, or be refracted. Suppose that the wave hits the building with an angle that is smaller than its critical angle. This time, the wave cannot enter the brick and all of it is reflected. If the wave struck the wall with an angle of 15 degrees, it would reflect back with the same angle from the other side. Since there are 180 degrees total, the reflected angle would be 165 degrees, 15 degrees measured from the other direction.

REFLECTION OF SOUNDAfter reading this section you will be able to do the following: Observe the experiment below and explain why the wave reacts differently depending on what surface it hits.

Discuss how echoes are made.

Questions1. 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? DiscussionReflectionWhen sound reflects off a special curved surface called a parabola, it will bounce out in a straight line no matter where it originally hits. Many stages are designed as parabolas so the sound will go directly into the audience, instead of bouncing around on stage. If the parabola is closed off by another curved surface, it is called an ellipse. Sound will travel from one focus to the other, no matter where it strikes the wall. A whispering gallery is designed as an ellipse. If your friend stands at one focus and you stand at the other, his whisper will be heard clearly by you. No one in the rest of the room will hear anything.Reflection is responsible for many interesting phenomena. Echoes are the sound of your own voice reflecting back to your ears. The sound you hear ringing in an auditorium after the band has stopped playing is caused by reflection off the walls and other objects. A sound wave will continue to bounce around a room, or reverberate, until it has lost all its energy. A wave has some of its energy absorbed by the objects it hits. The rest is lost as heat energy.Sound AbsorptionEverything, even air, absorbs sound. One example of air absorbing sound waves happens during a thunderstorm. When you are very close to a storm, you hear thunder as a sharp crack. When the storm is farther away, you hear a low rumble instead. This is because air absorbs high frequencies more easily than low. By the time the thunder has reached you, all the high pitches are lost and only the low ones can be heard. The best absorptive material is full of holes that sound waves can bounce around in and lose energy. The energy lost as heat is too small to be felt, though, it can be detected by scientific instruments.How does sound reach every point in the room?Since sound travels in a straight path from its source, how does it get around corners? You already know that if you and your friend are standing on either side of a wall and there is an open door nearby, you will be able to hear what your friend says. Because you would not hear your friend if the door was closed, sound is not traveling through the wall. Instead, it must be going around the corner and out the door.You hear your friend because of sound diffraction. Diffraction uses the edges of a barrier as a secondary sound source that sends waves in a new direction. These secondary waves overlap and interfere with each other and the original waves, making the sound less clear. Working together, diffraction and reflection can send sounds to every part of a room.

PULSE-ECHO ULTRASONIC TESTAfter reading this section you will be able to do the following: Explain what a pulse-echo ultrasonic test is measuring.

In general terms, explain how a pulse-echo ultrasonic test is completed.

Perform your own simulated pulse-echo ultrasonic test and be able to communicate what is happening.

Your Turn - Try this normal beam testA pulse-echo ultrasonic measurement can determine the location of a discontinuity with a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of the material. Then it reflects from the back or surface of a discontinuity and is returned to the transducer.The applet below allows you to move the transducer on the surface of a stainless steel test block and see the reflected echoes as the would appear on an oscilloscope.

What the graphs tell us?The ultrasonic tester graphs a peak of energy whenever the transducer receives a reflected wave. As you recall, sound is reflected any time a wave changes mediums. Thus, there will be a peak anytime the waves change mediums. Right when the initial pulse of energy is sent from the tester, some is reflected as the ultrasonic waves go from the transducer into the couplant. The first peak is therefore said to record the energy of the initial pulse. The next peak in a material with no defects is the backwall peak. This is the reflection from waves changing between the bottom of the test material and the material behind it, such as air or the table it is on. The backwall peak will not have as much energy as the first pulse, because some of the energy is absorbed by the test object and some into the material behind it.The amount of distance between peaks on the graph can be used to locate the defects. If the graph has 10 divisions and the test object is 2 inches thick, each division represents 0.2 inches. If a defect peak occurs at the 8th division, we know the defect is located 1.6 (0.2 x 8) inches into the test object. What if the thickness is unknown?If the thickness of the object is unknown, it can be calculated using the amount of time it takes for the backwall peak to occur. The thickness of the object is traveled twice in that time, once to the backwall and once returning to the transducer. If we know the speed of our sound, then we can calculate the distance it traveled, which is the thickness of the object times two.What happens when a defect is present?If a defect is present, it will reflect energy sooner also. Another peak would then appear from the defect. Since it reflected energy sooner than the back wall, the defect's energy would be received sooner. This causes the defect peak to appear before the backwall peak. Since some of the energy is absorbed and reflected by the defect, less will reach the backwall. Thus the peak of the backwall will be lower than if had there been no defect interrupting the sound wave.When the wave returns to the transducer, some of its energy bounces back into the test object and heads towards the back wall again. This second reflection will produce peaks similar to the first set of backwall peaks. Some of the energy, however, has been lost, so the height of all the peaks will be lower. These reflections, called multiples, will continue until all the sound energy has been absorbed or lost through transmission across the interfaces.

Review1. 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 to travel through a material, and then it is reflected from the back or surface of a discontinuity and is returned to the transducer.

ULTRASOUND AND ULTRASONIC TESTINGAfter reading this section you will be able to do the following: Define the acronym "NDT."

Explain how sound is used in NDT to find flaws.

Explain how sound is used in NDT to measure material thickness.

Why is it important to understand sound?There 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 must understand sound to design effective auditoriums, detectives can use sound to identify people, and many new types of technology apply sound recognition. Another use of sound is in the area of science called Nondestructive testing, or NDT.What is NDT?Nondestructive testing is a method of finding defects in an object without harming the object. Often finding these defects is a very important task. In the aircraft industry, NDT is used to look for internal changes or signs of wear on airplanes. Discovering defects will increase the safety of the passengers. The railroad industry also uses nondestructive testing to examine railway rails for signs of damage. Internally cracked rails could fracture and derail a train carrying wheat, coal, or even people. If an airplane or a rail had to be cut into pieces to be examined, it would destroy their usefulness. With NDT, defects may be found before they become dangerous. How is ultrasound used in NDT?Sound with high frequencies, or ultrasound, is one method used in NDT. Basically, ultrasonic waves are emitted from a transducer into an object and the returning waves are analyzed. If an impurity or a crack is present, the sound will bounce off of them and be seen in the returned signal. In order to create ultrasonic waves, a transducer contains a thin disk made of a crystalline material with piezoelectric properties, such as quartz. When electricity is applied to piezoelectric materials, they begin to vibrate, using the electrical energy to create movement. Remember that waves travel in every direction from the source. To keep the waves from going backwards into the transducer and interfering with its reception of returning waves, an absorptive material is layered behind the crystal. Thus, the ultrasound waves only travel outward. One type of ultrasonic testing places the transducer in contact with the test object. If the transducer is placed flat on a surface to locate defects, the waves will go straight into the material, bounce off a flat back wall and return straight to the transducer. The animation on the right, developed by NDTA, Wellington, New Zealand, illustrates that sound waves propagate into a object being tested and reflected waves return from discontinuities along the sonic path. Some of the energy will be absorbed by the material, but some of it will return to the transducer. Ultrasonic measurements can be used to determine the thickness of materials and determine the location of a discontinuity within a part or structure by accurately measuring the time required for a ultrasonic pulse to travel through the material and reflect from the backsurface or the discontinuity.When the mechanical sound energy comes back to the transducer, it is converted into electrical energy. Just as the piezoelectric crystal converted electrical energy into sound energy, it can also do the reverse. The mechanical vibrations in the material couple to the piezoelectric crystal which, in turn, generates electrical current.Your Turn - Try this normal beam testA pulse-echo ultrasonic measurement can determine the location of a discontinuity with a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of the material. Then it reflects from the back or surface of a discontinuity and is returned to the transducer.The applet below allows you to move the transducer on the surface of a stainless steel test block and see the reflected echoes as the would appear on an oscilloscope.

What the graphs tell us?The ultrasonic tester graphs a peak of energy whenever the transducer receives a reflected wave. As you recall, sound is reflected any time a wave changes mediums. Thus, there will be a peak anytime the waves change mediums. Right when the initial pulse of energy is sent from the tester, some is reflected as the ultrasonic waves go from the transducer into the couplant. The first peak is therefore said to record the energy of the initial pulse. The next peak in a material with no defects is the backwall peak. This is the reflection from waves changing between the bottom of the test material and the material behind it, such as air or the table it is on. The backwall peak will not have as much energy as the first pulse, because some of the energy is absorbed by the test object and some into the material behind it.The amount of distance between peaks on the graph can be used to locate the defects. If the graph has 10 divisions and the test object is 2 inches thick, each division represents 0.2 inches. If a defect peak occurs at the 8th division, we know the defect is located 1.6 (0.2 x 8) inches into the test object. What if the thickness is unknown?If the thickness of the object is unknown, it can be calculated using the amount of time it takes for the back wall peak to occur. The thickness of the object is traveled twice in that time, once to the back wall and once returning to the transducer. If we know the speed of our sound, then we can calculate the distance it traveled, which is the thickness of the object times two.What happens when a defect is present?If a defect is present, it will reflect energy sooner also. Another peak would then appear from the defect. Since it reflected energy sooner than the back wall, the defect's energy would be received sooner. This causes the defect peak to appear before the backwall peak. Since some of the energy is absorbed and reflected by the defect, less will reach the backwall. Thus the peak of the backwall will be lower than had there been no defect interrupting the sound wave.When the wave returns to the transducer, some of its energy bounces back into the test object and heads towards the back wall again. This second reflection will produce peaks similar to the first set of backwall peaks. Some of the energy, however, has been lost, so the height of all the peaks will be lower. These reflections, called multiples, will continue until all the sound energy has been absorbed or lost through transmission across the interfaces.ANGLE BEAM TESTINGAfter reading this section you will be able to do the following: Explain why it is important to know about sound refraction and Snell's Law when performing an angle beam inspection.

Explain what a shear wave is.

Often straight beam testing will not find a defect. For example, if the defect is vertical and thin enough, it will not reflect enough sound back to the transducer to let the tester know that it exists. In cases like this, another method of ultrasound testing must be used. The other method of ultrasound testing is angle beam testing. Angle beam testing uses an incidence of other than 90 degrees. In contact testing, an angled plastic block is place between the transducer and the object to create the desired angle. For angle beam testing in immersion systems, a plastic block is not needed because the transducer can simply be angled in the water. If the angle of incidence is changed to be anything other than 90 degrees, longitudinal waves and a second type of sound wave are produced. These other waves are called shear waves. Because the wave entered at an angle, it does not all travel directly through the material. Molecules in the test object are attracted to each other because solids have strong molecular bonds. The molecules carrying the sound are attracted to their surrounding molecules. Because of the angle, those sound carrying molecules get pulled by attracting forces in a direction perpendicular to the direction of the wave. This produces shear waves, or waves whose molecules travel perpendicular to the direction of the wave.

Angle beam testing and a change in the angle of incidence also creates further complications. Remember that when a wave hits a surface at an angle, it will be refracted, or bent, when it enters the new medium. Thus, the shear waves and the longitudinal waves will be refracted in the test object. The amount of refraction depends on the speed of sound in the two mediums between which the wave is traveling. Since the speed of shear waves is slower than the speed of longitudinal waves, their angles of refraction will be different. By using Snells law, we can calculate the angle of refraction if we know the speed of sound in our material.Review1. An angle beam test cannot be performed unless the angle of refraction is calculted using Snell's law, and the speed of sound must be known too.

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

IMMERSION ULTRASONIC TESTINGAfter reading this section you will be able to do the following: Explain what an immersion ultrasonic test is and why they are needed in NDT.

Another way to couple the sound from transducer to a test object is coupling the sound with water. This can be done with squirters where the sound travels through a jet of water or by immersing the transducer and test object in a tank of water. Both techniques are called immersion testing. In immersion testing, the transducer is placed in the water, above the test object, and a beam of sound is projected. The graph of peaks using the immersion method is slightly different. Between the initial pulse and the back wall peaks there will be an additional peak caused by the sound wave going from the water to the test material. This additional peak is called the front wall peak. The ultrasonic tester can be adjusted to ignore the initial pulse peak, so the first peak it will show is the front wall peak. Some energy is lost when the waves hit the test material, so the front wall peak is slightly lower than the peak of the initial pulse.Ultrasonic testing is an NDT test technique that interrogates components and structures to detect internal and surface breaking defects, and measures wall thickness on hard (typically metallic or ceramic) components and structures.How does ultrasonic testing work? Ultrasonic operates on the principle of injecting a very short pulse of ultrasound (typically between 0.1 MHz and 100 Mhz) into a component or structure, and then receiving and analyzing any reflected sound pulses.Conventionally, an operator scans a transducer over the surface of the component in such a way that he inspects all the area that is required to be tested by means of a scanning motion. The inspection relies on the training and integrity of the operator to ensure that he has inspected all that is necessary.----------Sound pulses reflected from features within the component or structure are conventionally displayed on a screen. The operator also has to interpret these signals and report if the component or structure is defective or acceptable according to the test specification that he is given.Typical detection limits for fine grained steel structures or components (hand scanning) are single millimeter sized defects. Smaller defects can be detected by immersion testing and a programmed scan pattern with higher frequency ultrasound (slower testing). Detection limits are in the order of 0.1 to 0.2 mm, although smaller defects (typically 0.04mm diameter) can be detected under laboratory conditions.Review1. Immersion testing is completed with squirters where the sound travels through a jet of water or by taking the transducer and test object and immersing them in a tank of waterPAGE 1