VIBRATION
Questions
- In each case, what is the energy that makes the sound happen?
The discussion feature allows you to summarize the concepts that you learn in the section above. Click the Discussion button below to start.
What is sound and how does it travel?
All of the sounds you heard on the previous page occurred because mechanical energy produced by your computer speaker was transferred to your ear through the movement of atomic particles. Sound is a pressure disturbance that moves through a medium in the form of mechanical waves. When a force is exerted on an atom, it moves from its rest or equilibrium position and exerts a force on the adjacent particles. These adjacent particles are moved from their rest position and this continues throughout the medium. This transfer of energy from one particle to the next is how sound travels through a medium. The words "mechanical wave" are used to describe the distribution of energy through a medium by the transfer of energy from one particle to the next.
Waves of sound energy move outward in all directions from the source. Your vocal chords and the strings on a guitar are both sources which vibrate to produce sound waves. Without energy, there would be no sound. Let's take a closer look at sound waves.
What do waves consist of?
Sound or pressure waves are made up of compressions and rarefactions.Compression happens when particles are forced, or pressed, together.Rarefaction is just the opposite, it occurs when particles are given extra space and allowed to expand. Remember that sound is a type of kinetic energy. As the particles are moved from their rest position, they exert a force of the adjacent particles and pass the kinetic energy. Thus sound energy travels outward from the source.
Sound travels through air, water, or a block of steel; thus, all are mediums for sound. Without a medium there are no particles to carry the sound waves. The word "particle" suggests a tiny concentration of matter capable of transmitting energy. A particle could be an atom or molecule. In places like space, where there is no atmosphere, there are too few atomic particles to transfer the sound energy.
Let's look at the example of a stereo speaker. To produce sound, a thin surfaced cone, called a diaphragm, is caused to vibrate using electromagnetic 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 diaphragm's position at different times. Think of a number line. We call the diaphragm's 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 diaphragm’s position as it vibrates looks like the sine graph, with its highest point when the diaphragm is the farthest right and its lowest point when it is farthest left.
Another graph can be made using the amount of force on the molecules versus time. The force is greatest when the diaphragm 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 diaphragm moves to the right, there is less and less force. At its rightmost position, it is exerting no force (due to pressure) and begins its trip the opposite way. Similarly, the diaphragm is exerting no force at its leftmost position. For our graph, we say the force is least when the diaphragm moves through its starting position heading the opposite way. When the force is exerting a pulling force, we assign negative values to it. A graph of the force versus time also resembles the sine graph.
More about compression and rarefaction
Compression and rarefaction are terms defining the molecules near the diaphragm. Compression is the point when the most force is being applied to a molecule and rarefaction is the point when the least force is applied. It is important to note that when a molecule to the right of the diaphragm is experiencing compression, a molecule to the diaphragm's left is experiencing rarefaction. For right side molecules, compression occurs when the diaphragm is in its original position, moving towards the right. This is where the molecule experiences the most force. Rarefaction happens when the diaphragm is once again in the center, this time moving towards the left. At this point, the molecule is experiencing the least force. Of course, this is the opposite for molecules to the diaphragm's left.
Longitude
Shear
Surface
Plate(Symmetric)
Plate(Asymmetric)
Different types of waves
As the diaphragm vibrates back and forth, the sound waves produced move the same direction (left and right). Waves that travel in the same direction as the particle movement 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 direction particle movement are called shear waves or transverse waves. Shear waves travel at slower speeds than longitudinal waves, and can only be made in solids. Think of a stretched out slinky, you can create a longitudinal wave by quickly pushing and pulling one end of the slinky. This causes longitudinal waves for form and propagate to the other end. A shear wave can be created by taking one end of the slinky and moving it up and down. This generates a wave that moves up and down as it travels the length of the slinky.
Another type of wave is the surface wave. Surface waves travel at the surface of a material with the particles move in elliptical orbits. They are slightly slower than shear waves and fairly difficult to make. A final type of sound wave is the plate wave. The particles of these waves also move in elliptical orbits but plate waves can only be created in very thin pieces of material.
THE SPEED OF SOUND IN AIR
- What conclusion can you draw about the speed of sound relative to the speed of light?
If 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 barrier
The speed of sound through warm air at sea level 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 most jet airplanes do not travel that fast. When a plane does go faster than speed of sound, it is said to break the sound barrier and a sonic boom is produced. 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 hear the loud shock wave of a sonic boom.
Why do we see lightning before the thunder?
The flash of light from lightning travels at about 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, the light arrives almost immediately (1/300,000 of a second) but it takes sound nearly 3 seconds to arrive. If you prefer to think in terms of miles, it takes sound nearly 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 MATERIALS
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 Sound
Material
|
Speed of Sound
|
Rubber
|
60 m/s
|
Air at 40oC
|
355 m/s
|
Air at 20oC
|
343 m/s
|
Lead
|
1210 m/s
|
Gold
|
3240 m/s
|
Glass
|
4540 m/s
|
Copper
|
4600 m/s
|
Aluminum
|
6320 m/s
|
- What happens when you change the material through which the sound travels?
- Through which material does sound move faster? Why do you think it is faster?
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 and the tighter their bonds, the less time 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 and more tightly bonded 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. The velocity of a sound wave is affected by two properties of matter: the elastic properties and density. The relationship is described by the following equation.
Where: Cij is the elastic properties and p is the density.
Elastic PropertiesThe speed of sound is also different for different types of solids, liquids, and gases. One of the reasons for this is that the elastic properties are different for different materials. Elastic properties relate to the tendency of a material to maintain its shape and not deform when a force is applied to it. A material such as steel will experience a smaller deformation than rubber when a force is applied to the materials. Steel is a rigid material while rubber deforms easily and is a more flexible material.
At the particle level, a rigid material is characterized by atoms and/or molecules with strong forces of attraction for each other. These forces can be thought of as springs that control how quickly the particles return to their original positions. Particles that return to their resting position quickly are ready to move again more quickly, and thus they can vibrate at higher speeds. Therefore, sound can travel faster through mediums with higher elastic properties (like steel) than it can through solids like rubber, which have lower elastic properties.
The phase of matter has a large impact upon the elastic properties of a medium. In general, the bond strength between particles is strongest in solid materials and is weakest in the gaseous state. As a result, sound waves travel faster in solids than in liquids, and faster in liquids than in gasses. While the density of a medium also affects the speed of sound, the elastic properties have a greater influence on the wave speed.
Density
The density of a medium is the second factor that affects the speed of sound. Density describes the mass of a substance per volume. A substance that is more dense per volume has more mass per volume. Usually, larger molecules have more mass. If a material is more dense 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 more dense object if they have the same elastic properties. If sound waves were passed through two materials with approximately the same elastic properties such as aluminum (10 psi) and gold (10.8 psi), sound will travel about twice as fast in the aluminum (0.632cm/microsecond) than in the gold (0.324cm/microsecond). This is because the aluminum has a density of 2.7gram per cubic cm which is less than the density of gold, which is about 19 grams per cubic cm. The elastic properties usually have a larger effect that the density so it is important to both material properties.
Air Density and Temperature
Suppose that two volumes of a substance such as air have different densities. We know the more dense substance must have more mass per volume. More molecules are squeezed into the same volume, therefore, the molecules are closer together and their bonds are stronger (think tight springs). Since sound is more easily transmitted between particles with strong bonds (tight springs), sound travels faster through denser air.
However, you may have noticed from the table above that sound travels faster in the warmer 40oC air than in the cooler 20oC air. This doesn't seem right because the cooler air is more dense. However, in gases, an increase in temperature causes the molecules to move faster and this account for the increase in the speed of sound. This will be discussed in more detail on the next page.
THE HUMAN EAR
For a interactive look at the human ear in 3D, open this link to theHealthline.com site.
THE COMPONENTS OF SOUND
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 byintensity, pitch, and tone.
Intensity
Sound is a wave and waves have amplitude, or height. Amplitude is a measure of energy. The more energy a wave has, the higher its amplitude. As amplitude increases, intensity also increases. Intensity is the amount of energy a sound has over an area. The same sound is more intense if you hear it in a smaller area. In general, we call sounds with a higher intensity louder.
We are used to measuring the sounds we hear in loudness. The sound of your friend yelling is loud, while the sound of your own breathing is very soft. Loudness cannot be assigned a specific number, but intensity can. Intensity is measured in decibels.
The human ear is more sensitive to high sounds, so they may seem louder than a low noise of the same intensity. Decibels and intensity, however, do not depend on the ear. They can be measured with instruments. A whisper is about 10 decibels while thunder is 100 decibels. Listening to loud sounds, sounds with intensities above 85 decibels, may damage your ears. If a noise is loud enough, over 120 decibels, it can be painful to listen to. One hundred and twenty decibels is the threshold of pain
Sounds and their Decibels
| Source of Sound |
Decibels
|
| Boeing 747 |
140
|
| Civil Defense Siren |
130
|
| Jack Hammer |
120
|
| Rock Concert |
110
|
| Lawn Mower |
100
|
| Motorcycle |
90
|
| Garbage Disposal |
80
|
| Vacuum Cleaner |
70
|
| Normal Conversation |
60
|
| Light Traffic |
50
|
| Background Noise |
40
|
| Whisper |
30
|
Pitch helps us distinguish between low and high sounds. Imagine that a singer sings the same note twice, one an octave above the other. You can hear a difference between these two sounds. That is because their pitch is different.
Pitch depends on the frequency of a sound wave. Frequency is the number of wavelengths that fit into one unit of time. Remember that a wavelength is equal to one compression and one rarefaction. Even though the singer sang the same note, because the sounds had different frequencies, we heard them as different. Frequencies are measured in hertz. One hertz is equal to one cycle of compression and rarefaction per second. High sounds have high frequencies and low sounds have low frequencies. Thunder has a frequency of only 50 hertz, while a whistle can have a frequency of 1,000 hertz.
The human ear is able to hear frequencies of 20 to 20,000 hertz. Some animals can hear sounds at even higher frequencies. The reason we cannot hear dog whistles, while they can, is because the frequency of the whistle is too high be processed by our ears. Sounds that are too high for us to hear are called ultrasonic.
Ultrasonic waves have many uses. In nature, bats emit ultrasonic waves and listen to the echoes to help them know where walls are or to find prey. Captains of submarines and other boats use special machines that send out and receive ultrasonic waves. These waves help them guide their boats through the water and warn them when another boat is near.
Tone & Harmonics
Another difference you may have noticed between sounds is that some sounds are pleasant while others are unpleasant. A beginning violin player sounds very different than a violin player in a symphony, even if they are playing the same note. A violin also sounds different than a flute playing the same pitch. This is because they have a different tone, or sound quality. When a source vibrates, it actually vibrates with many frequencies at the same time. Each of those frequencies produces a wave. Sound quality depends on the combination of different frequencies of sound waves.
Imagine a guitar string tightly stretched. If we strum it, the energy from our finger is transferred to the string, causing it to vibrate. When the whole string vibrates, we hear the lowest pitch. This pitch is called thefundamental. Remember, the fundamental is really only one of many pitches that the string is producing. Parts of the string vibrating at frequencies higher than the fundamental are called overtones, while those vibrating in whole number multiples of the fundamental are called harmonics. A frequency of two times the fundamental will sound one octave higher and is called the second harmonic. A frequency four times the fundamental will sound two octaves higher and is called the fourth harmonic. Because the fundamental is one times itself, it is also called the first harmonic.
How is this knowledge useful in everyday life?
The more harmonics a sound has, the fuller the quality the sound is. All the different overtones of a sound help give it a unique pattern. This is especially true for a person’s voice. Everybody in the world has a different voice print, or pattern of overtones. Detectives can track a criminal if they know his voice print just as they would use his fingerprints. Voice identification equipment is used in advanced security systems to recognize and let in only one authorized person. Voice prints are also used in modern technology, for example, voice activated telephones. In the future, if you want the lights on, it may be more common to say, “Turn on lights,” than to flip a light switch.
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, everyone’s 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 PITCH
- What happens when you make the string shorter? Longer? Thicker? Thinner? Tighter? Looser?
- What happens when you make the string out of different material?
Sound 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. Frequency refers to the number of vibrations that an individual particle makes in a specific period of time, usually a second. The frequency of a wave is different than the speed of a wave. Frequency refers to how often a wave passes through a certain point, while speed refers to how fast a wave passes through the point.
Particles 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 glass’s natural frequency!
Changing Pitch
A 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:
- 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.
- 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.
- 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.
- 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 a more dense material than the strings used for high pitches.
SOUND WAVE INTERFERENCE
- What is the difference in sound between the overlap area and the single color area?
- What is the difference in sound in the white area?
When 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 calledinterference. 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.
Dead spots
Waves can interfere so destructively with one another that they producedead 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 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.
Critical Angle
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 wave’s entering angle reaches a certain point, called thecritical 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 SOUND
Click on the experiment button below to open a Java applet.
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Make sure that your browser is set to allow you to see Java Applets.
- 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)?
- 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?
- What happens to the sound as the incident angle approaches being parallel to the surface?
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 (enters a different material) at an angle other that 90 degrees, it is bent from its original direction. This change in angle of direction is called refraction. 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 velocity of sound in each material is determined by the material properties (elastic modulus and density) for that material.
In the animation below, a series of plane waves are shown traveling in one material and entering a second material that has a higher acoustic velocity. Therefore, when the wave encounters the interface between these two materials, the portion of the wave in the second material is moving faster than the portion of the wave in the first material. It can be seen that this causes the wave to bend.
 |  VL1 is the longitudinal wave velocity in material 1. VL2 is the longitudinal wave velocity in material 2. |
REFLECTION OF SOUND
The Multi-Material Room- What happens when a sound wave hits a concave shaped surface?
- Is the sound reflected back to the source from a concave shaped surface more or less than that reflected from a flat surface?
- What happens when a sound wave hits the porous surface?
- What happens when a sound wave hits an irregular surface?
When 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 Absorption
Everything, 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.
ULTRASOUND AND ULTRASONIC TESTING
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 test
A 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.
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