Top 10 Demonstrations with Tuning Forks [W/Video] / Labs, Activities, and Other CoolStuff See Comments now! I have been using tuning forks in my classroom for 10 years, and in each of those years I have discovered several new tricks. I hope you can learn many of these from this publication. For a more complete treatment and my references, please see my article in “The Physics Teacher” March, 2013. 0. General Usage When a tuning fork is struck it will vibrate wildly in unintended ways. Imagine putting your arms straight above your head and clapping. That is the proper motion of a tuning fork. The problem is that it is also wobbling at the “elbows.” You can get rid of the unwanted vibrations by touching gently near the joint after striking the fork. The vibrating tuning fork should be almost silent when used properly. Hold the tines near your ear and you will hear it clearly. It is best to hit the tuning fork on a knee or the ball of your hand, avoiding metal on metal. This is because when tuning forks become chipped they change their inertia and will vibrate at different frequencies.

Spin the fork as you listen and notice that it is loudest right between the tines. (Constructive Interference.) 1. Water Dip Putting a tuning fork in water is one of the best ways to get students accustomed to handling it. Give a tuning fork to each student or every other student. Set out several cups of water. It is always a surprise to see the splash, students will gasp. These introductory activities are important for laboratory management because the tuning fork is a fun toy and does require some getting used to; the sensation of hearing the tines vibrating is new and somewhat alarming. It is also a good idea to have boxes or desks or the whiteboard cleared off for students to place the base of the tuning fork against and cause the vibration to resonate. 2. Strobe Lights A fun demonstration is to put the tuning fork in front of an adjustable strobe light (or a CRT computer monitor). The strobe light can be adjusted to make the vibration appear slower or even stop! This works better on a larger tuning fork. The flashing must match the frequency of the tines, or be very close. I found my 100 Hz tuning fork to be 99 Hz after investigation.

This effect comes from the strobe light “animating” the fork slowly through time by only making it visible after almost full cycle has passed. At that moment, the fork will look as though it has only moved slightly. The difference between the strobe rate and the tuning fork frequency determines the perceived rate of vibration. The CRT monitor can also act somewhat like a strobe light, but because of its trace across the screen, it causes a wobbly effects in the vibration. 3. Oscilloscope Verifying the frequency of the tuning fork can easily be achieved by using an oscilloscope. This is done by hooking a speaker removed from its housing to the scope’s leads. You will need to have a proper connection (usually a BNC connector with probe) to achieve this. You can also use a microphone. Hold the tuning fork up to the speaker and adjust the settings. You can see that the fork’s tone is a pure sine curve. Compare this with the human voice or other instruments such as flutes and kazoos. Also, try comparing tuning forks of various frequencies and noting the different periods & wavelengths. 4.

Resonance Two tuning forks that are the same frequency can be made to resonate audibly if the vibration is loud enough. For this purpose, I prefer using the large box-mounted versions. Most large glass or wooden objects will have so many resonance frequencies that any tuning fork will cause them to resonate. Tuning forks that are not the same frequency will not resonate. The important phrase to understand is “Forced vibration at natural frequency causes resonance.” Where “resonance” is high amplitude oscillation. We all experience resonance when singing in the shower; the longer notes resonate better and it makes our voice sound purer in tone. Also, when our wheels are not aligned in the car and we drive at the natural frequency of our shock springs the car will resonate up and down - but only at specific velocities. 5. Sound via Light Shine a laser on a solar cell from across the room, hook that solar cell to a set of computer speakers and demonstrate the transmission of sound via electromagnetic waves. This is analogous to radio signals that we listen to because they are also modulated electromagnetic waves. The laser’s color doesn’t matter much. I sometimes add smoke to enhance the demo visually.

You will get a less distorted sound if the fork is further into the beam rather than just barely touching it when vibrating. Clipping to the speakers may require some trial and error. The “male end” of a stereo cable has its tip going to the left speaker, the middle ring goes to right, and the inner metal goes to ground. Clip one end of the solar cell to either left or right, but you must clip the other to ground. A guitar amp will work fine, probably even better. Clip similarly to the plug of the guitar cord. 6. Interference Demonstrating interference is important because it is a property of all waves. In this case I am using two close frequency waves to show the phenomena called “beats.” Beats are sometimes also used to tune musical instruments (see #10). The beating frequency is the difference between the interfering frequencies, the note you hear is the average of the two original frequencies. This pattern can also be achieved by taking two identical tuning forks and heating one of them with a fire. (I demonstrate this in the introduction to the video.) Be sure to wear a hot glove! The heat reduces the Young’s modulus (similar to spring constant) of the aluminum and the vibrations no longer match.

You can easily tell the difference even with a non-musical ear. 7. Measure the Speed of Sound With a tube and some water in a bowl it is easy to measure the speed of sound by resonating it with a tuning fork. The wavelength of the sound must match the length of the tube, but the whole wave doesn’t have to fit inside for this to happen. Most commonly, the bottom is sealed and becomes a node (a place where the air can’t move) but the top is open and the air can vibrate liberally (anti-node). The smallest fraction of a standing wave that can fit in here is a ¼ wavelength. Multiplying wavelength and frequency gives the velocity of sound, usually within 1% error! If you don’t have a glass tube, this demonstration can also be done with a graduated cylinder that is being filled with water until resonance is achieved. 8. Smoke and Mirrors Reflecting light from the end of a mirrored tuning fork can lead to exciting effects. It gives us a chance to view the motion of the fork by amplifying it as the reflected light is projected across the room. In the video, I add smoke to help you see the beam.

Because the tuning fork’s motion is sinusoidal in time, it can be made to trace a nearly perfect sine curve in space when it is rotated smoothly at a point far away. Lissajous Figures are an old method by which tuning forks were tuned. Excess fork was shaved off to bring the frequency down. These days, Lissajous Figures are mostly they are used to analyze electromagnetic oscillations in LRC circuits, but originally they were produced by tuning forks reflecting light that is pointed at two mirror loaded forks vibrating at 90 degree angles. When the frequencies are in ratio you get a Lissajous Figure. They come in the shape of donuts, pretzels, fish, and other edible items. It is best to have the forks close, but the wall far away because that will increase the size of the figures and reduces aiming difficulties. 9. Strike a Chord Tuning forks come in various frequencies. You can use them to inform students that music is a branch of physics. With help you can create chords or even play songs with your students. Take time to notice that there are specific ratios between notes that are in harmony.

For example, between G and C there is a 3/2 ratio – this is called a fifth. Between E and C is a 5/4 ratio - this ratio is used in the C major chord. And between C and A is a 6/5 ratio which is used in the A minor chord. All octaves (such as middle C and the next C above middle C) are separated by a doubling of the frequency. These ratios apply to both scientific and musical tuning fork frequencies and it is a fun game to try to discover them by reading your tuning fork labels. 10. Tuning Tuning an instrument with a tuning fork can be done in many ways. Typically, the tuning fork is merely listened to or held to the body of the instrument while it is tuned by ear. But the fork can also be used to resonate the strings into vibration (if they are already in tune). A completely different method is to strike the note and listen for beats as the sound from the instrument interferes with the sound from the tuning fork. As the two are brought into tune, they will beat less and less frequently until they are matched with no beating. It is important to note that the scientific tuning forks do not match the musical frequencies.

For example, A 440 Hz is a musical note, whereas A 426.7 Hz is the scientific note. In the figure, my guitar tuner thinks my scientific tuning fork is flat by a half step. The scientific scale is arranged around middle C being 256 Hz (C is 261.6 Hz on the musical scale). The setting of the musical scale was done somewhat arbitrarily done by German musicians in the early 20th century. The scientific scale is convenient where all C notes are a multiple of 2; for example, the first C above middle C is 29=512 Hz. Many of the other frequencies are also whole numbers, such as G 384 Hz and D 288 Hz. Tarbut V’ Torah High School Irvine, CA, USA James Lincoln teaches Physics in Southern California and has won several science video contests and worked on various projects in the past few years. James has consulted on TV’s “The Big Bang Theory” and WebTV’s “This vs. That” and the UCLA Physics Video Project.

How Tuning Forks Work | HowStuffWorks How Tuning Forks Work ©iStockphoto/ Thinkstock The choir comes to a hush as the choirmaster steps into the room. "All right, let's tune," she declares to the assembled singers. Producing a two-pronged metal tool from her pocket, she strikes it against her knee. After a shrill whine, it begins to emanate a mysterious, angelic hum. Resting the base of the device against a wooden desk, the hum becomes louder. Listening intently to the small, singing piece of metal, the singers adjust their voices accordingly. Pianos lose their tuning, guitars fall out of key -- even church organs need to be tuned every now and then. For centuries, the only sure-fire way to tell if an instrument was in tune was to use a tuning fork: A two-pronged, all-metal device the length of a butter knife. Simple to make and easy to use, tuning forks guaranteed 100 percent pitch accuracy. In antique shops today, you can find 300-year-old tuning forks that emit notes just as clear and crisp as the day they were made.

Up Next How Radar Works The device was first invented in 1711 by John Shore, a trumpet player working for the British Crown. In concerts, Shore jokingly referred to his invention as a "pitch fork." Shore's device quickly became the must-have tool of musicians around the world. In portraits, choirmasters posed while brandishing a tuning fork. Young musicians were given gold-plated tuning forks upon successful completion of a conservatory exam. Today, the $3.5-billion Yamaha Corporation has a trio of tuning forks as their logo. Tuning forks were even behind some of telephone inventor Alexander Graham Bell's earliest experiments. In 1876, Bell was able to use a battery, an electrified tuning fork and a cup of acidic water to transmit sound over an electric wire for the first time. "Playing" a tuning fork is relatively easy: Taking care not to hold it by the prongs, you slap it against something hard like a tabletop -- or even the bottom of your shoe -- and then listen as it rings for about 5 seconds. Hit it hard or hit it soft, the note is always the same. It seems almost magical. Read on to find out how a hunk of metal makes music.

1 Hubble's Tuning Fork and Galaxy Classification | Astronomy 801: Planets, Stars, Galaxies, and the Universe Hubble's Tuning Fork and Galaxy Classification Print Coincident with his discovery of Cepheid stars in Andromeda, Edwin Hubble was working in the mid 1920s to study other galaxies in more detail (although at that point it was still common terminology to refer to them as nebulae). It is often the case with the discovery of a new class of objects that astronomers invent a classification scheme as a first step to try to understand these objects. Hubble is credited with creating a classification scheme for galaxies, which is usually referred to as his “Tuning Fork” diagram. Figure 9.2: Diagram of Hubble's Tuning Fork classification scheme from Hubblesite Credit: Space Telescope Science Institute The Hubble Tuning Fork has been reproduced many times with many different sets of galaxy images. A particularly detailed version was produced by the Spitzer Infrared Nearby Galaxies Survey (SINGS) group. The group wrote a brief article describing their work for you to read.

If you want to print this out as a full size poster for your classroom, they have made available a 4200x3600 TIFF file that provides enough resolution to do so. It becomes very apparent after observing just a few galaxies, like Hubble did, that there are a great variety of types. Some are featureless, while others have very distinct spiral arms. Below are links to a number of images of the various galaxy types. After you look at a number of different galaxies, we will discuss Hubble's classification scheme below. Try this with Starry Night! You should repeat some of what Hubble did when he developed his tuning fork classification system. You should investigate the images of several galaxies, and then compare and contrast their features. First, open Starry Night, and do the following (Note: The Digital Download core application does not have galaxy images, so if you have that version, you will have to skip this part of the exercise): Using the find tool, locate galaxies, M51, M81, M95, M61, M87, M49, M59 one at a time.

When you center on the galaxy, using the zoom tool, zoom in so that you can see the image at a reasonable level of detail. Keep a mental list of the similarities and differences between these 7 objects. Next, study some specific examples in detail to familiarize yourself with their differences: Within each of these groups, there are also sub-classifications. Spirals A spiral galaxy is classified as an S galaxy with subclassification a, b, or c, so for example, M51 above is classified as an Sb galaxy. The subclasses are divided based on the dominance of the bulge component of the galaxy. In Sa galaxies, the bulge dominates the galaxy, while in Sc galaxies, the bulge is much smaller. This is labeled on the SINGS Tuning Fork. The appearance of the spiral arms also changes between the a, b, and c subclasses. In Sa galaxies, the arms are tightly wrapped around the bulge, while in Sc galaxies the arms are much looser, and often appear to be more clumpy than the smooth arms of an Sa galaxy. Sb galaxies have intermediate properties between those of Sa and Sc galaxies.

We can classify edge-on spiral galaxies based on the appearance of their bulge, even without being able to see the spiral arms. So for example, the edge-on spiral M104 - the Sombrero galaxy, seen below, can be classified as an Sa galaxy. Figure 9.3: Hubble image of M104, an Sa spiral galaxy seen edge-on Here is another edge-on spiral galaxy, NGC 4565. Can you guess what type it would be assigned, Sa, Sb, or Sc? What is the most obvious difference between NGC 4565 and M104? Figure 9.4: Ground-based image of NGC 4565 Credit: NOAO / AURA / NSF Since the Milky Way is a spiral galaxy, you can consider its properties as illustrative of spiral galaxies in general. Spiral galaxies like the Milky Way often contain large amounts of gas and dust, and given the presence of gas, are found to contain many young stars and star forming regions. The colors of spiral galaxies vary from location to location, but because of the presence of O&B stars in these galaxies, their overall colors tend to be towards the blue end of the spectrum.

Barred Spirals In some spiral galaxies, the bulge has a well defined BAR that passes through the bulge. Hubble classified these galaxies by referring to them as SB galaxies with subclasses a, b, and c, just like the normal spirals. The spiral arms in barred spirals appear to originate at the ends of the bar, instead of in the bulge, like they do in normal spirals. We believe that the Milky Way is a barred spiral, perhaps an SBb or SBc type. The schematic image of the Milky Way that you studied in a previous lesson shows the Milky Way bar. Besides the presence of the bar, the properties of barred spiral galaxies are very similar to normal spirals. Ellipticals Elliptical galaxies (type E in Hubble's scheme) are featureless galaxies that appear to be just a ball of stars. They do not have obvious dust lanes and gas clouds like we see in spiral galaxies, either. There is no Hubble image tour of an elliptical galaxy, because there are no features to tour! With no gas and no dust, there are no young stars or star forming regions seen in typical elliptical galaxies.

The overall colors and spectra of elliptical galaxies are very similar to K stars, since they are dominated by these older, red stars. The subclasses of the elliptical class are assigned numbers based on how round they appear. The roundest ellipticals are referred to as E0 galaxies, and the most elliptical shaped galaxies that we have observed are labeled E7 galaxies. The numerical designation is determined by the ratio of the major axis (a) to the minor axis (b) of the object. The ellipticity is defined as e = 1 - (b/a), so a circle will have an ellipticity of 0, and a highly elliptical galaxy with a minor axis 30% of the length of its major axis will have an ellipticity of e = 1 - (3 / 10) = 0.7. An E0 galaxy has an ellipticity of 0, and an E7 galaxy has an ellipticity of 0.7, so to determine the Hubble classification, you multiply the ellipticity by 10.

Be aware that just like with spiral galaxies, our image of that object is a 2D projection of a 3D object, and so the classification only tells you how it appears from your point of view. If you consider a football as a decent analogy for an elliptical galaxy (see image below), if you look at it from the side view as in the picture, you can see its elliptical nature. However, if you look at it from the point of view of the tip of the football, it will appear to be circular. So an elliptical galaxy that appears to us as an E0, might appear as an E7 to an observer living in a distant galaxy with a different point of view from ours. Figure 9.5: American Football Source: Wikimedia There are some galaxies that appear to be intermediate between ellipticals and spirals. They are disk galaxies with bulges, but they have very little or no gas and dust like other spirals. Their disks do not show evidence of spiral arms, either. These galaxies are usually called lenticular galaxies or S0 galaxies. Irregulars Finally, we have the "miscellaneous" category.

Anything that is so unusual that it can't be fit into any of the normal categories is called an irregular galaxy. There are two subclasses in this group, too. Irregular I galaxies (like the Magellanic Clouds) appear to have some spiral structure, but it appears to have been disrupted. Irregular II galaxies are much more disturbed than Irr I galaxies and look like they have been victims of some type of violent event that has completely disrupted their original shape. Many irregular galaxies are found to contain many young stars and to be experiencing significant ongoing star formation. In conclusion... At this point, it should be noted that classification of galaxies is not a very precise enterprise. In fact, if you do any follow up research on some of the galaxies above, you will find a variety of classifications given for the same object. For example, M81 can be found labeled Sa or Sb. Depending on the type of observations made (e.g., optical observations vs. infrared observations), a barred spiral may have a prominent bar in one image, and it may be almost unobservable in another. So, you may find the same galaxy classified as an Sb or an SBb depending on how it was observed.

There are many other properties of galaxies we can discuss in some detail, but to conclude our discussion of normal galaxies, we should briefly discuss the size of typical galaxies. Among spirals and barred spirals, there is a reasonable amount of uniformity. Some spiral galaxies, like M33 , are measurably smaller than others, like M31 , which is approximately four times larger. However, in general the differences are small. Within the class of elliptical galaxies, however, there is a much larger range of sizes. Some galaxies are referred to as dwarf ellipticals because they share many of the properties of elliptical galaxies but are significantly smaller and less massive. NGC 205 is an example . If we take a census of the galaxies in the Universe, it appears that by number, dwarf ellipticals are the most common type of galaxy in the Universe. What is the best procedure to get a clean sine wave when using a tuning fork with the micr > Vernier Software & Technology Product Support FAQs Hold the tuning fork very close to the microphone, almost touching it. One of the best things to do to improve the signal from a tuning fork is to hold it so that the outside of the tuning fork is parallel to the face of the microphone.

Many times people try pointing the tuning fork at the microphone, but the side of the microphone will actually produce a cleaner signal. Another common problem is the timing of the experiment. When sampling at a high frequency, you are usually only collecting data for a very short amount of time. Be sure to strike the tuning fork first, get it in position, THEN quickly start your data run. This way, you know that the sound signal will already be reaching the microphone when your data run starts. Keep in mind the relationship between the sampling frequency and the signal. You need to be sampling at about 5 times the rate of the highest frequency signal you want to detect. So, if you are using a 400 Hz tuning fork, you need to be collecting data at at least 2000 samples/second in order to get enough of the sine wave to see it clearly on the screen. The method used to strike the tuning fork will directly affect how clean your waveform is. We suggest striking the fork about 2/3 of the way down the side towards the base. Strike it against an object that is not real hard. Many people use something made of rubber, such as a shoe sole or rubber mallet.

Other people prefer to strike the tuning fork on an object like their kneecap. Because the waveform will be a function of how the fork is struck, there is an element of experimental skill involved in getting a clean waveform. If the sound is bright to the ear, the strike was almost certainly a poor one that will give a waveform rich in undesired harmonics. An excellent alternative to using a tuning fork is to use an inexpensive electronic keyboard set to the flute sound. This sidesteps the problem of bad tuning fork strikes. We prefer a keyboard for demonstrations over tuning forks. The microphone is fairly sensitive. If you can do anything to reduce ambient noise, you will almost always get cleaner data. Need more assistance? Resonance_and_Speed_of_Sound_Lab Wavelength and the Speed of Sound in Air Experimental Determination of the Dimensions and Rate of Travel of Sound Waves Laboratory No. 1 How can we experimentally determine the length of a sound wave? How can we determine the rate of travel of sound waves in air? How does air temperature affect the propagation rate of sound waves? How can we better understand wave frequency and its calculation? How are the wavelength, frequency, and speed of sound waves interrelated?

Objective: Through the use of a resonance tube, a glass cylinder, and a tuning fork of known frequency, students will be able to measure the wavelength and speed of sound waves in air Introduction. A sound wave travels too fast to be timed directly with simple laboratory devices. The speed v of a sound wave may be measured indirectly, however, by first determining its wavelength from a sound source of known frequency. Then v may be found from the relationship v = fl where f is the known frequency of the vibrating (source) body and l is the wavelength of the sound wave. A tuning fork is a device that is designed to vibrate at a known and constant frequency which produces sound waves of the same frequency. The frequency of all laboratory tuning forks is stamped directly on the body of the fork, just above the connection between the handle and the tines. The wavelength of the sound waves produced by a vibrating tuning fork may be determined by holding the fork over the air column formed by a glass tube that is slipped into a larger diameter resonance tube partially filled with water while sliding the glass tube up and down. As such, the length of the air column is varied until a significant increase in volume (loudness) of the sound emerging from the tube is heard.

When this occurs, the air column is said to be in resonance with the tuning fork. This means that the air column has been set into vibration at its natural frequency. The vibration amplitude at resonance is so great that the loudness of the sound is multiplied many times. For a given tuning fork, especially one of high frequency, you may be able to find several resonant lengths. In this experiment, however, you will be primarily interested in measuring the shortest air column that will produce resonance. For the shortest resonance column, it ca be shown that the length of the air column is l/4. It is this relationship that will enable you to obtain the information you need to determine the speed of the sound wave in the air column. Throughout this experiment, you must be sure that the lower end of the glass tube is submerged beneath the surface of the water (see diagram below) so that you will be working with a tube that is closed at one end. d [Resonance Tube] Glass Tube Setup. Arrange the equipment as illustrated in the diagram above. The tall (1000 ml) graduated cylinder (resonance tube) should be nearly full of water and the glass tube should be able to easily slide within it.

Addition of a few drops of food coloring to the water column will make for easier observation of the resonant (water) surface. Strike a selected tuning fork on the green rubber striker to initiate vibration of its tines. Hold the vibrating fork at the level of the mouth of the glass tube with its tines vibrating toward and away from the water surface. Then slowly raise both the glass tube and the vibrating tuning fork, keeping the relative position of the tube and fork constant while listening for maximum tone. You may have to practice a number of times until you find the shortest air column length L' that produces resonance. The effect at resonance is unmistakable: the increase in tone loudness is very noticeable. To set the air column at exactly the right resonant length, move the glass tube up and down alternately, gradually reducing the motion until you have pinpointed the resonance position. To help you distinguish your particular sound from the multitude of other sounds coming from your classmates' equipment, remove the tuning fork, and then quickly replace it over the resonant air column. Experiment has shown that the length of the resonant air column determined as just described requires a small correction because of a phenomenon called end effect.

This phenomenon causes the air column at resonance to be slightly longer than the measured air column length (L'). The corrected length of the column is is obtained by adding 4/10ths of the inside diameter of the glass tube (d) to the measured length of the air column (L'). Corrected resonant length (L) is found as follows: L = L' + 0.4d where L is corrected air column (resonant) length (in cm), L' is measured air column (resonant) length (in cm) and d is inside tube diameter (in cm). Equipment. q Glass Cylinder (~46 cm minimum length) q Resonance Tube (~56 cm minimum length) or 1000 ml graduated cylinder q 256 Hz Tuning Fork q 512 Hz Tuning Fork q Tuning Fork Striker (green rubber) q Meter Stick (Note: All measurements are to be taken in cm, recorded to the nearest 0.1 cm.) 1. Note and record the frequencies of both tuning forks. 2. Using the green rubber strikers, set one tuning fork into vibration and find the shortest air column length that will produce intensified sound as outlined previously.

Measure distance L', which is the distance from the water surface to the top of the glass tube and record. Repeat two additional times and record. Average all three trials and record. Measure room air temperature in degrees Celsius and record. 3. Repeat Step 2 above for the second tuning fork. 4. Measure and record the inside diameter (d) of the glass tube. Data. Frequency of Tuning Fork 1 __________Hz Frequency of Tuning Fork 2 __________Hz Uncorrected Resonant Lengths (L'): Actual (True) Value of the Speed of Sound in Air [Show all Work] v = 331 + 0.6(T) where T is room air temperature in degrees Celsius, without units. The unit of speed in this expression is m/s. v = __________m/s Questions. 1. a. Calculate the Percent Error between your Experimental and Actual speeds of sound. b. On the basis of your calculated PE values above, is the resonant tube method a reasonable one for determining the speed of sound? Explain. 2. Does the speed of sound vary with changes in its frequency at constant temperature? Explain. 3.

To obtain the actual speed of a sound wave, modify the equation v = fl so as to permit you to substitute directly the values for frequency, uncorrected tube length of resonating column, and the end effect correction. Show all work. 4. State the relationship between frequency and corrected length of the resonant air column. 5. What effect does a decrease in air temperature have on the a. frequency of the wave. b. wavelength, c. speed of the wave, and d. length of the resonant column? Assembled Apparatus Air Column Strategy. For a tuning fork of known frequency (f), you will determine the length of a resonant air column (L') by listening for greatly intensified sound. After making a mathematical correction for the length of the air column, you will determine the wavelength of the sound wave from the length of the air column and then calculate the speed of the sound wave from the relationship v = fl. You will repeat these measurements with a tuning fork of different frequency and thus obtain a second value for the speed of sound wave.

After determining the accepted value of the speed of a sound wave at the temperature of your equipment, you will then be able to compare your measured (experimental) values with the accepted (true) value. Tuning Forks and Longitudinal Waves The Physics Classroom » Multimedia Studios » Waves, Sound and Light » Tuning Forks and Longitudinal Waves Longitudinal Waves and Tuning Forks Sound waves are produced by vibrating objects. Whether it be the sound of a person's voice, the sound of a piano, the sound of a trombone or the sound of a physics book slamming to the floor, the source of the sound is always a vibrating object. A tuning fork serves as a useful illustration of how a vibrating object can produce sound. The fork consists of a handle and two tines. When the tuning fork is hit with a rubber hammer, the tines begin to vibrate. The back and forth vibration of the tines produce disturbances of surrounding air molecules. As a tine stretches outward from its usual position, it compresses surrounding air molecules into a small region of space; this creates a high pressure region next to the tine.

As the tine then moves inward from its usual position, air surrounding the tine expands; this produces a low pressure region next to the tine. The high pressure regions are known as compressions and the low pressure regions are known as rarefactions. As the tines continue to vibrate, an alternating pattern of high and low pressure regions are created. These regions are transported through the surrounding air, carrying the sound signal from one location to another. In solids, sound can exist as either a longitudinal or a transverse wave. But in mediums which are fluid (e.g., gases and liquids), sound waves can only be longitudinal. The animation above depicts a sound wave as a longitudinal wave. In a longitudinal wave, particles of the medium vibrate back and forth in a direction which is parallel (and anti-parallel) to the direction of energy transport. In the animation above, the energy is shown traveling outward from the tuning fork - from left to right. The air molecules are vibrating about a fixed position from left to right and from right to left. This is what makes a sound wave a longitudinal wave. There is another important characteristic of waves depicted in the animation above.

A careful inspection of the particles of the air (represented by dots) reveal that the air molecules are nudged rightward and then move back leftward to their original position. Air molecules are continuously vibrating back and forth about their original position. There is no net displacement of the air molecules. The molecules of air are only temporarily disturbed from their rest position; they always return to their original position. In this sense, a sound wave (like any wave) is a phenomenon which transports energy from one location to another without transporting matter. For more information on physical descriptions of waves, visit The Physics Classroom Tutorial . Detailed information is available there on the following topics: Science Explorations: Journey Into Space: Sound and Ultrasonics | Scholastic.com Sound and Ultrasonics William C. Vergara The everyday world is filled with all kinds of sounds -- the murmur of voices, the blaring horn of an automobile, the distant putt-putt of a lawn mower, the buzzing of a mosquito. Each of these common sounds is different from the others. And it is easy to tell all of them from hundreds of other sounds. There are still other sounds, however, that are too high-pitched to be heard by the human ear.

These sounds are called ultrasonic sounds, or ultrasound. Some animals, such as dolphins and bats, make and hear these sounds. Scientists have built special instruments to produce and detect ultrasonic sound. In this article you will read how sounds are produced and why they are different from one another. You will also read how ultrasound has been put to use. How Sounds Are Produced All sounds are produced by a certain kind of motion. Suppose you stretch a rubber band and then pluck it. The band moves back and forth so rapidly that its motion appears blurred. The sound you hear comes from its motion. Such very fast, back-and-forth motions are called vibrations. Vibrations are the source of all sounds. Pluck a guitar string. You will hear a musical sound. If you place your finger lightly on the string, you can feel the vibrations. They produce the sound. When you stop the vibrations, you stop the sound. Sound Travels Through a Medium Sound travels from a vibrating object to your ear by means of a medium, or sound carrier. The medium may be a solid, a liquid, or a gas. Air, a mixture of several gases, is the most common carrier of sound as far as people are concerned.

If there is no medium to carry the sound to our ears, we cannot hear the sound. For example, the moon has no air. Astronauts on the moon cannot speak directly to each other but must communicate by radio. The Speed of Sound A person seated beyond the outfield of a ballpark sees a batter swing at the ball. An instant later the person hears the crack of a bat as it hits the ball. You usually hear a clap of thunder a short time after you see the bolt of lightning that caused the sound. Such common experiences demonstrate that the speed of sound is slower than the speed of light. Careful measurements show that the speed of sound depends on the medium. For example, sound travels through the air at a speed of about 1,100 feet (335 meters) a second. Sound travels about 4 times as fast in water as it does in air. Sound travels even faster in solids. In steel and in aluminum, the speed of sound is about 15 times faster than it is in air. Changes in temperature can change the medium enough so that sounds travel through it at a different speed. For example, sound travels slightly faster in warm air than in cold air.

However, at a given temperature all sounds in the same medium have the same speed. Sound Waves Sounds travel from a vibrating object to your ear by means of compression waves in the air. To understand what a compression wave is, you can try an experiment with a coil spring toy made up of many flat metal coils. (One such toy of this type is called a Slinky.) The spring stretches open easily and then returns to its original shape. This flexibility makes it useful for the experiment. First, find a yardstick or other stick about 3 feet (1 meter) long. Slide it through the center of the spring. Ask a helper to hold one end of the stick firmly. With your right hand hold your end of the stick and one end of the spring at the same time. With your left hand hold the free end of the spring. Now you can stretch the spring back and forth along the stick. If you slide your left hand back and forth along the stick in a regular motion, a wave will move down the outstretched spring from one end to the other. The individual turns of the spring move only slightly out of position, yet the wave motion travels all along the coils.

To understand how this happens, think about the movement of the spring in slow motion. When you first move the end of the spring toward yourself, the coils nearest your hand are compressed. When you let go, the coils immediately spring apart, or expand, which compresses the next section of coils along the spring. One such compression and expansion is called a cycle. Scientists call a wave made of many compressions and expansions a compression wave. Many compression waves pass along the spring as your hand continues to jiggle the end of the spring. Molecules of air behave in much the same flexible way. When pushed by a vibrating object, air molecules bunch up, or compress, and then expand. The effect is passed along to neighboring molecules as compression waves of sound. The invisible compression waves travel through the air, gradually becoming weaker and weaker until the sound waves die out altogether. Characteristics of Sound Waves A vibrating object sends out sound waves through the air in all directions. The waves travel outward in ever widening "shells," one inside the other. If you could see sound waves, they would look somewhat like round balloons of different sizes, one inside the other. The balloons would represent the compressions of air molecules, and the space between the balloons would represent the expansions.

The vibrating body, the source of the sound, would be in the center of the smallest balloon. Wavelength and Frequency The distance between one compression and the next is called the wavelength of a sound wave. The human ear can hear sounds whose wavelengths in air are from a little less than 3/4 inch (2 centimeters) to about 69 feet (21 meters) long. An object that vibrates 500 times per second causes a sound wave that vibrates at the rate of 500 cycles per second. This rate is called the frequency of the sound wave. Instead of cycles per second, the term hertz (abbreviated Hz) is often used. This unit was named for Heinrich Hertz, a German scientist. For example, a bell or a piano string that vibrates 250 times per second produces a sound wave with a frequency of 250 hertz. Depending on the rate of vibration, sound waves are described as being of low, medium, or high frequency. Thus, a 22-hertz wave is considered a low-frequency wave, while a wave of 7,000 hertz is a high-frequency wave. There is a relationship between the wavelength and the frequency.

Short wavelengths have a high frequency. Long wavelengths have a low frequency. There is also a connection between the frequency and the pitch. Pitch A whistling or screeching sound has a high pitch. A deep or rumbling sound, such as you get from the low notes on a piano, has a low pitch. The pitch of a sound depends on its frequency. A high frequency (many cycles per second) produces a high pitch. A lower frequency (fewer cycles per second) produces a lower pitch. You can show that the pitch of a sound depends on the frequency by plucking a stretched rubber band. If you pluck a loosely stretched rubber band, the frequency of the sound waves produced may be so low that you can see the vibrations of the rubber band. The sound you hear has a low pitch. Stretch the rubber band tightly and pluck it again. Its vibrations are so fast that you see only a blur, and the sound has a higher pitch. Think of how an electric fan sounds when you turn on the switch. At first the fan turns slowly, and the air particles around it vibrate slowly. A low-frequency sound wave is produced, and you hear a low-pitched hum.

As the fan whirls faster, the air particles vibrate faster. A higher-frequency sound wave results, and you hear a higher-pitched hum. Amplitude, Loudness, and Intensity A sound wave is a disturbance of the particles in the medium, or sound carrier. As each particle is disturbed, it moves back and forth a tiny distance. The distance each particle travels from its undisturbed position is called the amplitude of the sound wave. In general, the greater the amplitude, the louder the sound. Pluck a stretched rubber band gently. It vibrates gently and strikes the air particles gently. Each air particle is made to move only a short distance -- that is, the sound wave has a small amplitude. You hear a soft, gentle sound. If you pluck the rubber band with a greater force, each air particle moves a greater distance. The sound wave has a greater amplitude, and you hear a louder sound. Loudness depends on each person's sense of hearing. Loudness may not be the same for two people sitting next to each other and listening to the same sound. One person may have a keener sense of hearing than the other.

Therefore scientists cannot measure the loudness of a sound because it may be different for each person who hears it. Scientists can, however, measure the intensity of a sound. Intensity refers to the amount of energy in the sound, no matter who hears it. Think back to how you made a rubber band produce sounds of different loudness. When you plucked the rubber band gently, it produced a soft sound. When the rubber band was plucked with greater force, you put more energy into the sound. That produced a sound wave of greater amplitude. The sound had a greater intensity and was louder to your ears. It would sound louder to most people with normal hearing. Measuring Intensity of Sound Scientists can measure the intensity of a sound by measuring the amplitude of the sound wave. This intensity is called the sound level. It is measured in units called decibels. One decibel is the smallest change in intensity that can be detected by most people. Here are the decibel units of some sample sounds: Faintest whisper most people can hear: 0 decibels. Whisper loud enough to be heard 5 feet (1.5 meters) away: 20 decibels.

Ordinary conversation at a distance of 3 feet (1 meter): 65 decibels. Traffic on a busy street: 80 decibels. Noisiest spot at Niagara Falls: 95 decibels. A loud, amplified rock band: 100 decibels. A sound level of 120 or more decibels is painful to the ears of most people. If you stood near an 11-inch (280-millimeter) heavy military gun that was being fired, the sound would be very painful to your ears. The sound level is about 230 decibels. For this reason, soldiers usually use ear protectors when they are near a big gun or cannon that is about to be fired. Both the intensity and the loudness of a sound decrease rapidly as you move away from the sound source. Sound waves have energy. As the waves move away from their source, the energy spreads out over an ever increasing space. At a great distance from the source of the sound, the energy is so spread out that any one spot receives only a tiny bit of energy.

If you are standing a great distance from the source of the sound, only a very small amount of the energy reaches your ears, and you hear only a tiny sound. Behavior of Sound Sound waves can behave in ways that produce a number of different special effects. Some of these effects may cause the sound to be repeated or to be louder or blurred. If you have ever heard an echo, you have heard one of these effects. Echoes Suppose you shout across a canyon. Part of the sound wave you have produced with your voice strikes the opposite wall of the canyon and bounces back. The sound is reflected from the canyon wall. You hear the sound of your voice returning. This returned sound is what is known as an echo. In order for you to detect a definite echo, the reflecting surface must be at least 55 feet (16.75 meters) away from you. The reason for this has to do with the speed of sound and the way we hear. When you shout, at first you hear your own voice. This sensation of sound lasts in your mind for about 1/10 second.

Thus, if you are to hear a separate echo, the sound must take at least 1/10 second to cross the canyon and return. This is the minimum time that must pass before the sensation of the original shout has died out in your mind. In this minimum time of 1/10 second, sound travels about 110 feet (33.5 meters). If the canyon wall is 55 feet away, the sound will make a round trip of about 110 feet. You will be able to tell the difference between the shout and its echo, but you cannot do this if the wall is any closer. If the wall is more than 55 feet away, you may hear a clear, distinct echo of your shout as the sound wave bounces back. Reverberation The reflection of sound sometimes causes an effect called reverberation. In some large auditoriums and gymnasiums, for example, sounds are reflected many times from the floor, walls, and ceiling. When the sound waves bounce around between these surfaces, the original sound is repeated many times. But no distinct, separate echoes are heard. Each echo has mixed with the others that are reflected from various parts of the room.

This overall effect is called reverberation. Reverberation is measured by making a sharp sound, like the crack of a pistol, and measuring the time it takes for all the echoes and re-echoes to die out. This time is called the time of reverberation. If the time of reverberation in an auditorium is too long, speech sounds run together and it is hard to understand the speaker. If it is too short, the room sounds "dead" and not enough sound reaches the listeners. A certain amount of reverberation is necessary in auditoriums, especially for the enjoyment of music. Acoustics Acoustics is the science that is concerned with the behavior of sound. It also means the way sounds echo or reverberate in an auditorium or concert hall. People say, "The acoustics of this concert hall are very good," or "This auditorium has poor acoustics." Engineers and architects must take into account the problems of echoes and reverberation when they plan auditoriums, concert halls, and other public places. They must design the room so that there are no echoes. But they do want some reverberation, which is necessary for good acoustics.

The amount of reverberation can be controlled by placing special materials on the floor, walls, and ceiling of a room. Porous materials, such as wool, felt, and cork, are good sound absorbers. When sound waves enter the pores of these materials, the vibrations of the particles of the air are stopped. Sound-reflecting materials can be positioned in a room, especially auditoriums and concert halls, to help create desirable reverberations. Sound-absorbing tiles, called acoustical tiles, line the ceilings of many restaurants, reducing the clatter of dishes and the noise made by the voices of many people. Soft draperies, carpets, and wall hangings help to reduce reverberation in auditoriums and concert halls. The presence of people in an auditorium also affects the acoustics. The clothing and bodies of people absorb some of the sound waves. In an empty auditorium, listen to the way your voice sounds. It sounds different than when most of the seats are filled. For special experiments, scientists often use a room called an anechoic chamber. Anechoic means without echoes.

The walls, ceiling, and floor of such a room are constructed of sound-absorbing materials built into compartments of various sizes. Different size compartments trap sounds of different wavelengths. The result is a room in which almost no sound is reflected. Such a chamber reveals the true sound quality of instruments and voices. The anechoic chamber is useful for making very accurate measurements of sound and for testing microphones and other devices. Natural Frequency When you strike an object and make it vibrate, the force of your blow determines the intensity of the sound wave. But the frequency of the sound wave does not depend on the force of your blow. The frequency depends on the size, shape, and material of the object. Every vibrating object has its own frequency of vibration. This is called its natural frequency. For example, the metal of a bell vibrates at its own natural frequency no matter how hard the clapper inside the bell strikes the metal. A tuning fork -- a two-pronged "fork" used by musicians and singers to get a pure tone -- is constructed to vibrate at a certain frequency. If it is constructed to vibrate at 440 cycles per second, this is its natural frequency.

Sympathetic Vibrations Suppose two neighboring objects have the same natural frequency. One of the objects is made to vibrate. Its sound waves reach the second object and strike against it. Because the frequency of the sound waves is the same as the natural frequency of the second object, the waves start the second object vibrating. Both produce the same note. The second object is in sympathetic vibration with the first. If you have two tuning forks of the same frequency, you can hear sympathetic vibrations. Hold each tuning fork at its stem and make one vibrate by striking it against the edge of a table. After the tuning fork has sounded for a few seconds, stop its vibration by moving your hand up to the prongs. You can then hear the same sound coming from the second fork, because it is in sympathetic vibration with the first. Sympathetic vibrations can cause an avalanche. In a mountainous region, a large mass of rocks and snow may be piled loosely on a steep slope. The sound waves from a human voice or from some other source may cause some rock and snow particles to vibrate sympathetically. This small motion is sometimes enough to start the large mass of loosely piled material tumbling down the mountainside.

Resonance An effect called resonance occurs when a number of small repeated pushes cause a large vibration. Resonance increases the amplitude, and thus the intensity, of a sound. For an example of how resonance occurs, think of a child sitting on a swing. You give the swing a push, and it starts moving in an arc. If you continue giving a small push each time the swing is at its highest point, the swing soon moves through a wide arc. Although each push is small, you have timed the pushes so that their effect adds up, producing the large back-and-forth motion. The swing has a natural frequency at which it moves back and forth. Your pushes, if timed to this natural frequency, add up to make the wider arc. If you push at a faster or a slower rate, the swing comes almost to a stop. Resonance takes place only when you push at the natural frequency of the swing. The sounds made by wind instruments in music are reinforced (strengthened) by resonance. The resonance comes from a column of air inside the instrument. Thus, the sound of a trumpet or a bugle is reinforced by resonance of the air inside the instrument.

Forced Vibration Sometimes an object can be made to vibrate at a frequency other than its natural frequency. Its back-and-forth motion is then called forced vibration. A common example of forced vibration is in the eardrum. When sound waves strike the eardrum, they cause it to vibrate at the frequency of the received sound wave. You can use a tuning fork to produce forced vibrations. Strike the tuning fork to make the prongs vibrate, and listen to the sound for a moment. The sound is quite soft. Now place the stem of the fork against the top of a table. The sound at once becomes much louder. The vibrations of the fork force the tabletop to vibrate at the same frequency as the tuning fork. The table's larger size sets more air in motion than the tuning fork alone does, and you hear a louder sound. Many musical instruments are designed so that forced vibrations amplify their sounds. In a violin, for example, vibrating strings force the wood to vibrate -- thereby increasing the sound made by the strings. The forced vibration of the sounding board of a piano amplifies the sound made when a note is struck. Interference and Beats If two musicians play a tone of the same frequency, the two sounds combine.

The result is a louder sound. But if two tones of only slightly different frequencies are played together, a series of loud and soft sounds is heard. This effect -- of loud and soft sounds occurring one after the other -- is called beats. Beats are caused by the interference of two sound waves. When the compressions and expansions of two different sound waves reach your ear at the same time, they reinforce each other. The sound becomes louder. But when the compressions of one wave reach your ear at the same time as the expansions of the other, they interfere with each other. For a moment, almost no sound is heard. You can hear beats by using two tuning forks that have the same frequency. Change the frequency of one fork slightly by putting a thin rubber band around one of its prongs. Then strike the forks. You will hear the repeated loudness and softness as regular pulses. These are beats. It is annoying to hear more than four or five beats per second, and beats can spoil music. Musicians in an orchestra tune their instruments in unison so that all will produce the same pitch, or frequency.

This eliminates the possibility of beats on identical pitches, and the instruments are said to be "in tune." However, this applies only to notes of the same name, such as all "A" notes. In modern, or "tempered," tuning, all other intervals are very slightly out of tune and actually generate beats. This system is used to equalize the various intervals between the half-steps of the musical scale. These beats, described as "shimmers," give the characteristic richness to groups of notes played together as chords. Doppler Effect Have you ever noticed the change in pitch of a train whistle or automobile horn as the vehicle approaches and then passes you? As the sound source comes toward you, the pitch sounds higher. When the source is nearest you, you hear the sound at its normal pitch. Then as the source of sound moves away from you the pitch drops. This change in pitch is called the Doppler effect, after Christian Doppler, a 19th-century Austrian scientist. An automobile horn, like a train whistle, always vibrates at the same frequency. The apparent change in frequency in the sound wave is caused by the motion of the automobile.

When the vehicle approaches you, it is moving in the same direction as the sound waves coming from the horn. The sound waves are pushed closer together. Your ear receives more vibrations each second than the horn actually sends out. This gives the effect of a higher frequency, and you hear a higher-pitched sound. After the car passes you, the opposite effect takes place. The horn still sends out the same sound waves but is moving away from the waves. The sound waves are spaced farther apart, and your ear receives fewer vibrations a second than the horn sends out. This gives the effect of a lower frequency, and you hear a lower-pitched sound. The Doppler effect occurs whenever distance is changing between the source of sound and the listener. For example, you will hear a change in pitch if you are driving past a parked car whose horn is blowing. The amount of change depends on how fast the sound source or the listener is moving. If the speed of the sound source or the listener is great, the change in pitch is great. If the speed is slow, the change in pitch is less. Musical Sounds What determines whether a vibrating object will produce a noise, a pleasant sound, or a musical tone?

When compressions and expansions of a sound wave follow each other in an even, regular order, the sound is pleasant. People usually call it music. When the compressions and expansions follow each other in an uneven, irregular pattern, the sound we hear is a noise. Hammering on a steel tank produces a loud noise. The metal of the tank vibrates unevenly. It sends out compressions and expansions that do not follow each other in a regular pattern. A note played on the piano is a musical tone because the strings that produce the tone send out a regular pattern of compressions and expansions. A skilled pianist plays enjoyable music by producing a series of regular vibrations that blend well together. A child banging on the piano keyboard produces unpleasant noises. An exception to this rule is the sound of percussion instruments. Cymbals and most drums produce random combinations of frequencies, usually considered noises. However, when the bursts of noise are skillfully played in organized combinations, noise is converted into music. Fundamentals and Overtones A tuning fork is a specially constructed device that vibrates at only one particular frequency. However, most vibrating objects vibrate at several frequencies at the same time.

To understand how an object can vibrate at several frequencies at once, here's an example. A string is stretched between two posts. When plucked in the middle, the string vibrates as a whole . When plucked halfway between the middle and either end, the string again vibrates as a whole. But each half of the string vibrates separately as well. Depending on where it is plucked, the string can be made to vibrate as a whole and also in three or more separate parts. The parts vibrate at a different frequency from the vibration of the whole string. The tone produced by the vibration of the string as a whole is called the fundamental tone of the string. The tones produced by the vibrations of the parts are called overtones or harmonics. Musical instruments are designed to produce both a fundamental tone and overtones. In stringed instruments the strings vibrate as a whole and in sections, producing fundamentals and overtones. When a wind or woodwind instrument is played, the column of air in the instrument vibrates as a whole and in sections, producing both fundamentals and overtones. The metal or skin of percussion instruments produces overtones as well as fundamentals.

The different combinations of fundamentals and overtones give the sound of each musical instrument its special quality. Two instruments playing the same note sound different because each instrument produces different overtones. You might say that each has its own "recipe" of fundamentals and overtones. Like different musical instruments, each human voice has its own sound-wave recipe. The vocal cords in each person's throat vibrate with frequencies and overtones that depend on the size, thickness, length, and tension of the cords. The amount and force of the air passing between the cords as a person speaks also helps determine the kind of sound waves produced. This makes it possible for you to identify many different voices, just as you can tell the difference between the sound of a piano and a flute. Ultrasonics The human ear generally cannot hear sound waves whose frequency is higher than about 20,000 hertz. Such high-frequency waves are called ultrasound or ultrasonic waves. ("Ultra" means "beyond," and "sonic" means "sound.") Ultrasonic waves may have frequencies as high as millions of hertz. They can never be heard by a person. Ultrasonic waves can be produced in several ways. One method requires a thin slice of a quartz crystal.

(Quartz is a common mineral. It has the unusual property of responding to a changing, or alternating, electric current by vibrating in step with the current.) High-frequency alternating current is fed into the quartz. The crystal vibrates in step with the current and produces an ultrasonic wave. Another method makes use of an electromagnet with a special core usually made of the metal nickel. When a high-frequency alternating current is passed through the electromagnet, the nickel core expands and contracts slightly. This sets up ultrasonic vibrations in the surrounding medium. Uses of Ultrasound Ultrasonic waves have a very short wavelength. (Short wavelengths have a high frequency.) Because of this, many waves -- and their energy -- are packed into a small space. Scientists have found ways of putting this concentrated energy to use in scientific research, in industry, and in medicine. Ordinary sound waves move outward in all directions. But the very short wavelengths of ultrasonic waves make it possible to beam ultrasound in a straight line. Scientists have used straight ultrasonic beams in systems called sonar to locate submarines, schools of fish, and other solid objects underwater. In sonar systems, sound reflections, or echoes, are used to detect the solid objects.

A pulse, or burst, of ultrasonic waves is sent out from an instrument on a ship. The ultrasonic waves travel underwater in a straight line until they strike a solid object. The waves are then reflected and received at an instrument on board the ship. To measure the depth of the ocean a type of sonar system called a fathometer is used. An operator aboard a ship beams an ultrasonic wave down into the water. The time it takes for the echo of the sound wave to return to the instrument on the ship gives the operator the depth of the water. Ultrasound is used in industry to detect flaws hidden deep inside metals or other solid materials. An ultrasonic wave is beamed into the part to be tested. The way in which the beam is echoed back into a receiving device tells whether there is a defect in the material and where the defect is. An ultrasonic wave passed through a liquid makes the liquid vibrate very fast. The vibration can shake up paint to mix it thoroughly, homogenize milk by breaking up the fat particles, and clean tools, machine parts, dishes, and other solid objects. Ultrasound is used experimentally to break down the cell walls of bacteria.

In medicine it is used to help detect brain injuries, to trace blood flow through various parts of the body, and for certain kinds of surgery. In dentistry, a dental drill controlled by ultrasonic vibrations can neatly penetrate or cut into tooth enamel, creating very little friction or heat in the gum tissue. Certain ultrasound frequencies can be used to penetrate muscles and create heat, thereby stimulating circulation. An interesting application of ultrasound is in ultrasonic holography, a method of producing three-dimensional pictures with ultrasound. This is possible because ultrasound waves can easily travel through soft body tissues. When they reach denser layers of bone and cartilage, the waves are reflected back. Computers store and compare the images to generate three-dimensional pictures. With this technique it is even possible to safely detect the form of an unborn baby inside its mother's womb. Ultrasonics and Animals Some animals can hear ultrasonic sounds. For example, dogs can hear the high-pitched sound of a dog whistle that most people cannot hear. There are some animals that have built-in sonar systems. Dolphins have a sort of sonar system that they use to locate fish and obstacles in the water. Bats, too, have a sonar system.

They find their way about in darkness by uttering ultrasonic squeaks that are echoed back to them from solid objects like walls. Because animal sonar is so efficient, scientists are studying these animals to learn their "ultrasonic secrets."

Question: How many "tines" does a tuning fork usually have?
Answer: Two