Have you ever heard that old question, “If a tree falls in the woods and there is no one to hear it, does it make a sound.?”?  The answer depends on your point of view.  If sound is what our ears send to our brain after the ears detects the sound, and if no one is there to hear it, there is no sound.  On the other hand, if sound is something created by the fall of the tree that doesn’t depend on whether anyone has to be around to hear it, then there is sound, whether anyone hears it or not.  These experiments may not answer the question, but they will help you learn quite a bit about the nature of sound. After doing them, you may still not know the answer, but at least you will be able to debate the question intelligently!

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What Produces Sound?

 

Materials Needed: Plastic or thin wooden ruler; tabletop.

Procedure:   Place the ruler on the tabletop so that about 15 cm (6 in) extends off the table.  Place one hand on top of the portion of the ruler on the table to hold the ruler firmly in place.  With your other hand, push down on the other end of the ruler and let it go.  Listen carefully to the sound it makes.  Also, watch the ruler carefully.

Slide the ruler so that about 2 cm (1 in) more at a time on is the table and repeat.  What do you hear?
 
What Happened: When you released the ruler, it began to vibrate, or to move back and forth rapidly. It was the vibration of the ruler that produced the sound.   As the ruler vibrated, it moved the air around it and caused the air to vibrate as well.  This vibrating air spread out in waves much like a series of ripples spread out from a pebble thrown in the water.  When these vibrations reached your ear, they were interpreted by your brain as sound. These vibrations move out as waves, and in fact, are called sound waves. All sound waves are produced by vibrations.

You should have also noticed that the pitch of the ruler was higher as the length of the vibrating portion of the ruler (the part hanging off the table) was shortened.   The pitch of a sound is controlled by the speed of the vibrations.  The faster the vibrations, the higher the pitch of the sound.  The slower the vibrations, the lower the pitch of the sound.  In this experiment, the rate of vibration, and thus the pitch, was controlled by the length of the vibrating ruler.  This rate of vibration is called frequency and in measured in vibrations or cycles per second.  More about that later.

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Vibration, Pitch and Length of a String

Materials Needed: Piece of string about 125 cm (50 in) long; doorknob.

Procedure:   Tie one end of the string to a doorknob.  Measure 25 cm (10 in) from the doorknob, and pull the string taut.  Pluck the string.  As you do, listen to the sound, and observe the vibration.  Repeat at 50 cm (20 in), 75 cm (30 in) and 100 cm (40 in).

What Happened: As the string was lengthened, the pitch decreased.  When the length of a taut string is increased, the frequency (rate of vibration) is decreased, and as a result, the pitch decreases.

Going Further: You may observe how length of a string and pitch are related by examining a guitar.  You can change the length of the vibrating part of a string on the guitar by pressing the string down on a fret.  (The frets are the raised parts on the neck of the guitar.)  As you shorten a string on the guitar by pressing it against one of the frets, the pitch of the sound is higher.  As you lengthen the string, the pitch is lower.

You may also notice that some strings on the guitar are thicker than others.  The thickness of the string will also affect the pitch as we will see.

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Vibration, Pitch and Thickness

Materials Needed: Wooden board about 30 cm long; 4 nails; hammer; thick rubber band; thin rubber band of the same length.

Procedure:   Nail two nails 25 cm apart as shown. Nail two other nails parallel to the first two, also 25 cm apart.

Stretch one of the rubber bands between two of the nails 25 cm apart as shown. Do the same thing with the other rubber band.

Pluck each rubber band and notice the difference.

Save this board for the next experiment.

What Happened: The thicker rubber band has a lower pitch.  As the thickness of a vibrating material increases, the rate of vibration decreases, and the pitch also decreases.  Likewise, as the thickness of the vibrating material decreases, the pitch increases.
 
Going Further: Just as you saw how the length of a string affected pitch in a guitar, you can also observe how the thickness of a string affects the pitch.  When you pluck the strings without pressing any of the frets, you will notice that the pitch of the thicker strings are lower.

If the guitar is properly tuned, press the fifth fret of the top string.  Now pluck the top string and the second string.  If the guitar is tuned, the pitch should be the same. Because of this, you should see that the pitch may be controlled by either the length of a string or it’s thickness. 

There is one more factor that will affect pitch.

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Vibration, Pitch and Tautness

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed: Board with nails from the last experiment; two identical rubber bands; scissors.

Procedure: Cut each of the rubber bands to make a thin rubber strip.

Tie one end of one of the rubber bands to one of the nails and stretch it taut, but just barely.  Keeping it taut, tie the rubber band to the other nail.

Repeat with the other rubber band strip, but this time, stretch the rubber band as taut as you can without snapping it. 

Pluck each of the rubber bands and note the difference in pitch.

What Happened: The rubber band that was stretched tighter vibrated faster when plucked.  The faster vibration created a higher pitch. 

As any solid is drawn more taut, its rate of vibration will increase, and the pitch will also increase.

Going Further: This last factor that affects pitch may also be seen in a guitar.  If someone will let you use their guitar, pluck one of the strings as you loosen and tighten that string using the tuning nut at the end of the neck.  As you tighten the string on the guitar, the rate of vibration of that string increases, and the pitch increases as well.  Be careful not to tighten it so much that the string breaks!

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Vibration of Glass and Pitch - A Water Xylophone

Materials Needed: Eight identical glass bottles; measuring cup; water; spoon.

Procedure: Leave the first bottle empty.  Add 30 ml (1 ounce) of water to the second bottle.  Add water to the remaining bottles by adding 30 ml (1 ounce) more to each bottle than you did to the previous one.

Using your spoon, tap each bottle gently and notice the sound it produces.

Save this setup for the next experiment.

What Happened: The pitch of each bottle was different.  Up to this point we have used only strings to make sounds, but here, the sounds were produced by the vibrating glass.  

When each bottle was struck by the spoon, it began to vibrate.  The rate at which the glass vibrated was determined by the amount of water in each bottle. This is the same principle that a xylophone uses to make music.  On a xylophone, the sounds are produced when bars of metal are struck.  Each bar of metal is a slightly different length, so it vibrates at a different rate from all the others.  This causes each bar to produce a different pitch.

Going Further: If you are musically inclined, you can “tune” your water bottle xylophone by adding or taking water away from each bottle.  See whether you can tune your bottles to play a musical scale.

Materials Needed: Setup from the last experiment.

Procedure: Blow across the top of each bottle from the last experiment and notice the pitch produced.

What Happened: As you probably expected, a different pitch was produced by each bottle. This time, however, it was mainly the column of air inside the bottle that was being vibrated, rather than the glass.   Gases such as air may be vibrated directly to produce sound.  The rate of vibration, and therefore the pitch, is caused by the size of the column of air produced.
 
Going Further: If you tried to tune your bottles (See “Going Further” In the last experiment), do the tuned bottles produce a musical scale when you blow across them? Are the notes the same?

So far, we have seen how the rate of vibration is related to pitch.  As the rate of vibration increases, the pitch increases.  But what does the rate of vibration mean?   This rate of vibration is called “frequency” and is measured in Hertz or cycles per second as we will see in this experiment.

Materials Needed: Piano, electronic keyboard, or other musical instrument; someone who knows how to play it.

Procedure: Have the person to play a musical scale starting with “middle C”.  Listen carefully to the pitch as it changes.  Next, have the person to play the next octave above the scale.

What Happened: When middle C is played, the instrument causes air to vibrate at 264 times per second.  Each complete vibration is also called a “cycle”, so the rate of vibration is 264 cycles per second.  There is a unit called the Hertz which is used to measure frequency in cycles per second.  One Hertz is one cycle per second, so the frequency of middle C is 264 Hertz.

The frequencies for the notes in the scale beginning with middle C are:

An octave consists of the 8 notes on the musical scale. When the next note is played above that octave, the musical scale starts over with the same notes, but at a higher pitch. The next note played after the scale, was also “C”, but it was one octave higher.  The frequency was double that of middle C, or 528 Hertz.  The frequency of each note in the next octave is always double that of the same note in the octave below it.

So far, in all of our experiments studying sound, the sound we have heard reached our ears by causing air to vibrate.   The next two experiments will help us to understand how these invisible waves move through air.

Materials Needed: Sink; water; eye dropper or soda straw.

Procedure: Fill the sink with two or three inches of water.  Allow the water to become still.

Draw a little water into the eyedropper or straw.  Allow a single drop to fall on the surface of the still water in the middle.  Observe how the waves spread out from the drop.

What To Look For: In this experiment, be sure to notice the height of the waves as they spread out.

What Happened: The drop of water created waves that moved out in a series of circles. As the waves moved further away from the center, the height of each wave got smaller. This is because the energy in that wave is spread out over a larger area as the circle grows.
 
Sound waves move in somewhat the same way through the air, although they are Invisible to us.  Since a sound wave in air spreads out in all directions, the energy reaching your ear decreases the farther you move from the source of the sound, and the sound is not as loud.

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How Sound Waves Move (Slinky®)

Materials Needed: Slinky® or similar spring toy; a friend. (HINT: You can sometimes find small spring toys at a "dollar" store.

Procedure:   Take one end of the Slinky ® and have your friend to take the other end.  Stretch it by moving away from one another until it no longer sags to the floor in the middle.

Now squeeze several coils together near your end and quickly release them.  Observe what happens.

What Happened: You saw a wave in the Slinky ® move away from you, toward your friend, and it may have also returned back to you.  In fact, this may have happened several times.  However, if you looked carefully, you also saw that the a portion of the spring was squeezed together as the wave moved through it, followed by a portion that was spread out.

The Slinky ®  behaved something like molecules of air when sound travels through it.  When air is vibrated, it is first “compressed” or squeezed together.  The air is then “rarified” or spread apart.  This is repeated over and over as the source of the sound vibrates back and forth and causes the air to compress and rarify.  Of course, the sound wave continues to move away from the source unless something causes it to be reflected back. We’ll see an example of that in the next experiment.

Materials Needed: Yourself; a large building.

Procedure: Stand at least the length of several football fields from the building.  Face the building and cup your hands around your mouth.  Give a short yell and listen.  What do you hear?

What Happened: You should have heard an echo within about a second after you yelled. When you cupped your hands, that helped to channel the sound toward the building.  The sound waves moved away from you toward the building.  However, when they reached the building, some of the sound waves were reflected off the building and bounced back.  The returning sound waves produced the echo that you heard.

Going Further: If you live near a large cliff or mountain, or even a line of trees at the edge of a forest, see how far away you can stand from them and produce an echo that you can hear.

Materials Needed: Two small pieces of wood, such as two small blocks of 2 x 4 lumber; a large open area such as a football field; a friend.

Procedure: Have your friend take the two wood pieces and move 100 or more meters (yards) away from you.  Explain that you will hold one of your arms up, and when you drop it, you want your friend to clap the wooden blocks together. (Or, if you can get a pair of "walkie talkies", you can communicate back and forth without hand signals!)

Raise your arm and then lower it.  Notice what you see and hear when the boards are clapped together.

If you have enough space, keep moving apart until you can no longer hear the sound.

What Happened: You should have seen the boards clapped together a split second before you heard the sound.  This is because the light that allows you to see the boards clap together is moving extremely fast - 186,000 miles per second!  However, the sound moves much more slowly - about 1,000 feet per second through air.  The light reaches you almost instantly, but the sound is much slower.
 
Going Further: You can use the speed of sound to determine approximately how far away a thunderstorm is.  The thunder is produced by the lightning.  When the lightning is close by, you hear the thunder at the same time as you see the lightning.  However, the farther away you are from the lightning, the longer the thunder seems to follow the lightning.

CAUTION:  To avoid being hit by lightning, do not stand outside during a thunderstorm!

Since one mile is equal to 5,280 feet, and since sound travels through air at about 1,000 feet per second, you can count the time between the time you see the lightning and the time you hear the thunder.  For every second of difference, the lightning is about 1/5 mile away.

Sounds usually reach our ears by vibrating air, but they can be carried in other ways as well.  The following experiments will show you other ways sound may be transmitted.

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Sounds on a String - The Chiming Fork

Materials Needed: Metal fork; 1 meter piece of string.

Procedure:   Tie the fork to the middle of the string.  Press the ends of the string up to the opening of your ear and allow the fork to hang freely.  Gently swing the fork against a tabletop or wall and listen.  Move the string away from your ears and listen.  Now what do you hear?
 
What Happened: When the string was held against your ears, the fork sounded like a chime.  As the fork vibrated, the vibrations moved up the string to your ear.  Although some of the sound was transmitted your ear by vibrating air, most of what you heard was as a result of the vibrations of the string.

When you moved the string away from your ears, the sound reaching your ears was as a result of the vibration of air only and was much fainter.

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Sounds on a String - Tin Can Telephone

   

Materials Needed: 2 tin cans; 10 meter (40 ft) length of string; nail; hammer; 2 buttons; a friend.

Procedure: Punch a hole in the bottom of each tin can using the hammer and nail.  Work the nail around each hole to flatten the sharp edges as much as possible. If you have a small piece of board, you can stick it down inside the can and use the hammer to flatten the sharp metal even more. By flattening the sharp edges, the string will be less likely to be cut by one of the edges. (So will you!)

Run one end of the string through the bottom of the can and tie it through two of the holes of a button.  Do the same thing with the other can.

Take one can and give the other to a friend.  Stretch the string taut, but not so tight that the string will break.  Have your friend to speak into the can while you listen.  Then talk while your friend listens. What happens?

What Happened: The sound was carried between the cans by the vibrating string.

When you spoke into the can, the sound waves from the air vibrated the can, which caused the taut string to vibrate.  These vibrations traveled the length of the string and caused the can at the other end to vibrate. In turn, this vibrated the air, which produced the sound waves heard by your friend.  

Your voices could be heard by each other even if you spoke in a whisper because the energy of the sound wave was concentrated in the string rather than being spread out through the air.

Going Further: Try different using kinds of string to see which works best.  Also, try using nylon fishing line instead of string.

Materials Needed: You; a good quality digital or tape recorder; a friend.

Procedure: Record your friend’s voice and play it back.  Ask your friend whether the tape sounds like his or her voice.

Now record your own voice and play it back.  Does it sound like you?

What Happened: If you used a good quality tape recorder, the voice of your friend on the tape should have sounded very similar to that of your friend in person. However, when you heard your voice from the tape recorder, you probably didn’t think it sounded like you at all.  Here’s why...

When you speak, the sound of your voice is produced by membranes in your larynx or “voice box.”  These membranes are called vocal cords and they vibrate as air is forced across them.  The sounds are varied by varying the thickness of these membranes.  Sound familiar?

When others hear you speak, they hear sounds that are created only by the vocal cords. However, as you speak, these vocal cords cause the bones in your head to vibrate as well, and the sound of your voice as heard by you is a combination of the vocal cords vibrating the air and vibrating the bones in your head.  When you heard your voice on a tape recorder, the sound that was recorded is very similar to what others hear when you speak, because it records the part of the sound of your voice produced only by your vocal cords vibrating the air.  When this was played back, you heard your voice as others do - without the vibrating bones - and it probably sounded a little unfamiliar.  For the same reason, your friend probably didn’t recognize his or her voice on tape either.

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Concentrating Sounds With A Balloon

Materials Needed: Balloon; watch or small clock that ticks.

Procedure: Inflate the balloon and tie it off.  Put it aside for a minute.

Hold the watch or clock the same distance from your ear as the width of the balloon.  Note how loud or soft it sounds.

Now, hold the balloon firmly against your ear and hold the watch or clock against the other side.  How loud does it sound now?

What Happened: The air in the balloon is slightly compressed, and it carries sound much better than uncompressed air.  Also, some of the vibrations were transmitted by the tight surface of the balloon. Because of these two factors, the balloon was able to concentrate the energy of the sound waves, and the ticking sounded louder when heard through the balloon.

Going Further: Try this with an empty plastic 2 liter soft drink bottle; a plastic milk jug, an inflated plastic bag; and a glass jar.  Which works best? Do you have any idea why?

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Channeling Sound Waves

CAUTION: Never yell into a tube while someone else has their ear on the other end. This can damage their hearing and is very painful!

Materials Needed: A friend; a long cardboard tube such as a mailing tube or a wrapping paper tube.  (If you don’t have one, try rolling a tube from stiff cardboard such as poster board.)

Procedure: Have your friend to stand beside you while you whisper into one end of the tube.  Ask whether your friend can hear you.  Next, have your friend to whisper in the other end while you stand along side and listen.  Can you hear your friend?

Now whisper into your end while your friend has his or her ear up to the other end of the tube.  Again have your friend to whisper while you listen through the tube.  Can you hear each other now?

What Happened: You should have been able to hear the sound of your friend’s whisper much more clearly through the tube. The tube channeled most of the sound energy and kept it from spreading out in all directions.

Many ships used to have a network of “speaking tubes” that were used to communicate between different parts of the ship.

Going Further: Try this same experiment using a garden hose that has been stretched out on the ground instead of a cardboard tube.  (Make sure there is no water in the hose or you could get a wet ear!)  Also, make a couple of funnels from the tops of 2 liter plastic bottles, and tape one to either end of the hose. Does this improve your performance?  If you can talk back and forth with one garden hose, try connecting two. 

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Making a Stethoscope

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Studying Sound With A Computer

Materials Needed: Computer with Microsoft Windows operating system; Audio generator program.  (See procedure below.)

Procedure: Computer sound cards do an excellent job of generating sounds at a particurlar frequency with the right software.  A Romanian educational website has a free sound generating program.  You can download the program here.

The program downloads as as "zipped" file, and once unzipped, it is a single executable file that does not need to be installed on your computer.  

When you run the program, you will see that the controls are not in English, but they should be pretty obvious.  Volum = Volume, Timp/diviziune = Time division, and Frecvenba (Hz) = Frequency (Hz).

Now, run the program,  Use the slider control to sound each of the notes in the musical scale from the table below, Use the "Time division control to select one of the three options so that you will see a nice horizontal curved or S-shaped line.  For the musical scale, start with either 10 ms or 1 ms.  

Note          C D E F G A B C

Frequency (Hertz)   264 294 330 352 376 440 495 528

Notice the change in tone as the frequency increases.  Also notice that as the frequency increases, the number of waves in the curved line increases.  The curved line is called a sine wave, and it represents the actual sound waves.  The increasing number of waves that you see indicates an increase in frequency.  

Raise or lower the volume control.  Notice that as you do, the height of the waves changes.  The wave height is called the amplitude.  As the volume increases, so does the amplitude.

Going Further: This is just the beginning of what you can do with this little program.  Look online or in a reference book to determine what the normal range of human hearing is.  See whether you can hear frequencies in that range, as well as above or below the range.  Try this with your friends as well.

This is another one of those things that you could use to design a great science project around!

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There's lots more to do on the Experiments page!