Air Horns & Sonic BOOM!

Sunday November 29, 2009

f18Sound can change according to the speed at which it travels. Another word for sound speed is pitch. When the sound speed slows, the pitch lowers. With clarinet reeds, it’s high. Guitar strings can do both, as they are adjustable. If you look carefully, you can actually see the low pitch strings vibrate back and forth, but the high pitch strings move so quickly it’s hard to see. But you can detect the effects of both with your ears.


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The range of your ears is about 20 – 20,000 Hz (cycles per second). Bats and dogs can hear a lot higher than we can. The image (right) is a real picture of an aircraft as it breaks the sound barrier – meaning that the aircraft is passing the speed that sounds travels at (about 700 mph). The white cloud you see in the photo is related to the shock waves that are forming around the craft as it moves into supersonic speeds. You can think of a shock wave as big pressure front, which creates clouds. In this photo, the pressure from the shock waves is condensing the water vapor in the air.


There are lots of things on earth that break the sound barrier – bullets and bullwhips, for example. The loud crack from a whip is the tip zipping faster than the speed of sound.


shockwaveSo why do we hear a boom at all? Sonic booms are created by air pressure (think of how the water collects at the bow of a boat as it travels through the water). The vehicle pushes air molecules aside in such a way they are compressed to the point where shock waves are formed. These shock waves form two cones, at the nose and tail of the plane. The shock waves move outward and rearward in all directions and usually extend to the ground.


As the shock cones spread across the landscape along the flightpath, they create a continuous sonic boom. The sharp release of pressure, after the buildup by the shock wave, is heard as the sonic boom.



How to Make an Air Horn

Let’s learn how to make loud sonic waves… by making an air horn. Your air horn is a loud example of how sound waves travel through the air. To make an air horn, poke a hole large enough to insert a straw into the bottom end of a black Kodak film canister. (We used the pointy tip of a wooden skewer, but a drill can work also.) Before you insert the straw, poke a second hole in the side of the canister, about halfway up the side.


Here’s what you need:


  • 7-9″ balloon
  • straw
  • film canister
  • drill and drill bits

Grab an un-inflated balloon and place it on your table. See how there are two layers of rubber (the top surface and the bottom surface)? Cut the neck off a balloon and slice it along one of the folded edges (still un-inflated!) so that it now lays in a flat, rubber sheet on your table.


Drape the balloon sheet over the open end of the film canister and snap the lid on top, making sure there’s a good seal (meaning that the balloon is stretched over the entire opening – no gaps). Insert the straw through the bottom end, and blow through the middle hole (in the side of the canister).


You’ll need to play with this a bit to get it right, but it’s worth it! The straw needs to *just* touch the balloon surface inside the canister and at the right angle, so take a deep breath and gently wiggle the straw around until you get a BIG sound. If you’re good enough, you should be able to get two or three harmonics!



 


Download Student Worksheet & Exercises


Troubleshooting: Instead of a rubber band vibrating to make sound, a rubber sheet (in the form of a cut-up balloon) vibrates, and the vibration (sound) shoots out the straw. This is one of the pickiest experiments – meaning that it will take practice for your child to make a sound using this device. The straw needs to barely touch the inside surface of the balloon at just the right angle in order for the balloon to vibrate. Make sure you’re blowing through the hole in the side, not through the straw (although you will be able to make sounds out of both attempts).


Here’s a quick video where you can hear the small sonic boom from a bull whip:



Since most of us don’t have bull whips, might I recommend a twisted wet towel? Just be sure to practice on a fence post, NOT a person!


Exercises 


  1. Why do we use a straw with this experiment?
  2. Does the length of the straw matter? What will affect the pitch of this instrument?

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The Best Parent-Annoyer Ever

Sunday November 29, 2009

This is one of my absolute favorites, because it’s so unexpected and unusual… the setup looks quite harmless, but it makes a sound worse than scratching your nails on a chalkboard. If you can’t find the weird ingredient, just use water and you’ll get nearly the same result (it just takes more practice to get it right). Ready?


NOTE: DO NOT place these anywhere near your ear… keep them straight out in front of you.


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Here’s what you need:


  • water or violin rosin
  • string
  • disposable plastic cup
  • pokey-thing to make a hole in the cup


 
Download Student Worksheet & Exercises


Exercises


  1. What does the rosin (or water) do in this experiment?
  2. What is vibrating in this experiment?
  3. What is the cup for?

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Seeing Sound Waves

Sunday November 29, 2009

This section is actually a collection of the experiments that build on each other.  We’ll be playing with sound waves, and the older students will continue on after this experiment to build speakers.


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Here’s what you need:


  • a radio or some sort of music player
  • a balloon
  • a mixing bowl
  • water
  • your parent’s permission


 
Download Student Worksheet & Exercises


1. Turn on your music player and turn it up fairly loud. (Tell your parents that it’s for science!)


2. Take a look at your speaker. You should be able to see it vibrating. If there’s a song with a lot of bass, you should really be able to see it moving.


3. Put your hand on the speaker. Can you feel the vibrations?


4. ASK YOUR PARENTS if you can carefully put a half-filled bowl of water on top of your speaker. You should be able to see the water vibrate.


Remember that sound is nothing more than vibrating molecules. All speakers do is get molecules of air to vibrate, creating longitudinal waves. They push air. Your eardrums vibrate just like the speakers do when the longitudinal waves of sound energy hit your ears.


How to Feel the Beat

1. Inflate the balloon. Get it fairly large.


2. Turn the music on loud (the more bass the better).


3. Put both hands lightly on the balloon.


4. Walk around the room holding the balloon lightly between your hands.


5. Try to feel the balloon vibrating.


6. Does the balloon vibrate more for low sounds or high sounds?


7. If you have a synthesizer (piano keyboard) you may want to try turning it up a bit and playing one note at a time. You should notice that the balloon vibrates more or less as you go up and down the musical scale. At very high notes, your balloon may not vibrate at all. We’ll talk more about why this happens later.


What’s causing the balloon to vibrate? Energy. Energy causes objects to move a distance against a force. The sound energy coming from the speakers is causing the balloon to vibrate. Your ear drums move in a very similar way to the balloon. Your ear drum is a very thin membrane (like the balloon) that is moved by the energy of the sound. Your ear drum, however, is even more sensitive to sounds than the balloon which is why you can hear sounds when the balloon is not vibrating. If you ear drum doesn’t vibrate, you don’t hear the sound.


What to do this experiment but no speakers?

Here’s another version of the same idea – I’ll bet you did this experiment when you were a small baby! You need: a mixing bowl (one of those metal bowls), something to hit it with ( a wooden spoon works well), and water.


1. Take the mixing bowl and put it on the table.


2. Smack it with the wooden spoon.


3. Listen to the sound.


4. Put your ear next to the bowl and try to hear how long the sound continues.


5. Now hit the bowl again.


6. Touch the bowl with your hand a second or two after you hit it. You should hear the sound stop. This is called dampening.


7. Now, for fun, fill the bowl with water up to an inch or so from the top.


8. Smack the bowl again and look very carefully at where the bowl touches the water.


9. When you first hit the bowl, you should see very small waves in the water.


I want you to notice two things here. Sound is vibration. When the bowl is vibrating, it’s making a sound. When you stop it from vibrating, it stops making sound. Any sound you ever hear, comes from something that is vibrating. It may have vibrated once, like a balloon popping. Or it may be vibrating consistently, like a guitar string.


The other thing I want you to notice is that you can actually see the vibrations. If you put water in the bowl, the tiny waves that are formed when you first hit the bowl are caused by the vibrating sides of the bowl. Those same vibrations are causing the sound that you hear.


item4mIf your mom’s worried about making a mess with water (and it’s not bath night tonight) then try this alternate experiment: you’ll need a mixing bowl, wooden spoon, and rubber bands.


1. Stretch a few rubber bands around the box or the bowl. If possible, use different thicknesses of rubber bands.


2. Strum the rubber bands.


3. Feel free to adjust how stretched the bands are. The more stretched, the higher the note.


4. Try plucking a rubber band softly.


5. Now pluck it fairly hard. The hard pluck should be louder.


Again I’d like you to notice three things here. Just like the last experiment, you should see that the sound is coming from the vibration. As long as the rubber band vibrates, you hear a sound. If you stop the rubber band from vibrating, you will stop the sound. Sound is vibration.


The second thing I’d like you to notice is that the rubber bands make different pitched sounds. The thinner the rubber band, or the tighter it’s stretched, the faster it vibrates. Another way to say “vibrating faster” is to say higher frequency. In sound, the higher the frequency of vibration, the higher the pitch of the note. The lower the frequency, the lower the pitch of the note. The average human ear can hear sound at as high a frequency as 20,000 Hz, and as low as 20 Hz. Pianos, guitars, violins and other instruments have strings of various sizes so that they can vibrate at different frequencies and make different pitched sounds. When you talk or sing, you change the tension of your vocal cords to make different pitches.


One last thing to notice here is what happened when you plucked the rubber band hard or softly. The rubber band made a louder noise the harder you plucked it right? Remember again that sound is energy. When you plucked that rubber band hard, you put more energy into it than when you plucked it softly. You gave energy (moved the band a distance against a force) to the rubber band. When you released the rubber band, it moved the air against a force which created sound energy. For sound, the more energy it has, the louder it is. Remember when we talked about amplitude a few lessons back? Amplitude is the size of the wave. The more energy a wave has the bigger it is. When it comes to sound, the larger the wave (the more energy it has) the louder it is. So when you plucked the rubber band hard (gave it lots of energy), you made a louder sound.


I said this in the beginning but I’ll repeat it here, hoping that now it makes more sense. When something vibrates, it pushes particles against a force (creates energy). These pushed particles create longitudinal waves. If the longitudinal waves have the right frequency and enough energy (loudness), your ear drum antennas will pick it up and your brain will translate the energy into what we call sound.


Exercises 


  1. What is sound?
  2. How does the rubber band make different sounds?
  3. What difference does it make how hard or soft you pluck the rubber bands?

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Humming Balloon

Sunday November 29, 2009

You can easily make a humming (or screaming!) balloon by inserting a small hexnut into a balloon and inflating. You can also try pennies, washers, and anything else you have that is small and semi-round. We have scads of these things at birthday time, hiding small change in some and nuts in the others so the kids pop them to get their treasures. Some kids will figure out a way to test which balloons are which without popping… which is what we’re going to do right now.


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Here’s what you need:


  • hexnut
  • balloon
  • your lungs


 
Download Student Worksheet & Exercises


What to do: Place a hexnut OR a small coin in a large balloon. Inflate the balloon and tie it. Swirl the balloon rapidly to cause the hexnut or coin to roll inside the balloon. The coin will roll for a very long time on the smooth balloon surface. At high coin speeds, the frequency with which the coin circles the balloon may resonate with one of the balloon’s “natural frequencies,” and the balloon may hum loudly.


Exercises


  1. How does sound travel?
  2. What is pitch?
  3. How is frequency related to pitch?

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Good Old Telephone

Sunday November 29, 2009

telephoneThis is the experiment that all kids know about… if you haven’t done this one already, put it on your list of fun things to do. (See the tips & tricks at the bottom for further ideas!)


We’re going to break this into two steps – the first part of the experiment will show us why we need the cups and can’t just hook a string up to our ear.  Are you ready?


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You need:


  • Table
  • Spoon (or whatever is handy)
  • Partner

1. Sit at a table.


2. Have your partner sit at the other end of the table.


3. Have your partner very lightly scratch the table with the spoon.


4. Listen to see if you can hear it.


5. While your partner is scratching, put your ear on the table. Do you hear a difference in the sound?


6. Switch roles so your partner gets a chance to try.


Did you notice how you could hear the soft spoon-scratching sound (I love a good alliteration) quite clearly when your head was on the table? The sound waves moved quickly through the table so they lost little of the loudness and quality of the original sound. When sound travels through the air, the sound energy gets dispersed (spread out) much more than through the table, so the sound does not travel as far nor as clearly. This next one is an oldie but a goodie!


A Couple Cups of Conversation


1. Using the scissors or a nail poke a hole in the middle of the bottom of both cups. Get an adult to help you with this. Since this isn’t biology, no bleeding allowed!


2. Thread an end of the string through the hole in the bottom of the cup and tie a big knot in it to keep it from sliding through the hole.


3. Do the same thing with the other cup so that when you are done you have a cup attached to both ends of the string.


4. Take one of the cups for yourself and hand the other cup to your partner. Walk apart from one another until the string is fairly taut.


5. Have your partner hold the cup up to his or her ear while you whisper into your cup.


6. Can your partner hear you? If not, see if you can stretch the string a little more.


7. Switch roles and try again.



The string being a solid and having tightly packed molecules allows the sound wave to move quickly and clearly through it. You can talk very quietly in one cup and yet your partner can still hear you fairly well.


Tips & Tricks

You can try different types of cups (foam, plastic, metal (like tin foil), paper…) and also change the sizes of the cups – is bigger or smaller better?  You can also change the connection between the cups – have you tried yarn, wool, string, nylon fishing line, rope, clothesline, or a braided combination?  You can also stick a slinky in place of the string of ‘space phones’.


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Seeing Sound Waves using Light

Sunday November 29, 2009

After you’ve completed this experiment, you can try making your own sound-to-light transformer as shown below. Using the properties of sound waves, we’ll be able to actually see sound waves when we aim a flashlight at a drum head and pick up the waves on a nearby wall.


Here’s what you need:


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  • empty soup can
  • balloon
  • small mirror
  • tape
  • scissors
  • hot glue gun
  • laser or flashlight


You will be adjusting the length of string of a pendulum until you get a pendulum that has a frequency of .5 Hz, 1 Hz and 2 Hz. Remember, a Hz is one vibration (or in this case swing) per second. So .5 Hz would be half a swing per second (swing one way but not back to the start). 1 Hz would be one full swing per second. Lastly, 2 Hz would be two swings per second. A swing is the same as a vibration so the pendulum must move away from where you dropped it and then swing back to where it began for it to be one full swing/vibration.


The following information is for students in our upper level part of the science program:


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“Advanced students: Download your Seeing Sound Waves using Light


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Thunder and Lightning

Sunday November 29, 2009

When there’s lightning, thunder is not far behind.  Even if you don’t live in a tropical thunderstorm area, you can still simulate this experiment using the variations below and get the most out of the main ideas about sound waves and light waves.


For starters, let’s assume you’re waiting for a good storm. When one’s brewing, grab a timer and a pencil with paper and wait inside the house near a window.


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Here’s what you need:


  • two hunks of wood or a pair of baseball bats
  • a neighborhood block
  • a partner

1. Wait until you see a lightning flash (do this indoors please!).


2. Start your timer (or count “one Mississippi, two Mississippi”, etc.)


3. Stop counting when you hear the thunder.


4. Take whatever number you’ve reached and divide it by 5. That number is how many miles the lightning strike is away from you. So if you’ve counted for about 3 seconds the lightning strike is about a half mile away. If you’ve counted for 5 seconds the lightning is 1 mile away. If you’ve counted for 8 seconds the lightning is about 1.5 miles away.


Remember, sound travels at about 1000 ft/sec. A mile is a little over 5000 feet (5280 ft. to be exact). So it takes sound about 5 seconds to go 1 mile!



 
“Advanced students: Download your Thunder and Lightning”


Want to do this experiment without a storm?

You’ll need two hunks of wood or a pair of baseball bats, a neighborhood block, and a partner.


1. Give your partner the two bats or hunks of wood.


2. Have them walk a half a block away or at least 250 feet.


3. When they get there have them clack together the two pieces of wood (be careful not to smash fingers, you want to hear the wood, not the scream of you friend!).


4. Have them do it several times. Try to notice a difference between when you see the wood crashing together and when you hear it. If you don’t hear a difference, get farther apart from one another.


5. Trade places so that your partner can see the delay of sound (just like on one of those old Japanese movies).


Sound travels at about 760 mph. That’s the same as about 1000 ft/sec. If your friend was standing about 250 feet away from you, it took a quarter of a second for the sound to get from your partner to you.


The next time you’re at a baseball game or a fireworks display try to time the difference between the time you see something and the time you hear something. Remember that sound travels 1000 ft/sec. If the distance is great enough you may be able to figure out how far away it is and amaze your friends!


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Chladni Plates

Sunday November 29, 2009

chladniThis experiment is just for advanced students. Ernst Florens Friedrich Chladni (1756-1827) is considered to be the ‘father of acoustics’. He was fascinated by vibrating things like plates and gases, and his experiments resulted in two new musical instruments to be developed.


When Chladni first did these vibrating plate experiments (as shown in the video below), he used glass plates instead of metal. He was also one of the first to figure out how to calculate the speed of sound through a gas.


And it will completely blow your mind. Chladni patterns are formed with a metal plate covered in regular table salt is vibrated through different frequencies.


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Here’s what you need:


  • metal plate or un-rimmed cookie sheet (thinner is better, but you’ll have to experiment with this)
  • violin rosin
  • bass fiddle bow
  • two containers of salt

There are different ways of vibrating the plate – the easiest is by banging it, but this gives you only one frequency and usually makes a mess of the salt. You can alternatively bow the edge of the plate (clamped to a table) with a bass fiddle bow and specific points to get various frequencies… but you will need to practice to get this method to work.


These patterns can also be formed by setting the metal plate on a mechanical driver (like a speaker) controlled by a signal generator. (This way you don’t have to practice your bowing!).  The patterns you get this way are different from the bowing patterns, since you are vibrating it from the center instead of the edge.


chladniarray
Image Source: UCLA Physics Dept.


These patterns were made by attaching the center of the plate firmly to a speaker.  In the video below, you’ll see how the different patterns were made in a live physics demo from the physics department at WFU:



 


Tell us what you think! Write a comment below…


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Building Speakers

Sunday November 29, 2009

Alexander Graham Bell developed the telegraph, microphone, and telephone back in the late 1800s. We'll be talking about electromagnetism in a later unit, but we're going to cover a few basics here so you can understand how loudspeakers transform an electrical signal into sound.

This experiment is for advanced students.We'll be making different kinds of speakers using household materials (like plastic cups, foam plates, and business cards!), but before we begin, we need to make sure you really understand a few basic principles. Here's what you need to know to get started:

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For this experiment to really make sense, you'll need to complete the Telephone and the Seeing Sound Waves Experiments first. This will cover the basic mechanics of sound vibrations and waves.

Let's talk about the telegraph. A telegraph is a small electromagnet that you can switch on and off. The electromagnet is a simple little thing made by wrapping insulated wire around a nail. An electromagnet is a magnet you can turn on and off with electricity, and it only works when you plug it into a battery.

Anytime you run electricity through a wire, you also get a magnetic field. You can amplify this effect by having lots of wire in a small space (hence wrapping the wire around a nail) to concentrate the magnetic effect. The opposite is true also - if you rub a permanent magnet along the length of the electromagnet, you'll get an electric current flowing through the wire. Magnetic fields cause electric fields, and electric fields cause magnetic fields. Got it?

A microphone has a small electromagnet next to a permanent magnet, separated by a thin space. The coil is allowed to move a bit (because it's lighter than the permanent magnet). When you speak into a microphone, your voice sends sound waves that vibrate the coil, and each time the coil moves, it causes an electrical signal to flow through the wires, which gets picked up by your recording system.

A loudspeaker works the opposite way. An electrical signal (like music) zings through the coil (which is also allowed to move and attached to your speaker cone), which is attracted or repulsed by the permanent magnet. The coil vibrates, taking the cone with it. The cone vibrates the air around it and sends sounds waves to reach your ear.

If you placed your hand over the speaker as it was booming out sound, you felt something against your hand, right? That's the sound waves being generated by the speaker cone. Each time the speaker cone moves around, it create a vibration in the air that you can detect with your ears. For deep notes, the cone moves the most, and a lot of air gets shoved at once, so you hear a low note. Which is why you can blow out your speakers if your base is cranked up too much. Does that make sense?

Here's a video to help make sense of all these ideas. One of our scientists, Al, is going to demonstrate how to use a signal generator to drive a speaker at different frequencies. We even brought in specialist (with very good hearing!) to detect the full range of sound and used a special microphone during recording, so you should hear the same thing we did during the testing.

Download Student Worksheet & Exercises

How to Build a Speaker

Here's what you need:

  • Plates or cups made of foam, paper or plastic
  • Sheet of copy paper
  • 3 business cards
  • Magnet wire AWG 28, 30 or 32 
  • 2-4 neodymium or similar (rare earth) magnets
  • Index cards or stiff paper
  • Plastic disposable cup
  • Tape
  • Hot glue gun
  • Scissors
  • 1 audio plug or other cable that fits into your stereo / mp3 player
  • 2 alligator clip wires

Now you're ready to make your speakers. Note that these speakers are made from cheap materials and are for demonstration purposes only... they do not have an amplifier, so you'll need to place your ear close to the speaker to detect the sound. DO NOT connect these speakers up to your iPOD or other expensive stereo equipment, as these speakers are very low resistance (less than 2 ohms) and can damage your sound equipment if you're not careful. The best source of music for these speakers is an old boom box with a place to plug in your headphones. We'll show you everything in this video:

Sound waves can affect liquids also! Here’s what happens if you run sound waves through a non-newtonian cornstarch solution:

Exercises 

  1. Does it matter how strong the magnets are?
  2. What else can you use besides a foam plate?
  3. Which works better: a larger or smaller magnet wire coil?
  4. How can you detect magnetic fields?
  5. How does an electromagnet work?
  6. How does your speaker work?
  7. Is a speaker the same as a microphone?
  8. Does the shape and size of the plate matter? What if you use a plastic cup?

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Buzzing Hornets

Monday July 27, 2009

hornet1Sound is everywhere. It can travel through solids, liquids, and gases, but it does so at different speeds. It can rustle through trees at 770 MPH (miles per hour), echo through the ocean at 3,270 MPH, and resonate through solid rock at 8,600 MPH.


Sound is made by things vibrating back and forth, whether it’s a guitar string, drum head, or clarinet. The back and forth motion of an object (like the drum head) creates a sound wave in the air that looks a lot like a ripple in a pond after you throw a rock in. It radiates outward, vibrating it’s neighboring air molecules until they are moving around, too. This chain reaction keeps happening until it reaches your ears, where your “sound detectors” pick up the vibration and works with your brain to turn it into sound.


You can illustrate this principle using a guitar string – when you pluck the string, your ears pick up a sound. If you have extra rubber bands, wrap them around an open shoebox to make a shoebox guitar. You can also cut a hole in the lid (image left) and use wooden pencils to lift the rubber band off the surface of the shoebox.


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Materials:


  • index card
  • rubber band
  • 3′ string
  • small piece of craft foam sheet OR a second index card
  • hot glue and glue sticks
  • tape


Download Student Worksheet & Exercises


Why is this happening? When you sling the hornet around, wind zips over the rubber band and causes it to vibrate like a guitar string… and the sound is focused (slightly) by the card. The card really helps keep the contraption at the correct angle to the wind so it continues to make the sound.


Troubleshooting: Most kids forget to put on the rubber band, as they get so excited about finishing this project that they grab the string and start slinging it around… and wonder why it’s so silent! Make sure they have a fat enough rubber band (about 3.5” x ¼ “ – or larger) or they won’t get a sound.


Variations include: multiple rubber bands, different sizes of rubber bands, and trying it without the index card attached. The Buzzing Hornet works because air zips past the rubber band, making it vibrate, and the sound gets amplified just a bit by the index card.


Exercises 


  1. What effect does changing the length of the string have on the pitch?
  2. What vibrates in this experiment to create sound?
  3. Why do we use an index card?

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Harmonicas

Monday July 27, 2009

Your voice is a vibration, and you can feel it when you place a hand on your throat when you speak. As long as there are molecules around, sound will be traveling though them by smacking into each other.


That’s why if you put an alarm clock inside a glass jar and remove the air, there’s no sound from the clock. There’s nothing to transfer the vibrational energy to – nothing to smack into to transfer the sound. It’s like trying to grab hold of fog – there’s nothing to hold on to.


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Here’s what you need:


  • two tongue depressors
  • three rubber bands, one at least 1/4″ wide
  • paper
  • tape


 
Download Student Worksheet & Exercises


What’s going on? The rubber band vibrates as you blow across the rubber band and you get a great sound. You can change the pitch by sliding the cuffs (this does take practice).


Troubleshooting: This project is really a variation on the Buzzing Hornets, but instead of using wind to vibrate the string, you use your breath. The rubber band still vibrates, and you can change the vibration (pitch) by moving the cuffs closer together or further apart. If the cuffs don’t slide easily, just loosen the rubber bands on the ends. You can also make additional harmonicas with different sizes of rubber bands, or even stack three harmonicas on top of each other to get unusual sounds.


If you can’t get a sound, you may have clamped down too hard on the ends. Release some of the pressure by untwisting the rubber bands on the ends and try again. Also – this one doesn’t work well if you spit too much – wet surfaces keep the rubber band from vibrating.


Exercises


  1. What is sound?
  2. What is energy?
  3. What is moving to make sound energy?

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