Copyright © 2014, John Gregg, all rights reserved


Time: 45 minutes
Target Audience: middle school/high school
Table of Contents and Key

Sound, Waves, and Electricity

Can everybody hear me OK? So how does that work? I make sounds with my mouth, they invisibly zap through the air to your ears, and nerves carry a signal to your brain that tells you that I'm talking. But I want to focus on that invisible zapping part.

What is this invisible stuff called sound? Is it made of little particles like bullets, or is it made of rays or something?

(Waves)

We've all been to the beach and seen waves.

sine wave/cresting beach wave on the board
Why do we use the same word to talk about sound and waves at the beach? In what way are they alike? What is another word for a wave?

(Vibration/jiggle)

Both kinds of waves are really the same kind of thing. A wave is really just a vibration or jiggle. Waves at the beach are big and slow compared to sound waves, but a beach wave is just a jiggle in the water. Because the water has a flat surface, you can actually see the upwelling of water as it bulges above the normal surface, like the shape I drew on the board.

Waves generally travel in all directions, in a sphere: you could hear me speaking if you were above or below me, or even behind me.

A sound wave is a jiggle in the air. Sound can travel through water, too, or wood or metal. The sound is the jiggle, and it must always exist in something that can do the jiggling. You take some jello out of the fridge, and shake it, and it jiggles. But you can't have the jiggle without the jello. The stuff that is jiggling (the jello) is called the medium.

The sound itself is the jiggle. We say that sound travels through a medium. Most often for our purposes, air is the medium. So in a way, the sound is really made of the medium.

Does anyone know how fast sound travels?

About 750 MPH, but there is no 750 mile per hour wind in here. The jiggle moves through the medium, but the medium itself stays put.

In the same way, a wave at the beach is not water moving, at least not in the same way the wave itself moves. If you float a candy wrapper on the water, it will ride over the wave as the wave lifts it up and drops it again. In the same way, if you were to spill some ink into the water, and make a cloud of ink, if a wave came along the ink would not be carried along with the wave. The wave would moosh it around a little, but after the wave had passed, the cloud of ink would relax and still pretty much be where it had been before the wave hit it, just like the candy wrapper that rode on top of the wave.

Now think about that: it sounds like it doesn't make any sense. It is like saying that a car moves at 60 MPH, and the car is made of metal, but the metal isn't moving.

Does anyone know what air is made of at the smallest level?

(Atoms, molecules)

Normally, these molecules are floating around, randomly bumping into each other, but if something pushes a lot of them in one direction all at the same time, they push their neighbors, and so on, creating a wave.

Show Newton's cradle.
This is called Newton's cradle. Imagine that the balls are molecules, although in real life they wouldn't all be lined up in a neat row like this.
Set the Newton's cradle going.
Has everyone here been to a ballgame? Ever done the wave?
Make the kids to the wave: slowly sweep your arm across the class to guide the wave.

If one of you had tried to keep up with that wave you just made, they would have had to walk pretty fast. But none of you left your seat - you just stood up and sat down. This is exactly how waves work in real life, all waves. You were the molecules, except that instead of actually crashing into each other, you could just see each other and know when to stand up and sit down. But just like you guys, each molecule pretty much ends up right where it started, and the wave travels forward without carrying the molecules with it. The molecules get scrunched together into a sort of bulge of pressure, then they release by pushing the pressure onto the next bunch of molecules further out.

The important things here are that sound is just jiggles or vibrations in a medium, and that while the wave moves through the medium, the medium itself does not move other than to jiggle.

When I was young, but a little older than you are now, there was a scary movie about a monster loose on a spaceship. This movie's posters had a tag line that said "In space, no one can hear you scream." That is true. Even if there are lots of people around you, in space, no one can hear you scream. Who can tell me why that is?

(No air in space, vacuum, no medium. Nothing to vibrate or jiggle)

Who knows what this is?

Show phono player, plug in with extension cord, play record.
Pass around 45 RPM records.
show freebie magazine insert record and cereal box cardboard record.
How does this work?

There is a single spiral groove on the surface of each side of the record. There are bumps and irregularities in this groove. The player has sharp pin called a needle (sometimes with a tiny diamond at the tip) at the end of this arm here, and the needle rides along this groove and is dragged along its entire length and gets pushed this way and that by these bumps. Now you should guess by now that these bumps match the pattern of jiggles that make up the sound wave in the air that ends up being the music. The bumps jerk the needle around in exactly the same pattern that we want to jerk the air around to make the sound.

Next, some electronics in the arm that holds the needle does some stuff that I won't explain in any detail right now that causes each bump that the needle experiences to create a variation in the electricity on a wire in the player. As the needle is jostled and jiggled by the bumps in the groove, it causes the strength of the electricity in this wire to change, and these changes exactly match the pattern of jiggles that make up the sound wave we want to create at the end. It is important to understand that the electricity passing through the wire is completely silent, and completely still. It is the strength of the electricity on the wire that is changing. So the player has turned mechanical motion into an electrical signal on a wire.

After it passes into the body of the record player, the electrical signal gets amplified, then it is sent to a device that turns the electrical signal on the wires into sound.

What is this device called, that reads the electrical signal and jiggles the air to make the sound?

(speaker)

A speaker is a device that turns silent pulses of electricity into pulses of air, or sound. What if we were inventors, and we had to invent a speaker, a device that would make sound from an electrical signal? Well, since sound is just a jiggle in the air, first we have to jiggle the air. So let's try this - I'll try to jiggle the air and see if I can make a sound.

Spread both hands vertically in front of you, jerk them back and forth quickly.

Now why doesn't this make any sound?

It is too slow, for one thing. The faster the vibration, the higher the tone. The lowest tone the human ear can hear is about 20 vibrations per second. The highest is about 20,000 vibrations per second. I can only jerk my hands like this maybe 5 to 10 pulses a second, so I can't move my hands fast enough to make a sound we can hear. Some things can, however. The buzz of a fly is caused by its wings beating more than 20 vibrations a second, as is the hum of a hummingbird.

So if we were inventors given the job of inventing a speaker, we would need to push a bunch of air around very quickly. My hands aren't good air movers. So why don't I use a more or less flat surface, like a wall, and jerk it around really fast? But in order to jerk it back and forth precisely and very quickly, it better be very light.

What do you suppose they make speakers out of, that is flat and light?

(Paper)

Get out speaker, hold it up.
Now here is where the pattern of wiggles in the electricity is turned into the actual sound wave. The paper cone moves in and out. The electrical signal controls the motion of the cone. Because the movements of the cone match the pulses of the electric signal on the wire, the cone ends up pushing the air around in just the right way to (re)create the desired sound wave, and when it reaches our ears, we hear music.
Get out mutilated boom box, with one speaker disconnected and the other missing entirely, with its wires hanging out through the empty speaker hole, turn it on (dead silence), connect the dangling wires to the speaker and hear the sound come out. Let the wires go and show the sound abruptly stop.
As you look at the speaker, try to figure out how it does what it does - how does it jiggle the air in a way that exactly matches the strength of the electrical signal that comes in on the wire?

Here is a hint.

Take out steel bolts, dangle them from speaker magnet.
Pass speaker around, with warning about its fragility.
What we really want is an electric powered cone jiggler.
cross section of speaker on the board: cone, empty square on the bottom with two wires coming out, with a '?' in the middle of it.
Imagine that there is a little room at the base of the cone, and inside that room is a tiny elf with one hand on the tip of the paper cone, and this elf watches a needle that shows the level of electricity on the wire at each moment. When there is a lot of electricity, he pushes the cone hard, when there is a little electricity, he pushes gently, and when there is no electricity, he doesn't push at all. Now that's silly - there's no elf. But keep that image in mind, and try to figure out what there really is in there that does that elf's job.

We know that its a magnetic elf - there is a big magnet in here. Now, you've all played with magnets.

What happens when you put two magnets together?

Get out ceramic magnets and click them together.

They attract.

But what happens when you turn one of the magnets around and try to put them together?

Pull ceramic magnets apart, turn one around, and try to squish them together
They repel. They hate that. They really do not want to be anywhere near each other. Now keep that image in the back of your mind, next to the picture of the elf.

There is one missing piece of the puzzle. Does anybody have any guess as to what an electromagnet is?

It turns out that the forces in nature of electricity and magnetism are very closely related, and can easily be converted to each other. If you take a wire, and loop it around in a spiral then pass electricity through it, it will become a magnet.

coil
Demonstrate: nail and wire electromagnet attached to flashlight battery with one end of the wire taped to one end of the battery with electrical tape. Pick up paper clips, let go of one of the wires, see the paper clip drop. Pass the whole kit around.
Perhaps you have seen videos of those huge cranes at junk yards that can lift an entire car and drop it again. They are electromagnets.

If you pass a lot of electricity through the coil of wire, the magnet is strong, but if you pass only a little electricity through it, the magnet is weak.

If you put an electromagnet near a regular magnet in such a way that they repel each other, the electromagnet will push hard against the regular magnet if it is given a lot of electricity, and it will push gently against the regular magnet if it is given a little bit of electricity, and it won't push against the regular magnet at all if it is given no electricity. So now is the picture starting to come together of how a speaker works?

There is coil of very thin copper wire around part of the regular magnet in the base of the speaker. The paper cone is glued to the electromagnet. With no electricity, the cone just sits there. But if you pass a bunch of electricity through the wire, it turns on the electromagnet, and pushes against (repels) the regular magnet, pushing the cone outward. If there is a lot of electricity, the cone gets pushed a lot, if there is only a little electricity, the cone is only pushed a little. If there is a pattern of strength and weakness in the electricity, the paper cone will move, and push air, in that exact same pattern, making sound. So the elf in my incorrect little story just is an electromagnet.

Erase the '?' in the box in the picture on the board, and replace it with a picture of a coil connecting the ends of the wire. Color in a part at the bottom of the square and call it the regular magnet. Show the coil positioned above the regular magnet, and draw in the bottom of the cone attached to the top of the coil.
Does everybody understand this? Does everybody believe it?
Get a volunteer - get them to take apart the speaker and unspool the copper wire.
OK, now here's one more noise maker.
Pull out See And Say (old - not battery powered!)
Is there anyone here who does not recognize this? A few years ago I was at my Dad's house with my brother, and we were going through a box of old toys that had been ours when we were kids, 30 years before. We pulled one of these out, and it worked just fine. Then we started thinking about it. There is no plug, and no door for batteries. And no batteries would still work after being in a box for 30 years anyway. How does this work?
Show old ad with Nipper the dog: "His Master's Voice" .
Does anyone know who invented the record player? Thomas Edison invented the record player, and they became very popular in American homes over a hundred years ago, and those record players were not electric. Instead of the needle being jiggled and creating an electrical signal on a wire that was sent to a speaker, the speaker was in direct contact with the needle. So the jiggles of the needle immediately jiggled the speaker. It didn't work all that well by today's standards, and you could not turn it up and make it louder. This See And Say works on exactly the same principle.
Take apart the See And Say, show the insides, and pass it around.
This is the end of the presentation, and I will take questions while the insides of the See and Say are being passed around.

Equipment List

Key Words

Links


How to make a speaker from scratch
Different how to make a speaker from scratch
Retro mechanical phonograph
Relevant Calvin and Hobbes cartoon
Toy car/record player