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.

How to Build a Speaker

Here’s what you need:

• Foam plate (paper and plastic don’t work as well)
• Sheet of copy paper
• Magnet wire AWG 30 or 32 (RS#278-1345)
• 2-4 neodymium or similar (rare earth) magnets
• Disc magnet (1” donut-shaped magnet) (RS#64-1888)
• Index cards or stiff paper
• Plastic disposable cup
• Tape
• Hot glue gun
• Scissors
• 1 audio plug (RS #42-2420) or other cable that fits into your stereo (iPODs and other small devices are not recommended for this project – you need something with built-in amplifier)

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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15 Responses to “Building Speakers”

ok thanks.

2. Look in the shopping list for Unit 14 – they are the neodymium magnets. There should be an order link as well for easy purchase.

I couldn’t find the rare the magnets in my kit. Did you send them, or do I need to buy some? If so, where can I find them?

4. Jennifer Reynolds says:

I did the expirement without the springs and it works.

5. Carey Clark says:

Thanks

6. No – the crystal radio needs an earphone that is non-magnetic since the signal is sooo low power. You can read about the different kinds of speakers in Unit 6.

7. Carey Clark says:

Can I use this use this for the crystal radio?

8. Forgot to mention this earlier – you can attach a plug from Radio Shack and then you’ll be able to plug it into any stereo. Simply attach the end of each wire from your homemade speaker to the end of each wire from the plug and it should work just fine.

Or, if you’re like me and you don’t want to wait to go to the store, attach one alligator clip lead to the end of each of the two wires from the speaker and attach one to the outside metal part of the jack and the other shoved into the center (be gentle so you don’t break anything). That should do the trick. Let me know how it goes!

9. Juliet White says:

The only stereo that’s close to no plug is one that takes plug and battery’s. Will that work?

10. You connect the two wires from the coil to the speaker inputs at the back of the stereo. You’ll need ones that don’t require a plug – the kind where you can simply insert bare wire into the contacts. Flip your stereos around and see if anything looks like it would be a terminal (contact) you can connect bare wire to. If you still have trouble, you can always attach an alligator clip to the bare wire and clip the other end of the alligator clip lead in the hole – just make sure you’ve got a good metal-to-metal contact.

11. Juliet White says:

Question… how do you plug it in to the radio!

12. Yes, that’s right – only it moves so quickly you often don’t see it (but you can feel it). Watch the first video and you’ll see what I mean. The great experiment for a science fair project would be to build several speakers and vary just one aspect of them, like the power of the magnet, or the number of coils, or the size of the speaker, etc.

To show how sound waves move through the air, fill a long tube halfway with water, wrap each of the ends with something flexible like saran wrap (so it resembles a drum head), and stretch a rubber band over one end. Lay the tube on its side (make sure it does not leak!) and then snap the rubber band on the drum head and watch the waves move down the tube on the surface of the water. You can talk about how bats use this principle to “see” at night…

13. Cynthia Shaver says:

I’ve always wondered why speakers have magnets in them, do the magnets help the speakers to move?
And I’m going to do a science project on sound waves, and I can’t decide what experiment really would be the best to use to help explain the topic. Do you have any suggestions?

Rian

14. Look like our server had a hiccup. Go ahead and try again.

15. Anita Mullins says:

Hi,

The videos are not coming up for me; however, the YouTube videos are coming up okay. The videos for Unit 5, Sound, came up just fine for me yesterday. I have tested with new browser sessions under IE8, Chrome, and Firefox.