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:
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
- 3 business cards
- Magnet wire AWG 30 or 32
- 2-4 neodymium or similar (rare earth) magnets
- Disc magnet (1” donut-shaped magnet)
- 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:
Exercises
- Does it matter how strong the magnets are?
- What else can you use besides a foam plate?
- Which works better: a larger or smaller magnet wire coil?
- How can you detect magnetic fields?
- How does an electromagnet work?
- How does your speaker work?
- Is a speaker the same as a microphone?
- Does the shape and size of the plate matter? What if you use a plastic cup?