Welcome to Fun with Less Kilowatts! We believe that science experiments at home can be a creative way to engage kids in learning while having fun. They can be educational AND great activities to keep your kids busy and away from the television. Each month, we’ll feature a new science experiment that can be a great resource for parents and teachers.

Build a Bottle Speaker

Transducers convert physical energy movement directly into electrical energy. A loudspeaker converts electrical current into vibrations that produce intelligible sounds. The first moving-Coil transducer was invented by Ernst W. Siemens in 1874. A coil of wire was supported in a magnetic field so that it moved when electricity ran through the coil. While Siemens didn’t invent it for sound, the design was used by Alexander G. Bell who added a paper cone to the design so that it could pick up and emit sound for his telephone. Siemens modified the design further and before you knew it, the rush to build better speakers was on. Eventually, the dynamic loudspeaker was developed by Edward W. Kellogg and Chester W. Rice in 1925 and has been in use ever since.

The Basic Speaker

Basically, a dynamic speaker consists of a Voice Coil, magnet, cone, frame, and a spider. The voice coil is a coil of wire formed around a Mylar-covered tube. The ends of this coil connect to the audio amplifier. For the coil to vibrate, it needs to be kept within a magnetic field, which is why speakers have big, powerful magnets attached to them at the back. For the vibrations to be heard, the voice coil is usually glued to a cone of heavy paper, though plastic is also now common. But to keep the voice coil suspended in the magnetic field, the voice coil is attached to stiff folded fabric that provides a spring-like suspension and attaches to the lower part of the speaker frame (or basket) to pull the voice coil back. The top edge of the cone is glued to the top of the frame to pull the voice coil up. In this way, the voice coil is kept suspended in the magnetic field so that it will produce the most sound. Most loudspeaker magnets are designed with a circular slot just big enough for the edge of the voice coil to move in and out of.

As any electric guitarist (like me) will tell you, shredding at max volume is screaming good fun —until your speaker “crunches” and dies. Speakers essentially fail in two ways:

• Too much complex vibration (that is, it’s really, really loud and very distorted) that makes the voice coil tear itself apart. If you push against the cone gently and it doesn’t easily move back and forth or it makes scraping noises, then this is what has happened.
• Too much electrical current so that the coil and magnet overheats. The very fine wire leads of the voice coil get so hot that vibration easily snaps them or sometimes the wires just melt.

Build Your Own Speaker

Building your own speaker is really easy. And if you want to build a standard paper/plastic cup version, just follow these easy directions.

But as I was thinking about this experiment, I got to wondering if I could make a Bottle into a speaker. After all, bottles are hollow and they vibrate. Why not give that a try?

As it turns out, you can actually make a wine bottle or a 2 liter soda bottle into a working speaker in less than 10 minutes! Here’s how it goes:

The Materials

• A bottle— I first made a bottle speaker using an empty wine bottle but a really good choice is a plastic 2 liter bottle.
• Coil wire — About 5 feet of 22-24 AWG enameled magnet wire.
• Three 1/2” or 3/4” rare earth magnets. These (and the wire) are available at local hardware stores for under $10. WARNING—These are EXTREMELY powerful little magnets. Keep them away from credit cards and cell phones as their magnetic fields can damage this stuff.
• A cork and some super glue.
• A pair of alligator wire leads— to connect from coil leads to the output terminals on your audio amplifier.
• An audio amplifier— You’ll need something fairly powerful — 75 watts or better.

The Directions

1. Sand off about 1/2 inch of the enamel coating from both ends of the wire. Next, you’re going to wind the wire tightly around the bottle neck below the collar or finish. Leave 2 inches of one end of the wire sticking straight up and use a little tape to hold the wire in place vertically against the bottle. Now, tightly coil the wire around the bottle neck below the finish part of the bottleneck, being careful not to overlap each pass of the wire. You want to have nice rows of wire in your coil. When you get to the end, leave 2 more inches sticking out and use some tape to hold the wire in place.

1. Test fit the cork to make sure it fits snuggly into the bottle. Next, put a little superglue in the center of one end of the cork. Put your stack of rare earth magnets onto the glue. Let the glue set for about 10 minutes.
2. Insert the cork with the magnet side down into the bottle. You’ll want to be able to push the cork in far enough so that the magnets will line up with the bottom edge of the coil.

1. Connect your alligator leads to the two coil leads and the power output on your audio amplifier.
2. Turn on the amplifier and tune in a radio station.

The Result

You may have to turn the volume up pretty high but you should hear sound coming from the bottle!

The Science

We already know from our motor project that when you put electrical energy though a coil, it turns into a magnet. The electricity coming from the amplifier follows the wave form of the sound it’s carrying. That means, as the wave has peaks and valleys, the electricity changes polarity— causing the voice coil to act as an electromagnet that changes polarity. Since the the three rare earth magnets have a fixed polarity, they attract the voice coil when it has an opposite charge and repels the voice coil when it has the same charge. This push-pull causes the voice coil to vibrate. Vibration is transferred from the voice coil to the cone.

In the case with our bottles, the vibration is transferred into the glass or plastic. The great thing about this is that you can touch the coil while it’s operating and feel it vibrating. You’ll also notice that it gets a little warm from the amount of electricity moving through it.

A note on performance—

Though it is strange and marvelous to see a 2 liter bottle radiating Mozart or Beyoncé, don’t expect great audio quality. The vibration of the glass wine bottle puts out a higher frequency sound. The 2 liter bottle’s plastic produces a lower, mid-range sound which is much easier to hear. While the rare earth magnets worked well, electrical resistance on both bottle speakers was .1 ohms, so I believe a much larger magnet would produce much higher volume level for less power. Feel free to experiment and see how far you can push it!

Do you have any fun and kid-friendly science experiments you’d like to see us try for Fun with Less Kilowatts? Share with us in the comments!