Introduction: Electromechanical Transducer Out of a Polystyrene Conical Section!

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About: I am an enthusiastic scientist and amateur engineer sharing my passions with students for the past thirteen years. I believe that the challenges ahead are never as great as the science within and work to enab…

"A what?" you ask. An "electromechanical transducer" refers to the type of speakers we are most familiar with; a permanent magnet and an electromagnet wildly vibrating to produce sound. And by "polystyrene conical section" I mean plastic cup.

Whatever this is, it is not an Instructable on how to callously rip apart your roommate's computer speaker and glue the driver into some other object. I show how to build the actual transducer unit (commonly called a speaker driver) with a few simple objects. The speaker is super easy, extremely impressive, and so cool that it even makes Kenny G. sound good.

If you are abhorred by reading, feel free to cut to the meat of the how to on step 3. But the theory I present in the first few pages may help you build a better speaker, and... (dramatic pause)... may even make you smarter (Egad!)

There are a couple risks (other than learning) so please read the Safety Page.

Step 1: Theory: What Is Sound

The first concept to wrap your rubbery little mind around is the idea of sound. Sound isn't an object. Your boom box isn't firing little particles of magic sound dust to tickle your ears with M.C. Hammer. Instead, sound is the transfer of energy.

A source (such as the speaker on you boom box) is receiving electrical energy and converting it into mechanical energy. If you'll kindly place your fingers against your throat and scream the phrase, "someone's already made a movie about a giant singing plant," you'll feel that mechanical energy in the form of vibrations. You'll also have noticed those vibrations when you stand really close to a drum set or those cheap speakers your ex-girlfriend blasts Smash Mouth on.

That mechanical vibration acts like a piston pushing particles forward when it moves outward and pulling particles backwards when it pulls in. Like I said, sound isn't an object; it is a transfer of energy. Those particles are not hurling towards your ears. The first particle touches the next particle and moves it a bit. That particle moves the next particle a little bit, and so on until that movement, that energy, reaches your ear. How fast those particles transfer energy (the speed of sound) is determined by what type of particle it is. In air, sound moves at 343 meters per second. In your secret underwater sea lab it moves at 1533 meters per second (I won't tell anyone).

I know you implicitly understand this, because you're super smart, but small sources move a small number of particles and big sources move a big number of particles. If the mechanical vibration is small (if the piston only moves a short distance), it doesn't transfer much energy to the particles so the sound is small. If your speaker is really athump'n (the piston moves a large distance) it is transferring large amounts of energy and it produces large sound.

One last note on the concept of sound, we say sound is a wave. But it isn't one of those up and down waves like a jump rope or those sine graphs your algebra teacher makes you draw. It is a back and forth sort of wave featuring a series of particles pressed really close together and particles spread far apart. If you stretch a good slinky out on the ground and give it a push (a push not a wiggle! a push I said!) you'll see another example of this type of wave.

Step 2: Theory: Converting Electrical Energy to Mechanical

Signal sources: 8-track player, cassette player, AM radio, mp3 player, what have you (with possibly the exception of a record player) all work on the same principle. They read a code and send out impulses of electricity, the electrical impulse transfers energy through wires to an electromagnetical transducer (speaker driver) and sound is produced. It's like ants in an anthill. The anthill is the signal source sending ants (electricity) out to a picnic (the speaker). We won't concern ourselves with the politics of anthills or explaining exactly the movement of ants. We just have to answer two questions to build a good speaker: How many ants reach the picnic in certain amount of time? And what are the ants doing at the picnic?

How many ants reach the picnic in a certain amount of time is different than asking how fast the ants go. Ants basically go just one speed. What I'm referring to is how close together the ants are. Did they come out of the anthill one right after the other? Or did they wait a couple seconds between each ant? This refers to the frequency of ants. If the ants are frequent visitors (one right after another) to our picnic (speaker) the sound produced will be a high frequency sound (high pitched) like the squeal of teenage girls... the kind of noise that shatters glass and ear drums alike. If the ants don't go by very often they are said to be a low frequency and the sound they produce is a low thumping base.

Frequency is extremely important in designing speakers. Some materials and sizes are just better for producing different sounds. You'll notice speakers that produce low sounds (sub woofers) are really big, while high sounds are made by little speakers. This Instructable only describes one size of speaker that is going to be doing its best to produce all frequencies of sound... but a better system can be made when the electrical impulses (ants) are filtered so that the low sounds go to a big speaker and the high sounds are directed to a small speaker.

Now what's happening at our picnic? Ignore the young couple that are rolling around and just focus on the ants. They are picking up bits of food right? In speaker terms the electrical impulses are producing magnetic impulses. Part of the speaker is becoming an electromagnet in a certain frequency determined by the frequency of the ants.

Holy Lorenz force Batman! How does electricity produce a magnet? Electricity and magnetism are closely related. In fact, if you spin magnets around something that conducts electricity (such as a bit of copper wire) you can produce electricity... but you knew that... you're smart, it's called a generator. The reverse is also true. If you make electricity spin in a circle (by wrapping wire into a tight round coil) it produces a magnetic field.

The signal source is reading a code and sending electrical impulses at a frequency. The electrical impulses travel down a wire to a coil of wires where it produces a magnetic field that is changing at the same frequency. To produce mechanical energy we now simply move a permanent magnet near our electromagnet. As the electromagnet turns on and off, it will be moving the permanent magnet back and forth. Back and forth, by definition is mechanical energy. If these magnets are glued down to something like the bottom of a cup, the cup bottom will be moving at the frequency sent by the signal source. You will feel the cup bottom vibrate and sound will be produced. Yeah baby!

Step 3: Materials

Be sure to read the end of this section where I explain alternatives and where to obtain these items.

Items for the Speaker

1 Plastic cup
4 5/16" round x 1/8" thick disk neodymium magnets
40 inches of 16 gauge enamled copper wire
Super glue (thick "gel" type works best)
Tape
Signal source with audio wire

Tools

Wire snips or heavy scissors to cut the wire
Sand paper or a sharp edge
Something pointy
AA battery (or a round object of similar thickness)

A good hook up to a signal source may be the most difficult item to obtain. If you're careful you can strip the wires out of old head phones so that your speaker can be plugged into your iPod. You can buy speaker wires that have a plug on the end and are bare on the other to plug into a radio. I used the bared ends of sound wire running out from an old TV. They don't need to be soldered to your speaker (unless you want to) as long as they are bare and you can twist/hold/tape to make a good connection.

Just about any size of plastic cup will work. And it doesn't necessarily have to be plastic. Real speakers use paper, silk, composites, etc. Experiment with paper plates, ice cream containers, Styrofoam cups... anything that is flexible and has a slight cup shape to magnify the sound.

The magnets don't have to be exactly 5/16" round or 1/8" thick. I used 8 5/16" round x 1/16" thick ring magnets. Just be sure that they are a good, powerful magnet that is smaller in diameter than the AA battery.

Enamel wire, also called magnet wire, is copper wire that is coated with a thin layer to prevent it from shorting. Buy it or strip it out of an old speaker for free. It doesn't have to be exactly 16 gauge... just a nice size to work with.

Step 4: Safety

Super glue can cause skin irritations. Take care when using it. If it comes in contact with your skin be sure to flush with water. If you have a known allergy to super glue, try an alternative such as small dabs of hot glue or simply using tape.

Rare earth magnets are extremely powerful! And they can wreck electronic things like your favorite mp3 player. Be careful where you place the magnets (near your digital camera... a big no no) and don't let them snap together to quickly. They may break or pinch fingers.

Shock Hazard Never attach your speaker to the signal source while it is powered on. Never touch the bare connections while the power is on.

This involves some sharp tools to cut wires and poke holes. Never hold the point or edge toward your body when making holes.

Step 5: Voice Coil

Use wire snips to cut a 40 inch length of 16 gauge copper wire. Leaving a 5 inch tail, wrap the wire around a AA battery (or similar sized object). Make 14 to 16 wraps total. It is important to make as tight and neat of coil as possible.

Tip - Wire crinkly, bent and hard to work with? Pull the wire tight with both hands and run gently over a sharp edge to straighten.

Technical Terms - This coil will serve as our electromagnet. In speaker terms it is called a voice coil.

Step 6: Secure the Coil

Carefully slide the coil off the battery and secure with a couple small pieces of tape.

Very important step To get a good connection between the speaker wire and speaker, the enamel insulation has to be removed from the two tail ends of the coil. With a piece of sandpaper or the edge of a shape knife, gently scrape the coating off the tail bits of wire on the coil

Step 7: Coil to the Cup

Use something pointy, such as a paper clip, to poke a small hole near the base of the cup. Set your coil in the cup and slide the wire tails through the hole.

Squeeze the super glue into a small circle in the center of the cup. Press the coil onto the glue and hold for ten seconds.

Split your magnets into two groups. Hold one group against the outside of the cup right under the center of the coil. Toss the second group in the cup so they attach in the center of the coil to the magnets on the outside.

Step 8: Finish Up

A piece of tape will hold your speaker in place. With the power off, attach the signal source to the speaker by taping or twisting. Be sure that the two wires do not touch each other at the bare connections.

Power up and rock on.

For further experimentation try different size cups, better glue, different materials, larger magnets, and different connections. This is an ugly utilitarian build just to show off the basic construction principles. But go ahead and knock yourself out making it good looking. Build a iPod speaker that looks like an old phonograph, build a giant sub woofer, or build a whole home theater system using decorated cardboard boxes for speaker cases. Go nuts you mad scientist you. Best of luck!