I designed this speaker as an easy, inexpensive way for artists to embed sound in their work. I also created it as part of a larger (somewhat quixotic) quest to knit working electronics. The speaker can be made with or without knitting. Instructions for both designs are included.
The Basic Design
Rows of parallel magnet wire, glued between two pieces of paper, carry the audio signal. The wires are positioned over the mid-section of the magnet.
This creates an effective speaker because hard drive magnets are dipolar.
The broad face of the magnet has both a north and south pole.
When the red line on the paper rests on the red line on the magnet, it produces a clearly audible speaker.
Step 1: Build the Speaker
- masking tape
- 32-36 AWG magnet wire
- tracing paper
- cooking parchment paper
- metal leaf spray adhesive
- hard drive magnet
- two 8 ohm 20 Watt resistors
- Class D or T amplifier
Find a jig to wrap your wire around.
Tape one end of the wire down on the jig. Wrap the wire tightly around the jig to create at least 10 parallel rows of wire. Avoid twisting. Once you're done wrapping, tape the other end of the wire down as well.
In the photos, I'm wrapping the wire on a warping board, but a large square of cardboard will work just as well. Place a piece of double sided tape along opposite edges of the cardboard to hold the wire in place as you go.
The total width of the wires should be narrower than the length of your hard drive magnet.
Step 2: Tape and Cut the Wire
Press the wires firmly together as you add low-tack tape on one side. The rows should be as tightly aligned as possible, without overlapping each other: a single layer of parallel wires.
Cut all the wires on one end.
You now have a long "ribbon" of wire.
Step 3: Glue It
Flatten the tracing paper out and tape it in place. Cut a smaller piece of tracing paper to lay on top of the wire.
Spray a generous stripe of glue down the length of paper. Let it dry for 2 minutes.
Press the wire down the stripe.
A silicone wedge works well for this (my fingers got super sticky!).
Lay the smaller piece of tracing paper over the the wire and burnish with the wedge. Press out excess glue.
Place a sheet of parchment paper over it, and top with a heavy, flat weight while it dries.
Once the glue dries, discard the parchment paper.
Solder the wires together on each end. The two ends are your speaker leads.
Step 4: Amplify It
Use a Class D or Class T amplifier to drive your speaker. These amps have overdrive protection. Do not power this with your home stereo-- you could damage the amplifier.
I use a Class T Lepai LP2020-A. If I turn the volume up too much, the audio signal clicks off. To avoid this, the amp needs to power a speaker with more resistance. So I fake that additional resistance with a non-inductive 8 ohm 20 Watt resistor. Two of these resistors in parallel work great. (One resistor also works.)
Once you have everything wired, monitor the temperature of the amp, your speaker, and the resistor. If the amp or speaker get hot, unplug and let it cool down.
Your resistors may get hot-- attaching a large heat sink will help. Otherwise, turn down the volume.
Position the wire between the magnetic poles
Tension the paper
Experiment with both and you'll notice big differences in sound.
Please note: high-end speakers are heavily engineered, bulky objects, costing hundreds, if not thousands, of dollars. This is a scrap of paper with wire glued to it. It's inefficient and it's not going to sound like your home stereo. But.... it's still really cool!
Step 5: Design Your Own
Designing your own speakers from unconventional materials is a great way to explore speaker design and acoustics. It's also a fun way to add sound to art projects.
What follows are tips for adapting the design to your favorite craft, plus an explanation of how and why it works.
The speakers pictured include my experiments with different materials and methods, but all use the same design.
Instructions for the knitted speaker are at the end.
Step 6: Your Materials
The copper wire conducts your audio signal.
- More conductors = louder sound. Use thin gauge wire (32 to 36 AWG) to fit more in.
- Electrically isolate conductors from each other. Use magnet wire (coated with non-conductive enamel) or leave an air gap between conductors.
- Firmly attach the conductor to the paper.
- For this particular design, all the conductors should be wired in parallel.
Choose stiff, light material. The bigger, the better.
Tracing paper works well if it is stretched taut, like a drumhead.
Styrofoam cut from a food container works exceptionally well. You might also try cellophane or clear mylar.
Look for materials that crinkle loudly.
Use a thin coat of glue. Too thick and it will dampen the sound (see image).
Experiment to find the right glue for your speaker materials.
Some glues hold the paper but the wire eventually vibrates loose. 5 minute epoxy held both paper and wire, but was so stiff it dampened all sound. I had great results with spray adhesive for metal leaf (which makes a lot of sense, if you think about it). E6000 tested well, but went on thicker. I also had success with white glue on parchment paper.
Step 7: The Magnet
Place the wires between the North and South poles of a magnet.
This is tricky: most bar magnets have one pole on each side, so only one pole can be pressed against the paper at a time.
However, while experimenting with hard drive magnets, I realized they were dipolar. Each face has two poles. So it's easy to place the wire between the poles.
- Place the paper directly on the magnet so the wire is effectively sitting on the surface, right between the two poles.
- Keep the wire as close to the magnet as possible. Magnetic force decreases sharply with distance.
Step 8: The Magnetic Fields
A clear box of iron filings over the magnet makes the fields visible.
The bar magnet has only one pole on its face.
The hard drive magnet has two poles.
The fields between them form a visible line. This line is the sweet spot for the wires.
*You could also create a strong field by mounting two identical bar magnets side-by-side, with opposite poles facing out.
**If you're experimenting with magnets, buy a $5 bottle of iron filings. A few tablespoons in a plastic case is a great tool to compare magnet strength and locate poles.
Step 9: Magnet Safety
You can scavenge magnets from dead hard drives or on ebay.
Always handle them with care.
Powerful magnets are dangerous. They will jump out of your fingers to stick to another magnet or ferrous metal.
- Wear thick gloves to handle magnets.
- Keep loose magnets separate.
- Store magnets stacked with slips of cardboard between them.
- Mount magnets on a firm surface with hardware -- screws, nails, or upholstery staples. (Hard drive magnets are often mounted on brackets with hardware mounting holes.)
- Don't use hot glue. It's not strong enough, and heat can damage a magnet.
If you're using more than one magnet, cover the faces with layers of tape while you work. Take the tape off after all magnets are securely mounted.
Step 10: How and Why It Works
Electric current running through a copper wire produces an electromagnetic field. If this wire is placed in an existing magnetic field, it experiences physical force.
The directions of the current, the magnetic field, and the physical force are all perpendicular to each other. Fleming’s left-hand rule uses your left hand as a mnemonic for this.
The thumb, forefinger, and middle finger are held to form an x, y, and z axis.
- The first finger is the magnetic field (B), flowing from north (knuckles) to south (fingertip).
- The middle finger is the electric current (I) traveling from positive (knuckles) to negative (fingertip).
- The thumb is the physical force (F), the direction the wire moves.
Step 11: Conventional Speakers
You can see this principle at work in a conventional, dynamic loudspeaker.
The coil of wire (voice coil) fits into a circular slot, the sides of which are magnetic. The outer ring is one pole. The middle piece (pole piece) is the other pole. The magnetic fields (blue arrows) run through the coil (green arrows). The result: the coil pushes out (red arrows), perpendicular to the cone.
When designing your own:
Think of the wire and paper as a drumstick striking a drum head-- you want the wire to move perpendicular to the paper surface.
The paper serves as the speaker's diaphragm. When the wire vibrates the paper, the paper moves air molecules, with audible results.
Step 12: Knitted Version
Instructions assume knowledge of knitting and electronics.
For a standard gauge 4.5mm Japanese machine (i.e.Brother 930 or 940)
T2 (or the tightest tension you're comfortable with).
Cast on waste yarn, needles L15-10, R10-15. You can vary the gap between the two two columns, but leave at least a few inches, to create a wide ladder up the middle.
Switch to main yarn (2-3 wires held together) and knit 3-4 rows.
Load this pattern as a repeated motif:
Set the machine to slip.
Knit til your piece is the length of the distance between your magnet's poles. (An inch should be fine.)
Cast off with waste yarn.
Solder together the inside stitches on both sides of the ladder.
Hang your claw weights on waste yarn.
Tug gently every row to make sure the wire is knitting off. It may help to tug while you move the carriage.
Don't sweat dropped stitches -- this can easily be fixed with solder later.
Go slow and watch your wire tension.
You could do this without slipstitch. I use it because it gives me a double density of wire floats across the ladder, which greatly improves the sound.
*I don't hand knit and I haven't tried this. So... this is a request for hand knitters to try it out, let me know how it works, and improve on these instructions. Also, it doesn't include the slipstitch patterning.
Use 2-3 strands of 36 AWG magnet wire held together. Cast on 2-3 stitches in the tightest gauge you can stand.
Leave a wide ladder, and cast on a second column, also 2-3 stitches. Keep knitting til the length of your piece = the distance between the two poles of your magnet. Cast off.
Very important : Solder the wire columns down each side.
Follow the previous instructions to glue the paper and add a magnet.
Step 13: Building on Ideas
I was inspired to explore the science of speakers by the embroidery of Hannah Perner-Wilson, the papercraft compositions of Jess Rowland, and the knitting of Karla Spiluttini and Piem Wirtz. At the suggestion of my colleague, Dr. Dominique Cheenne, I also looked at commercially-made flat magnetostatic loudspeakers, and discovered the DIY LaFolia Planar Speaker website. If you're interested in flat speakers, all of them are worth checking out.
Sharing this project with other artists has led to interesting discussions about the materials of their chosen media, an intersection of art and science. I'm grateful to venues that create a space for these interactions. For instance, the Craft/Work Project which hosted my speaker workshop last year (pictured). (Thanks to founders Nora Renick-Rhinehart and Rachel Wallis!)
And Chicago makerspace Pumping Station: One, where Laurel Sache Garrett shared her expertise with glue and paper with me.
I first documented this project on my blog last year and continue posting as it evolves.