I've been working on a design for simple, efficient speakers that can easily be embedded in textiles. Pictured are two of my working prototypes. The one above is knitted, and uses hand-made paper. The one below is a no-frills version that I'll use to illustrate the design concept here.
[ETA 6/4/14 Here is a video of several speakers in action, taken at a speaker-making workshop I led in March.]
This speaker consists of four pieces of magnet wire, glued between two pieces of paper, positioned precisely over a magnet from a hard drive. The 4 pieces of wire are soldered together at both ends so that they carry audio signal from a small amplifier in parallel. The wires are placed just 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. (Most bar magnets have just one pole per side, and aren't as effective for a flat speaker design.) Additionally, hard drive magnets are extremely strong.
When the wire is placed directly over the boundary between the magnet's two poles (i.e. the red line on the paper rests on the red line on the magnet), it produces a clearly audible speaker.
How and Why It Works
Electric current running through a copper wire produces an
electromagnetic field. If this wire is placed in a magnetic field, it experiences physical force.
The directions of the current, the magnetic field, and the physical force are all perpendicular to each other. A good way to remember this is Fleming’s left-hand rule, which uses your left hand as a mnemonic.
The thumb, forefinger, and middle finger are held perpendicular to each other, forming an x, y, and z axis. The first finger is the magnetic field (B), flowing from north (knuckles) to south (the fingertip). The middle finger is the electric current (I) traveling from positive (the knuckles) to negative (the fingertip). The thumb is the physical force (F), the direction the wire moves.
You can see this principle at work in a conventional speaker:
image: Tony DiMauro
The middle piece is the north pole, and the outer ring is the south. So the magnetic fields run perpendicular through the coil, with the result that it pushes out, in the direction of the cone. Very efficient!
Flattening the Speaker
A coil is great for speakers, but not particularly flat, as it sticks out perpendicular to the resonator.
So I started with a straight length of wire, glued between two pieces of paper. I centered it between the two very powerful poles on the face of the hard drive magnet. I attached a resonator, the paper, so the wire
vibrates the resonator, which vibrates the air much better than a piece of
wire. The result is an audible speaker. (The wire-between-two-magnetic-poles will look familiar to those who know the work of sound artist Alvin Lucier.)
However, one piece of wire doesn’t move the paper very much. To increase the volume, I attached several pieces
of wire, glued parallel to each other across the paper. I also sent the current running through the wires in
parallel (this is very important for increasing volume). Now all the wires vibrate the paper in sync.
My development of this design is on-going, and I am currently refining my knitting machine fabrication techniques. Stay tuned for more documentation.
Hannah Perner-Wilson's Kobakant - resource for e-textiles
Jess Rowland - foil-and-paper speakers using parallel wiring
Dr. Dominique Cheenne - my Columbia College colleague who suggested I look at planar speakers as an alternative model for embedded speakers
LaFolia Loudspeaker Project - a site for diy planar speakers