Embedded Speakers

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.

image: Jfmelero

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 coil of wire (“voice coil”) fits into a circular slot, the sides of which are a magnet.

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

Karla Spiluttini and Piem Wirtz at V.2 - a previous knitted-speaker project

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

Magnepan - manufacturers of the first planar speaker, Magneplanar, invented in 1969 by Jim Winey
4 responses
Very cool. Why are dipolar magnets more effective? Is it that they conserve momentum by making the wire/paper flex like a sine wave instead of just moving to and fro? Is it better to have stiff or limp paper? Foil? So many questions!
Great questions! My knowledge is more practical than theoretical but I'll attempt an answer. Magnetic fields run between North and South poles, and there's a sharp drop-off in strength with distance from the magnet. So the ideal position of the wire is evenly suspended between two very close poles (such as with the conventional speaker design.) You could do this with wire-in-paper if two magnets were also embedded in the paper (or projecting into it) with an opposite pole on either side of the wire. However, for fabrication reasons, I wanted to explore placing the magnet behind the resonator. So I arrived at this design: with the thin paper placed on top of the dipolar magnet, the embedded wire is situated in a strong field between the two close poles. Placing a wire across the face of a bar magnet that only has a North pole on the face does not place the wire within a strong magnetic field as the corresponding South pole is "far away" on the back side of the bar facing the opposite direction. (At least, that's my understanding. Any physicists reading this are invited to elaborate...) By the way, if you don't have a hard drive magnet, you can create a dipolar surface by gluing two bar magnets to a solid surface side by side, one with North facing up and the other with South. In answer to your other question... resonators reproduce sound most accurately when they're stiff and lightweight. A heavy resonator is hard for the wire to move. A limp resonator will absorb the movement instead of transferring the energy to air molecules as sound. (Think of hitting a pillow vs. hitting a golf ball.)
How thin versus how stiff is that handmade paper resonator? I would expect that stiffness would be paramount.
Nice work, Jesse! Thanks for all of the references as well.