Chladni Plate

The first step is to tear apart a speaker to get at the good bits. We’ll need the magnet assembly and the former, which is the cylinder that the voice coil wraps around.

Cut through the speaker cone around its outside edge and around the dust cap in the center. Remove the cone.

Cut the outside edge of the “spider,” the flexible ring-shaped fabric membrane under the cone that runs between the former and the speaker’s frame.

Lift the spider and former out of the ring magnet and speaker. At some point you’ll need to snip the braided wire that leads into the coil.

If your speaker coil is deep enough to let you mount an extension and has wire beefy enough to handle some high-voltage driving, you can skip the next step and add the riser (Step 5) to the existing coil. Otherwise it’s a better bet to wind your own coil as follows.

Gently untangle, extract, and unwind all the old wire from the former. Carefully prod the former into good shape. If the former has rough spots, sand them down. If it’s bent, unbend it. If it’s sticky, rub on graphite powder to seal it.

Create a new former by wrapping a tight loop of packing tape around the old former with the sticky side out, so you can slip it off later. It may take several tries to get a clean, smooth cylinder.

Wind a new voice coil onto the tape. Start by taping the end of your magnet wire securely to the spider on both sides, leaving some extra length. Then wind a tight, single-layer coil around the former with 50 to 100 loops. Don’t make multiple layers of wire, or the coil will jam in the magnet or overheat.

When winding a coil, the first loop is the most important, because it sets the standard for the rest. To make winding easier, pull the wire off of its spool tangentially, so it doesn’t kink. To help with this, I used a large Allen wrench in a vise as a spool holder. Spin the former in your hand as you pull the wire and lay it down firmly.

After you’re done winding, spiral the outer end of the wire down the former and tape it to the spider. Cut the wire and strip both ends.

Use squares of aluminum tape to cover the coil and at least ¼” of the packing tape that sticks out underneath, but leave a small gap down 1 side so that the aluminum doesn’t create a conductive ring. Make sure no tape is sticking to the original former.

Use several small squares of tape rather than 1 piece, because aluminum tape is tricky to manipulate, and a single cylinder of tape would fit poorly on the conical-shaped cup later.

Slide your new coil off the former.

Calculate your coil’s resistance by multiplying its length by the wire’s resistance. For example, my coil had a 2″ diameter, 60 windings, and 30-gauge wire whose resistance is about 0.1Ω per foot. This gives (2×π×60) × 0.1/12 = about 3Ω.

The plastic-cup riser makes a strong connection between the coil and the Chladni plate.

Insert the narrow end of a plastic cup into the coil until it stops, and draw a line around it along that edge.

Cut away the wide part of the cup so that it fits through the coil. Use small bits of tape to tack the cup onto the coil, with the wire coil side toward the cup and the empty tape side away from the cup.

Use aluminum tape to tape the cup to the coil from the inside, covering all the wire, ¼” of the packing tape, and at least ¼” of the cup. You can add more aluminum tape to the outside of the cup for strength, but don’t tape very far down past the coil or it may become too thick to slip into the speaker.

This coil/cup connection will take a huge amount of abuse during operation, so make it strong; we don’t want it to slip or buckle.

Use a sharp knife to cut a clean edge through the 3 layers of tape about 1″ beyond the coil. Remove the excess tape.

Cut a ½”-thick styrofoam disk that fits snugly into the bottom of the plastic cup, and use epoxy on its face and edge to glue it in.

The plate’s mounting point is built from a plastic tube, tapped for a screw mount, nested inside a rigid metal tube. Cut the metal and plastic tubing to 3″ and epoxy them together. After the epoxy sets, tap the plastic tube for #10-24 threads

Mark the center-bottom of the cup and drill a ¼” hole. Epoxy the tapped mount into the hole so that it’s straight and nearly flush with the outside of the cup. Your driver assembly is complete.

The stop ring around the voice coil holds the weight of the Chladni plate so that the coil doesn’t sink too far down into the magnet.

Test-fit the driver assembly into the speaker magnet. Find a reasonable position with most of the wire coil inside the magnet, and mark that position on the driver.

Make a plastic ring by cutting another plastic cup ¼” down from its rounded lip.

Fit the ring over the driver assembly with the cut edge pointing up toward the threaded tube, and the lip facing the magnet side and running along the marked line. Cut the ring as needed to fit, and epoxy it into place, holding it with small squares of tape as it cures.

Take the metal sheet and cut a clean square about 24″ on a side. Lay a straightedge across each diagonal pair of corners and draw an X through the center with a thick marker. Use a knife to make a finely scribed X in the dark ink.

Use an awl or nail to dent the precise center of the X, then drill a ¼” (or so) hole through the plate.

(Optional) If you have enough metal, you can mark and cut a disk, a violin shape, or any other shape.

(Optional) Paint the Chladni plate black, and then un-sticky the surface with “the poor man’s Teflon” by rubbing graphite powder into the dried paint.

To find the center of balance of a flat shape, hold any point along the perimeter with a plumb bob hanging down, mark along the plumb line, then rotate the shape and repeat at another perimeter point. Where the lines cross is the center of balance. You can also simply find where the shape balances flat on 1 fingertip.

Using a nylon #10-24 bolt, fasten the plate to the driver.

Connect your signal source to one of your amplifier’s inputs and the speaker wires to an output.

Finally, gently settle the driver into the magnet, down to the stop ring. That’s it!