Mechanical Wave Driver for Chladni Plate

I volunteered to help a friends child with her science fair which we decided to do on sound. After seeing the chladni plate displays on YouTube I thought this would be a great display for the project. I assumed you could drive the plate with just a simple speaker but the lower frequences just bounced the salt right off while the higher had to be unbearable loud to even get the salt to move. Overall a regular speaker produced limited results for the loudness you had to deal with.
A lot of people use a piezo but I found this to be to small to produce a good effect. I ran across a website Cymatics which had a link to a wave driver you could buy and after looking it over I realized it was just a modified speaker which could be easily made for cheap. This is my first instructable.

Step 1: Materials and Tools and Misc

Materials I used:
A discarded 8" speaker with its coil intact
small sink strainer
cardboard (an old cereal box)
heavy canvas (optional)
8-32 X 6" threaded rod (1)
3/8 X .171 X 1 Nylon spacer (1)
1/4" plexiglass
8-32 Insert Lock Nuts (4)
Standoffs (4)
Metals spacers (4)
8-32 screws (8)
9 1/2" X 9 1/2" metal plate (cut from old VCR casing)

Tools:
Safety Goggles
Gloves
Nut driver
Wrench
Vise-Grips
Hot-glue Gun
Razor Blade
Scissors
Sabre Saw with Sheet Metal Blade

WARNING
This project involves cutting metal and plexiglass with a sabre saw, which can be dangerous.
Plexiglass is not easy to work with at this thickness. It cracks, shatters, and melts. You might want to use wood or metal instead of plexiglass.

Step 2: The Old Speaker

I started with an old speaker I found in the trash about two years ago. The cone was blown out and torn but the coil and wires were still intact. You may be able to use the stiff fabric used to support the coil. I decided to go with cardboard hoping it would give more support and a broader range of frequencies. I cut away all the old paper, cut the braided wires attached to the terminals, cut away the stiff fabric used to support the coil, and pulled the coil up out of the slot it fits in Be careful not to tear the little wires connecting the coil to braided wire going to the terminals. Leave a small ring of paper around the coil form.
Measure the bottom inside of the speaker. Use this measurement to cut a circle of cardboard out the same diameter as the lip running around bottom inside of speaker. Center the coil form on the circle of cardboard and trace around with pencil. Then use a razor blade to cut out inside circle. I also cut out a circle of canvas to sit on top of cardboard. which is really not needed but looks better. You should have a cardboard doughnut now.
There should be a lip with some paper still attached around the coil form. Insert the coil into the canvas and the cardboard and then push them flush against the lip of the coil form. Hot glue into place. Then set the coil form with attached cardboard back into slot. Check to make sure coil does not bind in slot, it should move with a bit of pressure. The cardboard sits on lip of speaker frame. Hot glue outside of circle onto frame.

Step 3: Support Frame and Plexiglass

The support frame and plexiglass provides a slot for the vibrating rod to ride in. I used 4 standoffs with 4 metal spacers to mount a plexiglass circle about 4" above top of speaker. Metal or wood are easier to cut though. WAIT to drill the outside mounting holes on plexiglass.
Flip the speaker over face down on the plexiglass and trace around with marker. My speaker had mounting holes around the outer edge so I used those to mark out the center of the circle. Just mark holes then use a ruler to draw X through circle connecting the hole marks.
A sabre saw with a metal cutting blade cuts plexiglass easy enough the only problem is melt-back. As soon as you cut past a place it melts back together! So cut around your circle leaving a groove of melted plexi. Then fill this groove with metal cutting oil and go back around very slowly adding oil when necessary. (If anyone has found an easier way please tell me!)
Drill a 3/8" hole in exact center of plexiglass disc. I used a bolt to chuck the disc in a drill and then clamped the drill using it as mini lathe to grind/sand down the plexi edges smooth.

Step 4: Vibrating Rod

Back to the coil form. I used a sink strainer as the support cone for the rod. It fit perfecty on top of the lip of the coil. Pull the little knob in the middle of the strainer out with a pair of pliers while trying not to bend up the strainer. Use the hole left to insert your threaded rod. You only want the very bottom of the rod inside the strainer. Attach the rod with a lock nut on each side of strainer. Use vise grips to hold rod in place. Make sure the threaded rod is setting straight up/down. Should look like little bell.
Now place strainer on top of coil form in speaker. Make sure the metal is not shorting speaker wires. Use hot glue to level out and then hot glue into place.

Step 5: Mounting Plexiglass, Testing

Place a 3/8 X .171 X 1" nylon spacer in the hole in the middle of the plexiglass. Mine holds just fine with compression but you may need to use a little superglue. With the standoffs and spacers mounted on the outer rim of speaker, gently place the plexiglass disc over the threaded rod. It should slide comfortably over in the nylon spacer. Make sure the rod is sitting straight! Press gently down on the cardboard to make sure the rod can move thru the nylon guide. Mark your screw holes on the disc above the standoffs then remove to drill out. I would drill one hole at a time then put the disc back on to be sure of alignment. After all holes are drilled, screw plexiglass into place. Put just a small dab of oil where rod slides thru nylon guide.
Now would be a good time to test on a Function Generator with an amplifier. I use an old 30W amp but will probably make a smaller one for the science fair.
The rod should vibrate freely in the guide.

Step 6: Chladni Plate

For the vibrating plate I used an old VCR housing. Just measure out the dimensions and cut with a sabre saw. Find the center by drawing line through oppisate corners and drill hole to fit rod. Holding the rod gently in place, use locking nuts on top and bottom of plate to mount on rod. Be careful not to let rod turn and pull strainer off coil!
Most people use salt or sand to create the different figures. I have found that frequencies between 800 to 2000 hz work really well. Go slow on the dial to find the sweet spots and you will have to play with amplitude a bit to, but its not that hard.
This picture is just below 1000 hz.