Sunday, January 4, 2015

Chladni plate

Yet another project to start while others aren't finished!

I'm looking into what it takes to make a Chladni plate.  Toni posted something to FB with bouncing salt, and then Corey and I struck up a conversation considering building one.

Basics of a Chladni plate: back in Ye Olde days, a guy named Chladni messed around with harmonic vibrations of a metal plate, and saw that it made neat patterns.  In those days, he used a violin bow or similar, pulled across the edge of the plate at specific points.  Nowadays, people use electronic vibration generators or wave generators connected to a piece of sheet metal.

Various web sites are out there on instructables and the like.

The link here:
http://www.instructables.com/id/How-to-make-a-Chladi-plate-vibrating-membrane/
shows a way to hook up a speaker, connect it through a "cone" that traps the audio, and then put a membrane over the cone.  So for them, it's kind of like having a speaker connected to the air chamber of a drum, and the drum skin is where the particles bounce around.  That's not quite the same thing as having a plate atop a post, where the vibration energy is focused at one point.
but that's pretty complex, IMHO.  Basically, it it saying "kill a speaker, unwind its magnetic wire, and then wind it up again" which basically turns the speaker into a different magnetic coil.  I couldn't quite grok what the new electromagnet would be driving, though.  The pictures weren't that clear.
This one is more informative, but doesn't have great pictures.
but that's where the project hits at $250 cost.  Not in my budget!There are also some devices one can buy for a pretty penny.  The basis of the machine is the vibration generator, and those will run you $200 or more.  Then you can mount square or circular plates above them for $40 or $50 more.
That would go along with one of these, I think: a vibration generator:
http://www.djb.co.uk/ppm_vibgen.html
There are also devices that can be ordered from Pasco.

So off we go, exploring a DIY approach to this, and perhaps providing better documentation than what I've seen on other sites.

The DIY approach to this is to use an old speaker as the wave generator.  Question one then is: what is a speaker, and why can it work as the wave generator?  I found this link to be informative:
http://www.bcae1.com/speaker.htm

Here are the two speakers I have that could be victims for this project.  The first is a 6" coaxial speaker meant for use in a car stereo system.

This is the speaker with grille assembly:





Looking closer, you can see the tweeter is on a post that goes through the center hole of the woofer's cone.

The main signal wires connect through the woofer cone loosely, and the connection holes are sealed, allowing it to push air with little interference from the wires.  The wires end up connecting through a capacitor, which I assume is used to allow only the high frequency signals to hit the tweeter.



You'll find that you can push gently on the woofer cone, and it springs back.
Push down:
 Lift up:

This movement of the cone is caused by an electromagnet being attracted to or repelled by the speaker's circular magnet at the base.  It springs back into a resting position because of the "spider" which in this case is a webbing material connected to the base of the cone.  The spider is the brown/orange material seen here:

The voice coil and VC Former should be inside there, attached to the spider, and together they move up and down, pushing the cone.

The DIY Chladni plate pages say to cut away the cone, and at least one page says you can cut away the spider, though if you do, I don't quite see how the voice coil + VC Former would go back to a proper resting position.

After they take away the cone (and maybe spider), they attach something like a short tube or cap where the cone had been connected. But if you do that with this coaxial speaker, you'll run into tweeter.

Instead, let's look at another speaker.  This is from an M-Audio studio speaker system that I found in e-Waste.  It has its own amplifier, which I'm thinking I'll reuse.  There were two problems with the speaker set.  First, its tweeter dome was crushed in.  Second, its volume knob wasn't well constructed and caused an awful, loud scratchy sound as it would be adjusted.  (As it turns out it's a ganged A502 5kOhm potentiometer, so I can probably fix that.)  The picture here shows the volume knob and headphone output circuit removed.


After some disassembly, which included some nasty approaches to removing glued wires, it looks like this.  Word of warning for anyone messing around similarly: this has exposed wiring to its internal 115v transformer.  I'm being safe with this, only working on it unplugged.
The woofer on this one is a plain woofer with no center tweeter post.  (Audio pages will tell you that having separate speakers provides better audio fidelity.)

Here's a picture of the spider in this speaker

and here's a side-by-side comparison of the two speakers.  It's interesting that the M-Audio one has a deeper can, and they're both about the same when comparing the can diameters.



05-JAN-2015
I viewed this video last night:
https://www.youtube.com/watch?v=-Jvosadq2ao
It's a person creating a "vibration speaker".  Basically, he takes a coaxial speaker, turns it upside-down, and then removes the tweeter and puts the tweeter back on top (facing upward).  Then, he replaces the stock woofer cone with a post-and-flange assembly that allows the woofer's voice coil to convey the vibration energy to whatever's underneath.  He puts the whole thing atop a hollow box, which then serves as a cavity for capturing and reflecting the air pressure waves.

There are a several interesting parts about the video.  First, it's relevant.  In essence, he's doing the same thing as what we're trying to do here, connecting a voice coil to a post.

Second, it is interesting that you can remove the tweeter and mess with it, but for us it's not entirely relevant.  It's likely we will not use a coaxial speaker, instead using an old, blown component speaker (or two).

Third, the assembly process is interesting.  He constructs a plexiglass mount point for the rod and flange first, and joins that to the speaker's normal mount points.  He then adds the rod, and glues it down to the cone, which allows the inner part of the cone to act as a bowl where the glue hardens.  Only after the glue is hardened does he remove the cone.  I think it's arguable that that makes things easier, compared to attaching a cylinder to the voice coil assembly, and it's also arguable that it reduces the overall mass that the coil is trying to push.

Yet another similar link for a DIY "Bass Shaker" (which is different than the topper of a Fish Tales pinball machine) but if you watch this, the informative part starts after about 1 min 42 secs into the video.  Before that, it's just silly.
https://www.youtube.com/watch?v=m1GS0uIFVfE

I also love this youtube video both for linguistic reasons and for the high def animations:
https://www.youtube.com/watch?v=DIBDeC3G3_4


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