Sound is a key part of the world we live in, but what you might not know, is the way in which sound can affect the motion of a structure. For any object, there are certain "resonant" frequencies of sound that cause it to vibrate more violently and with a specific pattern compared to other frequencies. In this instructable, we'll repurpose an old speaker to visualize resonant frequencies, shown as patterns in salt.
Step 1: What You'll Need
Speaker: This one got stood on, but it still works.
Utility/Exacto knife: We'll use this to cut away the speaker cone.
Scissors: Optional, also handy for removing the speaker cone.
Pliers: We'll use these to remove the dust cap.
Glue gun: For sticking things together.
Hacksaw: Or even better, a dremmel.
Plastic tube: The base from this will connect to the speaker's voice coil, as a means to attach the plastic plate. Bottle caps or other small disks should work just as well.
CD case: The clear plastic from this will form our vibrating plate, any stiff plastic/metal plate should work.
Amplifier: To drive the speaker once we're done.
Figure 8/Speaker wire: Used to connect to the amplifier.
Salt: You can experiment with other powders, but it seems to work pretty well.
Step 2: Remove the Speaker Cone and Dust Cap
Firstly, using the utility knife, cut around the outside of the speaker cone until it is completely detached from the outer metal frame.
Next, carefully run the knife blade under the dust cap (that's the little shiny dome in the middle) until its all detached from the cone. Be extra careful around the two black plastic pieces stuck to the speaker cone, they contain the wires that make the speaker run, so be sure not to cut them.
Now you can cut away all of the speaker cone EXCEPT the part containing the wires.
Now use the pliers to carefully lift up and remove the dust cap.
Step 3: Cut Away the Speaker Frame
Use the hacksaw (A Dremmel is easier, but I don't have one) to cut away the outside frame of the speaker, cutting at the 4 innermost black lines as shown. Be careful not to cut too close to the speaker itself, the inner cone and voice coil are very fragile.
I then used a grinding stone in a drill press to blunt the areas cut, a file or course sandpaper could also be used.
Step 4: Prepare and Glue the Plastic Disk
This disk will serve as an attachment point for the plastic plate. A bottle cap or some other small disk may also be used.
First, cut the base off of the plastic tube using the hacksaw, and clean up the edges of the cut with some coarse sand paper.
Then using the hot glue gun, apply a liberal amount of hot glue on the inside of the disk, in a circular shape that matches the voice coil of the speaker.
Turn the disk over and press it onto the voice coil, center it as best you can, and hold it down for a few seconds, until the glue has dried.
Step 5: Attach the Plastic Plate
Open the CD case, and simply pull the clear plastic cover away from the rest of the case, it should detach quite easily.
After applying a large drop of hot glue onto the plastic disk from the previous step, place the clear part of the CD case on top, centering it as best you can, and press it down gently for a few seconds, until the glue is dry.
Step 6: Connect It to the Amplifier, and Pour the Salt!
If your amplifier is already connected to speakers like mine, make sure you disconnect all of them first, turn the sub-woofer volume all the way down if it has one built in as well. Now we can connect one end of our speaker cable to our new speaker, and the other end to the Amplifier (in my case, I connected it to wires that another speaker was using).
Now we can pour some salt (or some other powder) on top of the speaker.
Note: I highly recommend placing the speaker on a tray, as much of the salt will fall off the speaker while it is turned on.
Step 7: Play Some Tones!
Using this website to generate your desired frequency, I recommend starting at a ~50Hz Sine wave, using the up/down arrow keys to gradually increase the frequency. start the volume low, and experiment.
You'll find at certain frequencies, the plate will resonate, causing the salt to form interesting patterns, that will become more complex with higher frequencies.
The areas that contain little/no salt, are the ones vibrating the most, as the salt will settle in areas where there is little vibration.