Chladniplate

For our final project of the course Design Enginering for Fysici (BSc Applied Physics, TU Delft) we decided to build a Chladni plate. The main requirement was that a high school teacher could replicate it together with his/her students.

So how does a Chladni plate work? By vibrating the plate at certain frequencies, the plate starts to resonate. The frequencies at which the plate starts to resonate are called the natural frequencies. At these frequencies a pattern of nodes and anti-nodes is formed. By sprinkling salt on the plate, this pattern becomes visual because the salt piles up at the anti-nodes.

To build the Chladni plate you will need the following materials:

• Speaker;
• Function generator;
• Aluminium plate (1mm thick, as big as possible);
• Fine sand or salt;
• Cables (To attach the speaker to the generator);
• Endcap to attach the conus of the speaker to the plate;
• 5 M6 rods about 20cm long;
• M6 bolts, 26 pc;
• M6 lockwashers, 26 pc;
• Wooden plate as base;
• Lexan sheet.

And the following tools:

• Clamps;
• Metal saw;
• Scissors;
• Wrench for M6 nuts;
• File;
• Drill;
• Spirit level.

Before assembling the setup, some of the materials need to be cut into the right size. After completing the following steps, the setup can be assembled.

1. The rods need to run through the entire setup, therefore it is important that everything is aligned properly:

   1. First of all, it is necessary to drill holes into the sides of the speaker (If these do not already exist). The holes need to be as big as the threaded rods, and they need to be in a perfect square;
   2. Cut a square from the lexan sheet approximately the size of the speaker. Use the holes in the speaker to determine the location of the holes for the lexan sheet and the wooden base;
   3. Drill the holes in the lexan sheet and the wooden base at the locations determined in step 2.
2. Saw a square from the aluminium plate using the metal saw. We cut a square of around 20x20cm (different sizes can be used). After that, drill a 6mm hole in the centre of the aluminium plate.
3. Cut the rods to the right size using the metal saw. The exact length is not very important, but four of them need to be the same size, and one of them slightly shorter (to attach to the cone).
4. At last, a 6mm hole needs to be drilled into the end cap.

The following steps indicate how to put the setup together:

1. Thread the rods through the wood en secure with bolts and lock washers;
2. Attach the speakers to the threaded rods using bolts and lock washers. Make sure the speaker is levelled;
3. Put a rod through the hole in the endcap and attach with bolts and lock washers;
4. Apply glue to the end cap. Use the lexan sheet to guide the end cap to the centre of the cone;
5. Secure the lexan sheet with bolts and lock washers;
6. Attach the aluminum plate to the central rod;
7. Tighten the bolts using a wrench.

This is the moment to test whether the Chladniplate works or not.

(For our experiment we only used salt.)

There are two things that have to be done before the test begins.

1. A bin should be placed under the plate, to catch the salt that falls off;
2. The frequency and the volt peak-to-peak should be set to the lowest value possible. Otherwise, you might blow up your speaker or you start with an annoyingly high frequency.

Now the test can begin:

1. With a hook cable the generator can be connected to the speaker;
2. Sprinkle some salt on the plate and vary the frequency on the generator, this should be done till the salt vibrates;
3. Turn the frequency up very slowly, until the resonance frequency is reached;
4. The salt should now form a pattern on the plate. By turning the volt peak-to-peak up, the effect can be amplified.

The patterns on the pictures are found at the following frequencies:

• Oval: +/-160kHz;
• Cross: +/-210kHz;
• Circle with quarters circles in the corners:+/-500kHz.

If the setup works fine with the small aluminum plate, some adjustments can be made.

With bigger plates of different shapes, other patterns can be created. This is because the resonance of a plate changes when the shape does too.

To make the plates, you just have to draw the form and saw it out. Find the middle of the plate (preferably the mass middle point, put a finger under the plate and see where it is balanced) and drill a hole there.

Black paint can be used to see the patterns on the plate better.