How a guitar vibrates:
Chladni patterns

 Ernst Chladni (1756-1825) came up with the concept of placing small particles (like dust or powdered talc) on a vibrating surface to see what areas were vibrating and in what patterns. The areas that end up free of particles are the areas that vibrate the most. The areas where the particles collect are the areas where the movement is least (nodes). Rather than spend a lot of words trying to explain all this, I present the following photos so you can discover the concept and on your own. It's up to you to get as much or as little from this as you want. Have fun !

 The following thumbnails can be clicked on to view full size images.

Chladni patterns generated on Canadian Cyprus soundboard sections before joining and bracing.

A few pages from my shop notebook, showing simple Chladni patterns generated on a thick styrofoam sheet, a section of Englemann Spruce and Canadian Cyprus (aka Yellow Cedar). This shows simple patterns on rectangular pieces of wood, known as plates. The styrofoam is a uniform material, the wood is not.

  Notebook pages for the top pictured below; unbraced, braced as shown below, braced and glued sides and finally with the back attached. This is so you can see the changes to the patterns when bracing is applied and when the guitar body is assembled.

Photos of Chladni patterns generated at the frequencies noted on my #216 guitar. The top has been braced and attached to the sides. Input was a sine wave frequency generator, amplifier and 3" speaker. 

1. The top bracing under all the photos below. Braces are Fir, Spruce and Balsa

 2. 204Hz (main) also visible at 28, 41, 69 and 198-212Hz.

3. 253Hz (main) also visible at 83 and 219Hz

4. 344Hz

5. 446Hz

6. 512Hz (main) from 504 to 540Hz

7. 601Hz

Not shown is the single mode, which was apparent at 19, 25,35, 44, 59 and between 168 and 180Hz.

So a summary can be shown by the number of vibrating areas and the frequencies they occur at. I have included in the last column the main frequency where it appeared after the back was glued to the sides, enclosing the body. The air resonance at this point was 91 Hz.

Notes:

1. The above data is meant to inform on general principles. It is not intended to infer that the instrument this data is from is good, bad or indifferent. 
2. The body shape is the NBN classic/concert model. The top is Western Red Cedar ~2.5mm thick
3. Some instrument makers claim that each pattern appears at only one frequency. This is not true. However, certain patterns emerge:
4. At lower frequencies (under 100Hz) the patterns tend to be very specific to the frequency(s) they appear at, but do so at several frequencies
   
5. At higher frequencies, patterns tend to manifest over a range of frequencies and tend not to reappear outside that range.
6. For the data as presented above, I have shown the frequency where the pattern is the most defined, not the only frequency it appears at.