Some simple equipment used to visualize guitar sound patterns. Every soundboard and back vibrates at certain frequencies. It doesn't really matter how these frequencies are generated to discover what they are, although different methods of driving the "plates" will show slightly different results.
The pattern above was generated from sound directed into the sound hole via a 3" speaker driven by a frequency generator. The bridge shown is a Morrison "ring" bridge which extends the range of the patterns significantly over the standard classical guitar "stick" bridge.
Above is the bridge driver I use to activate the top's resonant frequencies. Driving the bridge shows similar results as driving the air, but acts in a manner that is more similar to strings driving the top. The arm is hinged to the base and a large hose clamp secures the speaker to the arm. The dominant patterns display with the weight of the speaker coil. The arm extension to the right can be weighted to reduce the pressure on the top. This allows more sensitive patterns to develop.
The center of the cut-away speaker cone driving the bridge at the saddle placement point.