Here is a cool series of pictures obtained at the Cardiff University. They used the holographic interferometry method to get these pictures (Google will help the most curious to know more about this method). These pictures show how the soundboards of stringed instruments vibrate.
The 3 last pictures of the series are particularly interesting as they show the guitar soundboard vibrations changes when the frequency changes from low (first picture) to higher frequencies (the 2 others). The brightest lines are nodal lines i.e. no vibration occurs there.
It’s funny to think that this is a view of the voice of an instrument that we are watching!
More pictures at BBC News.