I am a Luther building violins, mandolins, and guitars. I have been using this method of tuning the plates (back and front of instruments) for some time now. Placing sand on the plates and letting them vibrate to specific notes is called "tuning the plates" The method was developed to try and duplicate the sound from the violins made by the old masters. the plate is suspended over a speaker and covered with sand or some other medium and then made to vibrate to a specific note. removing wood from the plate changes the vibration pattern. the plate is thus adjusted to the specific note the builder is looking for. This is a simple explanation, and if you wanted to know more just google 'tuning the plates."