"There is a theory in bridge-making that says that you should NOT have a 'direct path' from string to top. If you look at a violin bridge, for instance, there are holes and 'partial' holes (the two outside strings are over holes with missing sides) under each string. Solid mandolin bridges without holes sound different from ones with holes."

Ah - something I do understand.

The direct path uses 'compression wave transmission': the bridge with holes relies on using other vibration modes, the names of which JiK will probably advise, as I can't think of them just now. The different modes will transmit each of the various overtones in a different manner providing a degree of 'filtering'.