Though I’m interested in all types of viols, I’m currently focused on late 16th through late 17th century English and the small number of French viols made with "bent fronts" or tops.
These were made by bending 5 or 7 thin, narrow strips of spruce to form the complex, arched shape of the front, which are joined and then carefully carved to the final shape and thickness, requiring the removal of minimal amounts of wood. This is unlike other types of viols and almost all violin family instruments which have fronts made from two thick, "book matched" wedges joined in the center and then roughed out with heavy gouges before being carefully carved to final their shape and thicknesses.
This type of construction was only identified with certainty in surviving instruments about 40 years ago but is now well documented. One can speculate, but nothing was written down as to tell us exactly why this method of construction was used but bending and assembling strips of bent wood was a fairly common technique used in those days, barrel and ship making being obvious examples and was also in use in the construction of other instruments, like the backs of lutes.
The question is, was it done because most of the larger trees had been cut down and wood wide enough for two piece tops was hard to come by, or were the makers of these instruments so skilled at the art of bending and joining pieces of wood that it was less work than fully carving from thick wedges? Or was it to economize, where 2 or even 3 bent fronts could be made from the same amount of wood needed for a single front, fully carved from 2 wedges?
Whatever the reason, from the engineering point of view, this is a very significant feature. The wood fibers of the front follow through the complex, curving shapes, almost as if it had grown that way, making it stronger than a fully carved top which has areas of "run out" ...places where the fibers are are not parallel to the top, passing in one side and out the other, which weakens it and requires more thickness to achieve the same strength. (illustration coming soon, 9/18/2017)
The natural virtue of the design allows for making the tops thinner and therefore with less mass than the fully carved version, yet capable of making durable, resilient tops that are resonant and responsive ...and that ultimately have their own characteristic sound. A subtle thing, not better than a fully carved top, just different and appropriate for reproducing the sound of the time.
There are many other elements that combine to make a successful instrument and all of those must be done well, the "setup" including the right strings types and gauges. But It was a design that must have been well appreciated, for whatever reason, as it was used by all the best known makers of it's day.
Once the top pieces are joined and the final shape and thickness achieved, strips of linen or vellum are glued inside the viol body to reinforce all the joints before finally "closing" the box.