Measuring Top Compliances

Measuring the Mechanical Compliance of Instrument Tops With Thanks to Bartolini & Bartolini

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It is sometimes popular among Lutherie blog writers to declare that “We know nothing about how a guitar really works” and “Only the great masters have control over the creation of extraordinary instruments” and “One must build hundreds of instruments before one can hope to make something that even a good player would want”. I suggest that most of the writers of such drivel are abysmally ignorant of existing literature on the subject of musical acoustics in general and guitar acoustics in particular. There is a great deal known and available to anyone with access to the Internet and standard library services. The paper discussed below is a classic example of the type of powerful and applicable material that exists.

One of the most eclectic papers written about the acoustical properties of guitars is by W. and P. A. Bartolini in Issue #6 of the Journal of Guitar Acoustics. Among the many fundamental acoustical and mechanical properties measured on a variety of guitar tops was mechanical compliance. For further details, I recommend reading the article itself.

In all honesty, I hadn’t appreciated the significance of this measurement until Ross Gutmeier, an East Coast classical guitar builder, told me that he had had success using this approach in building his instruments. A useful discussion ensued and I applied the compliance measuring concept to building my ukuleles and guitars. After an initial trial and error period, a set of useful measurements were achieved. In this webpage, I will link to several pages which describe the simple compliance measuring setup that I use and show it in operation.

But what is compliance? Compliance is a measure of the deformation of a body (for example a plate) under load. If a weight is gently placed on the center of a guitar top, the deformation of the top is the mechanical compliance relative to that particular weight and its placement. By measuring the compliance of the top, we can obtain a quantitative estimate of its mechanical stiffness. So, if you had a guitar which sounded great and whose top hadn’t caved in as a result of string tension and bridge torquing, you could make a map of the mechanical compliance of the top. This map would then be a powerful building tool, since one could aim toward reproducing the compliance map by brace carving and top thinning.

Let’s first look at an overview picture of the process. Then we’ll look at the actual pieces of the apparatus. Finally, we’ll return to the measurement process and look at some data.

The first picture (Figure 1) shows the basics of the process: an instrument body with the top but not the back attached to the sides; to the left a sort of lever arm thing with a little platform and ring on the end of it; and to the right a U-shaped wooden frame with holes the length of it and a dial indicator in one of the holes. Missing is the weight that sits on the platform and causes the deformation.

Figure 1: Mechanical compliance set-upoverview

If you think it looks pretty simple, then I have achieved my purpose. You can build this setup in an hour or two and have the whole business cost only a little more than the cost of the dial indicator itself. Any shop has scraps from earlier projects lying about and this is mostly what I used.

Please follow these links for making the jig, understanding how to make a measurement and then making measurements on an actual instrument:

Assembling the Compliance Jig

Making Compliance Measurements

The Compliance Mapping Process

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