Compliance testing

[Craig]

After reading of Mark's soundboard problem , I think it a good moment to introduce this topic. This is a technique I have been discussing with fellow member and good mate, David Hurd( AKA hilo kawika) ( Author of the book " Left Brain Lutherie " ), although I understand there are others who use a similar technique .

The advantages are that you can build instruments of similar top deflections and thereby gain similar sound characteristics. The testing Dave uses is simple enough that anyone would be able to set it up

http://www.ukuleles.com/Technology/compliance.html

I wonder if Dave could hop in here on this thread to discuss this and maybe help answer any querries that might turn up ?


Cheers Craig

[hilo_kawika]

Thanks very much for the introduction of this subject Craig. I appreciate it a great deal.

Compliance testing is a technique in lutherie which although not a panacea for top optimization, deserves to be among the arsenal of useful luthier tools.

Several points are in order. First, this isn't a new idea. It's been used on and off for perhaps 30 years or so for guitar family instruments. Second, in its simplest form, no mathematics or theory are required before jumping in to make useful measurements and in all likelihood more consistent sounding instruments, assuming that's a desirable goal. Third, it's also been used in instruments such as mandolins for more than a decade.

Basically the technique seeks to map the mechanical compliance or stiffness of the instrument top. A simple frame which rests on the edges of the guitar or ukulele has vertical holes across its length in which singly rests a dial micrometer. The second part of the device is a sort of balance beam with a removable weight on one end. The end of the beam with the removable weight also has a small open section that the tip of the dial micrometer fits loosely into.

In practice, the balance beam is so positioned so that the tip of the beam is at a place of interest for measuring top stiffness. The frame and dial micrometer are then positioned such that the tip of the micrometer is in the open section. The weight is temporarily placed on the end of the beam and the deflection of the top as indicated by the dial micrometer is noted. The measurement is repeated several times for consistency. In the building process, all this is done with the top attached to the sides but the back still not on. The bridge may also be double-stick taped to the top during the measurement process.

But how do we know whether the measurement is meaningful/useful? If we have first collected a set of measurements from a "successful" instrument (good tone and stability) then we have a proper data set to compare to. If we then carve braces until the point at which we are near the "successful" top, we can then make other judgements. Should the stiffness be symmetrical or asymmetrical? Was the other instrument a little too tight or loose? Since we have the actual stiffness numbers of the "successful" instrument to compare to, we can make an intelligent decision about how much more (or less) to work on the braces or do other subtle tuning procedures.

I have used a ~ 2 lb weight for nylon string guitar family instruments. I suggest that a heavier weight - perhaps 3-4 lbs - be used for steel string instruments in order to give large enough deflection measurements.

Clearly, different sized instruments, different string tensions, different wood types will all have to be considered when comparing stiffness maps for actual utilization. But at least it's a simple set of measurements to make and think about and discuss among ourselves. Please consider it.

aloha,

Dave Hurd