Official Luthiers Forum!

Since there is a thread about top testing after joining but before any other work, it might be interesting to explore testing of completed tops on guitars. I do the former and it is a good starting point, have not tried the latter, but have been thinking about it and wondering if anyone else has. It seems to me this might be "the proof is in the pudding" stage. I know about the refs. in the Gore book but would like to know what else is happening.
Tom

_________________
A person who has never made a mistake has never made anything!!!

Again, see David Hurd's "Left Brain Lutherie" for information on that.

Gone up since advertised on his site, Looks packed full of stuff but after the Gore/Gilet books I might have to wait for a while until my building catches up.

Hi Tom
Since I started building a few years ago I've been looking for ways to quantify different aspects of guitar building in order to predict the tonal
outcome of what I'm trying to build. For me, being able to calculate the specific mobility of the top on the closed box is an important factor for building a responsive guitar. The Gore/Gilet books are right up my alley. I like to measure and see the numbers, so to speak. But, I haven't
built a lot of guitars yet so I am still forming opinions on the best approach for me. I too, am curious as to what others think with regards to this
topic.
Ken

The problem I see is I do not think the data will transfer over to just braced tops. The stiffness of the rims, Glued on FB extension, glued on bridge, border purflings and even finish will affect these measurements in unrepeatable ways.

_________________
Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Here's my deflection testing gear
It rests directly on the edge of the soundboard and the load is applied directly through the shaft of the dial gauge.

Nice!

Jeff: Wonder if you are just measuring deflection for a certain weight or if you are carrying through to figure Specific Mobility as per Trevor Gore's book. Wonder also how must weight you are showing and the deflection you are getting.
Tom

_________________
A person who has never made a mistake has never made anything!!!

?????? You totally lost me here. How can you apply a load through the shaft of the dial gauge? Isn't that part supposed to measure the deflection?

And you want to correlate this with the guitar sound? How many hundred guitars are you going to measure and how is this single measurement supposed to relate...seems like only one of very many interrelated factors?

Grant

Yes for specific mobility
I think I had about 450 grams probably would be better with 1000 grams. deflection is quite small, don't have it at hand.

The Beam supports the body of the gauge only. The nose of the shaft rests on the bridge and the extension of the shaft out the other side has the small platform attached to it, It is important with the very small deflections being measured to avoid any backlash or slop and this is very effective.

No I don't correlate this single measurement with the guitar sound.
Used in conjunction with the effective soundboard mass (derived from this measurement of stiffness, and the soundboard uncoupled monopole frequency) you can calculate a number called "Specific Mobility" which is an indicator of the responsiveness of the guitar.

Jeff: Thanks, I thought so much with the Specific Mobility. I'm going over it in the book but having a bit of a time. The book was written by an engineer................I'm treading in deep water. Assume it is all done in the metric system. Have not tried it yet but assume again the equation works out to the numbers shown on the chart for various guitars.
I check my raw tops for deflection the same way with the weight resting on the top of the indicator shaft. Works slick, got it from you or Trevor.
Tom

_________________
A person who has never made a mistake has never made anything!!!

Hurd actually measures deflections on a grid of points, and maps out deflection contours. He trims the braces to obtain a smooth overall deflection pattern, with not big bumps or dips. He also, of course, looks for a deflection in a particular range of values: neither too stiff nor too loose.

At one point he asked me to send out the body of my 'corker' test mule, and ran deflection tests on that. What he got was exactly the sort of map he looks for. I determine the stiffness distribution in the top by using Chladni patterns, rather than a deflection test. It does not surprise me at all that we'd end up getting the same result using our different methods, though.

I´ve read this topic with much interest as I am just building something similar to what Jeff Highland shows here.

As i really don´t understand to much at all about "soundboard uncoupled monopole frequency", or "Specific Mobility" i did a little search
and found this: http://www.speech.kth.se/music/acviguit4/part2.pdf
Maybe it is of some help or interest to others..

Somogyi does something similar only he does it sonically and not by deflections.
Thanks for the info Alan, I always get something from your posts. You are one of the top providers of what might be called scientific info. It's much appreciated.
Tom

_________________
A person who has never made a mistake has never made anything!!!

There is a very interesting picture of Jimmy D'Aquisto pushing the top with his thumb,
to know when it's the right thickness,
in Irving Sloanes book "Steel String Guitar Construction".
So, he probably has built so many git-boxes,
that he knew when it was right.
There is also a very cool pic of him chiseling out the dovetail joint on the body,
with the guitar propped against his workbench.
Amazing builder!
So, you are sort of starting from square 1, Tom,
but good luck with that.
Maybe you could test some of your gits,
note the ones that sound best,
and go from there.
What I do is push down on the top,
and see if it gives,
and if it does,
I'm done.