Deflection and Resonance Testing
 

I have a guitar that I'm willing to sacrifice for the sake of science.

I'm thinking of shaving the braces, but I want it to be a learning experience. I want to understand what is happening to the guitar as I change the brace shapes and sizes.

So I'd like to rig up a deflection gauge and some way to measure the resonance frequencies of the top, back, and air cavity.

Any suggestions on cheap rigs?

For a deflection gauge, I assume one with 1" travel and 0.001" resolution would be sufficient. Various sources seem to have them for under $20.

For resonance testing, I assume doing a slow frequency sweep via PC sound card / speaker and measuring the amplitude with a PC-based microphone should work, right?

I believe that is how Woody makes his tops. If he see's this I'm sure he will have some ideas.

I'm also following this path and have found these links to be helpful:

http://www.siminoff.net/pages/siminoff_books.html

I own The Art of Tap Tuning, and the Luthiers Handbook and have found them to be extremely helpful.

http://www.strobosoft.com/index.cfm?...ry=122&sub=189


This software will handle all your tap tuning needs and comes in two versions, lite and complete -both are under 100 bucks.

To do this the right way you'll need a force gage, the ability to build some fixturing, and a inch pound torque wrench. Also you'll need to build a master soundboard because you will need some sort of refrence to compare your experiments to.

There's a lot of smoke & mirrors about tuning tops.

personally I hold Siminoff's book in the same area as homeopathy.

scientifically I cna poke HUGE holes in it and I'm open to be proven wrong but there are a LOAD of luthiers who hold similar (if less forthright) views!!

when tuning a top..... and carving the braces........

you have a top that is bigger than the sides...... and is free to resonate (ie. not attached to the sides) so you carve and sand til it resonates at a D. then you glue it to the sides and cut an inch off all the way round......

you want to guess what it'll resonate at, because I haven't got a clue!!

Sam Irwin (one of Lowdens original team and now running a luthiers class in Belfast) says forget tuning to a NOTE, just go for a nice musicality of a resonance. if it sounds right then it prolly will be.

it IS good to experiment and repeated builds and meticulous note taking will mean that eventually you will be able to predict what a top will sound like once it's complete.

This is a bit different in that I'm looking at fully assembled guitars.

Today, I sang into three different guitars and measured the resonant frequency of the air cavity.

It was interesting in a few respects:

1) They all resonated strongly at certain pitches, and not at all for others.

2) A parlor resonated at B, one 000 at A sharp, and another 000 at D.

3) I like sound of the one that resonates at D the best.

4) It seems that the air cavity resonates at a lower octave than the open strings. I'm not sure if this is by design, but it seems to keep the sound fairly balanced across the strings.

Question: when I sing into the guitar and cause it to resonate, is it resonating at its Helmholtz frequency? I.e., is it just the air that's getting excited?

I assume that shaving the braces won't have any effect on this resonance frequency.

it will very slightly as you'll have removed wood, therefore increasing the amount of air.

I think!!

removing wood from the braces will lower the resonant frequency of the top.