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-   -   Deflection and Resonance Testing (https://www.acousticguitarforum.com/forums/showthread.php?t=177046)

gitnoob 02-16-2010 01:48 PM

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?

Coke_zero 02-16-2010 06:53 PM

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

arie 02-22-2010 10:45 AM

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.

martinedwards 02-23-2010 09:57 AM

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.

gitnoob 02-23-2010 06:12 PM

Quote:

Originally Posted by martinedwards (Post 2137343)
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!!

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.

martinedwards 02-24-2010 04:22 PM

Quote:

Originally Posted by gitnoob (Post 2137980)

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!!

arie 02-25-2010 12:41 PM

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


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