A few resonance challenges

[Munchkin]

I've recently completed my 7 string (Roger McGuinn style) dreadnought guitar - she's X braced with a live back and has been constructed using many of the principles in Trevor's book.



The plates were thicknessed according to the formulae in the book and after initial tapping I added 70g of side mass to get main resonances of 95Hz, 169Hz and 195Hz.



As expected the sound was a bit muddy because the back resonance was pretty low (in fact it was G!) so I scarffed a shaped piece of spruce onto the lower bout back brace as suggested to stiffen the back in an effort to get the back resonance up to 215Hz. I've obviously overdone it though as the back resonance has disappeared....



Before I proceed in shaving down the brace I'd appreciate any input you can give me on what may have happened here. I had to use 5 minute epoxy to glue the brace filler due to the clamping challenges and a somewhat inadequate glue joint .
The new resonances are 97.5Hz and 175Hz ....

It's great to have a scientific way to analyse your tone!

Thanks Guys!

[woodrat]

Martin...Did you put side mass attachment points into your dread?...You could use those to drop the 175 top to 170Hz pretty easily and that may also shift the air down nearer to 95Hz. Otherwise you could play around with some blu-tac on the bridge and see how much mass is needed to push it down to the required 170.

Lovely looking guitar BTW.

John

[Trevor Gore]

Nice build, Martin.

The first thing to ask yourself is which version sounded better? Get people on both sides of the guitar to give a view, though that might be hard if you didn't do a sound check prior to the back brace mod. The basic rule is don't fix things that don't need fixing.

Next thing is to tap the back directly and see where the B(1,1) sits. Tap the middle of the lower bout and you should see one major peak which should be close in frequency to the T(1,1)3 and indicates where you should look for it in the top response.

Ask yourself whether you need a T(1,1)3. It's main purpose (to my mind) is to limit the amplitude of the T(1,1)2 so as to avoid wolf notes. If you don't have that problem you might just leave the back as it is, with its stiffened brace. Getting a 4 semitone separation won't make the guitar sound magically better, it just keeps the peaks far enough apart to avoid "phasing" type problems and over attenuation of the T(1,1)2, yet close enough to actually do something to the T(1,1)2 whilst spreading the width of the T(1,1)2 / T(1,1)3 couplet to give a little more volume in that region.

Remember that what's important is what it sounds like, so your ears (and your audience's ears) are the ultimate judge.

[Munchkin]

Thanks for the responses!

I was very happy with the T(1,1)1 and T(1,1)2 at 95Hz and 169Hz prior to stiffening the back. 195 for T(1,1)3 was the challenge - I actually braced the back with 3/8" spruce so she came out more flexible than planned.

As I mentioned, I did bolt on 70g of lead mass to get to 169Hz so there's a lot of flexibility there.....

The guitars voice is not as pleasant now to my ears so I'll be aiming to reduce the brace height (after determining the back resonance as Trevor suggests). Looks like the strings are in for it! I'm also due to install a K&K Pure Mini pickup http://www.kksound.com/puremini.html on the bridge plate so I'll do that sooner rather than later as it may change the top slightly....

The customer who commissioned the guitar is busting a gut for delivery!

I'll report back...

Martin