Double Top, Quilted Sapelle.

[tlguitars]

So hello to Darryl Young!!!!!!

So, a big I'm sorry, you were completely right and I was in total denial. After getting it all boxed up the guitar didn't make a sound, nothing. You could beat and beat on it and it would thud and suddenly stop. So after months of shame, denial and procrastination I finally put the big boy pants back on; and cut and decorated some circles.

It sounds great now. I now have it voiced correctly and my purfling has arrived so I can get back to it. Here's pics.

[Rodgers Piano]

Hello Everyone,

My name's Dave Rodgers. I've been rebuilding pianos the last 20+ years and have installed my share of piano sound boards. But for about the last six years I've been intensively studying guitar building of all kinds and doing a lot of thinking. I'm in the middle of building my first two instruments for better or for worse!

I was really surprised to read that the sound of the guitar was disappointing. The idea and the physics I'll mention were solid as a rock. I can't help but wonder if it would still work great without the extra holes if the inner board were also made of hardwood, very thin, sturdy but majorly flexible and then finished with a very thing layer of epoxy (West System pops to mind) as it works great as an actual finish. The epoxy would help push air better than raw wood, the flexible inner portion would move readily but the hardwood could provide the spring you want. Don't know. I'm still puzzling it out technically. It should have worked really well. Obviously the inner sound board panel was absorbing energy instead of reflecting it. I think you're very close and that you just need to discover why the air spring you created in the back didn't "spring" but rather it absorbed.

I just had to put in a tidbit of physics into the double back you all've been talking about. There was a certain amount of concern about the lack of a second sound hole in the inner back. Well, I've got ideas on that myself. I think the builder is on the right track by not putting in a sound hole. He just needs to tweek it so as to take advantage of the "air spring". I think he's making a greater thing than he initially may realize once he hammers out the details. With it spruce front and back it is somewhat more like the old Loar Vivitones. Loar was no fool. But the history of hardwood backs has its reasons. At the very least, you will keep players bellies off your sounding boards and that is a good thing indeed. They hold an instrument and kill its sound and then blame the builder for the tone...

What is sustain all about anyway? Isn't it a thing I've heard called "restoring force?" When I'm making a piano or designing a new piano, I'm always dealing with spring effects in unlikely places. We put energy into a string, the energy flows off (hopefully) into our sound board. If our board has enough energy stored up inside of it (the reason we crown and the reason string tension is applied to the tops with fixed bridges in classical and flat top guitars) the sound board will take the strings energy and start to move, then as its internal resiliance takes effect, it returns that energy back to the string which, in turn, gives it back to the sound board. The longer we can keep this game going, the more sustain we have in our instrument.

Imagine, if you will, that the sealed air chamber between the Rosewood back and the spruce inner panel is a spring. As the inner sound board is pushed toward the spring, the air pressure increases. But physics always wants to return things to a state of rest, therefore, the air chamber that is sealed will push back to try to reestablish its own balance. But in doing so, it drives the inner sound board which, in turn, pushes on the air inside the guitar which, in tern exits the sound hole with more steam than usual or smacks into the actual top and forces additional restorative energy being pumped back into the string. The lack of an air hole on the inner board is definitely a worthy thought! I see from your efforts that the inner sound board must be brittle, delicate and yet strong and springy or sprung if you prefer. The weight of the inner board and all the bracing, lack of finish and species choice is what I believe let you down. Not the application, installation or general thought put into it.

Funny as it sounds, I have never read of this technique until I came across it on line today. My two guitars that I am building both have similar design methods planned into them without ever having been told about them. They're obvious if you understand physics at all. Which you guys clearly can. But I, like you, had planned holes in the actual rosewood back like you installed and I planned on a brittle or, maybe fragile, hardwood inner sound board.

So glad to see the great work you're doing. You inspire me to work hard to make sure all my tools (especially my hand made jigs and guitar sized presses are perfect so as to make the perfect result.) I already have huge stuff for making piano sound boards but I have to be even more careful with guitar work as the woods are more delicate and the details so much more observable. Wish me luck. You, obviously, don't need any!

Hopefully, your new friend,
Dave Rodgers
Rodgers' Piano Rebuilding
http://www.pianorebuilding.com/frames.htm" onclick="window.open(this.href);return false;

[Chris Paulick]

Welcome aboard Dave!
And Happy New Year !

[Nick O]

Well it's been a long time since I've posted over here and I apologise for not being a more frequent visitor!

Imagine, if you will, that the sealed air chamber between the Rosewood back and the spruce inner panel is a spring. As the inner sound board is pushed toward the spring, the air pressure increases. But physics always wants to return things to a state of rest, therefore, the air chamber that is sealed will push back to try to reestablish its own balance. But in doing so, it drives the inner sound board which, in turn, pushes on the air inside the guitar which, in tern exits the sound hole with more steam than usual or smacks into the actual top and forces additional restorative energy being pumped back into the string. The lack of an air hole on the inner board is definitely a worthy thought! I see from your efforts that the inner sound board must be brittle, delicate and yet strong and springy or sprung if you prefer. The weight of the inner board and all the bracing, lack of finish and species choice is what I believe let you down. Not the application, installation or general thought put into it.

Welcome Dave! thanks for your thoughts. Your sealed spring theory is sound however the way I see it is that if it is sealed(as it is in Todd's case) this rear 'chamber' becomes, in effect, coupled to the outer back. Air hates to be compressed unless there is a much greater force making it do so, it will compress to varying amounts depending on the size of the container it's held in, a larger container will allow more 'spring' whereas a smaller one much less. During the simple action of playing the guitar, it's held against the body (or in my case rather 'healthy' gut ) which imparts a dampening action against the outer back, this inturn partially compresses the air in the small chamber between the plates, which when you think about it, imparts a 'preload' if you like onto the inner back which, in combination of the braced inner back, resists the much weaker sound waves in the box. It becomes more of a 'reflector' rather than a live plate, coupled with the fact that Spruce has less density than a hardwood & 'sucks up' some of the energy in the sound box. I think this is why Todd found it didn't have as much life as he thought it would, but by putting vent holes in the rear, he's allowed it to become more like the live back he wanted, without the player dampening it's sound.
My take on it anyway.

[Oryza5]

Todd, thanks so much for posting this double top build and your experience making it work. I own two single topped Contreras Sr. guitars (5-fan model from early 1970s and a 7-fan model from 1982). They are very nice instruments but I have been very interested in the double top model made by Contreras Jr. I'm told that the Contreras II double top voice is very powerful loud, but I'm not aware that either the inner top or the back plate are ported as you have done in this example. Do you or anyone else have any insight? I believe the Contreras II double top uses cedar for the inner top, not a stiffer hardwood as one reader suggested. If you have any additional learnings now that a few years have passed since you finished the instrument. I also play steel string archtop and wonder if this concept could work there as well? Advice is greatly appreciated.

Update 1/18/16. I was inquiring about features on a Contreras II Double Top for sale on eBay. The seller kindly answered my questions and confirms there is no sound port on the inner top. Instead the seller describes bracing and a block connected to the inner top. This seems more like a sound post on a bowed instrument which connects the top to the back. On a violin, most of the back plate is free to vibrate with the top. If you've ever played a violin without a sound post you know what a difference that wedged-in stick of wood makes. I can envision a wedged-in sound post approach working on an archtop guitar using a tailpiece with only downward pressure on the bridge (here the sound post pushes both plates away from each other). But on a flat top with a glued bridge & saddle the top will naturally lift up under string tension. A wedged sound post would have an undesirable effect, pushing the top up more. So one way Cotreras II may have solved this by pulling the two tops towards each other with the sound post, then both tops could still be lightly braced and vibrate together. This could be done with two dovetail type joints or v joints, one in the block described by the seller, the other in a similar block on the underside of the top. I'm just guessing here folks, but remain intrigued by this natural concept.