What is it about back bindings?

What is it about back bindings? (Full Version)

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NorCalluthier -> What is it about back bindings? (Aug. 24 2017 17:54:37)
Hello All, The following exchange took place on the "What is it about a negra?" thread, and it seemed like an interesting enough topic to merit a new thread: quote: "Using Spectra Plus audio software allows me to tell very accurately what the effect has been of what I have just done to a brace. The effects of gluing on the back and installing the binding can be tracked in the same way, and are quite predictable." estebanana: "Brian, I'm curious what you find on binding channel - before cutting , after cutting and after gluing and scraping the binding down. After you cut the channel the bottom falls out of the sound, have you found any difference relative to binding channel size of a more or less drop out? I've made dozens of guitars of the same model with the same size binding and the same thing magically happens, the whole box goes slack, then comes right back up after up glue the binding in. It's the darndest thing. It happens to every guitar anyone makes with the Spanish style. Any numbers on how consistent your process is in tracking before and after binding?" "Hello Stephen, Excellent question! I hadn't thought of running a test with just the binding rabbet cut---that should tell us something. I have two guitars right at that stage now, and will run tests on them. You're right about the dramatic effect of gluing in the back binding. Cheers, Brian"

NorCalluthier -> RE: What is it about back bindings? (Aug. 24 2017 18:40:46)
Hello Stephen, Well, I've just run a test on a rosewood and spruce classical, with the back binding rabbet cut, and as expected the effect is quite dramatic. As you put it, "the whole box goes slack". I would say that the whole bass end of the box's response almost disappears, and the mid range and treble are severely weakened. The lowest resonance is the "air" or "cavity" resonance, and about 40% of the sound output of the guitar is dependent on that one resonance. It's produced by the movement of a combination of the top, the back, and the air contained within the box. The top and back are moving in opposite directions---technically, they are "out of phase". The whole box is working like a bellows, so that resonance is called "an air pumping mode". When I make a guitar vibrate at the frequency---around 100 Hz---of its air resonance, I can actually feel the air whuffing in and out of the sound hole with my hand! I just ran a "response curve" on a rosewood and spruce classical with its binding rabbet cut, but no binding or purfling installed. I had a response curve that I took just after the back was glued on, for comparison. With the rabbet cut, the air resonance went up in frequency by 23 Hz---over 2 full steps---and dropped in volume by 18db! It's going to take a few days to get this classical and a negra through the binding process, but I'm going to run tests with the binding full, after being scraped flush, and after rounding over the corner. With the binding full---actually a bit oversize---the change should be very large from having the rabbet cut with nothing in the channel, I'm interested in the subtler effects of reducing the size of the binding just a bit. I've always made quite small bindings, like the old timers used, and I'm wondering if larger, more modern, bindings might be better in some way. The great beauty of having the acoustic analysis program tracking all this stuff is that I can see the effects of very small changes in an instrument. I figure that the difference between a really good guitar and a great guitar is quite small. We the players can sure tell it by how much of a thrill it is to play a great one. I want to figure out how to make great ones! Cheers, Brian

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