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Most folks will tell you there are two types of backs. One that is flexible and interactive with the top and one that is rigid and basically does not reinforce the top's output. Given the normal range of thicknesses of backs, I'm beginning to think there is only the interactive type. But they vary from quite supportive to not so much, but they all add something to the tops output. Wonder what other think, one way or the other?
Tom

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To me, it's about whether there is a significant T(1,1)3 peak in the tap response spectrum. If there is, I class it as "live", if not, it isn't. Of course, the back doesn't "switch on" in a digital way, you can have more or less of it, as you choose. And there are higher modes of the back that contribute, too. But something like a Smallman style classical (~5mm laminated arched back) doesn't get close to what I'd be calling live.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Trevor: Are you saying a certain DB level on the read out, or a comparison between the levels of back and top?
Tom

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A person who has never made a mistake has never made anything!!!

Well, you can't say a certain dB on the tap response, because that will vary based on how hard you tap. Your tap may be much harder than Trevor's, and so even on the same guitar you will get different dB levels.

So you could compare the dB difference between the peak responses of first mode of the top and the first mode of the back. You might come up with a criteria, say, 10 dB. If the difference is greater, then the back plays a very small role.

Or perhaps an even better measure is whether you can even find the first mode of the back. It may be so low in amplitude that it is buried under the tails of other nearby modes, in which case it definitely doesn't play a significant role. Or it may occur at such a high frequency that it is lost in the sea of higher order top and air cavity modes. I think it is safe to say that if you can't even find the back response on your Fourier plot, then you probably can't hear it either.

Jake has pretty much covered it, referring to the size relationship between the T(1,1)2 and the T(1,1)3. There's more detail and specific criteria on p 2-40 and p 2-41 of the Design book. But definitely, if you can't identify a back response in the frequency response chart measured with a microphone, I doubt your ears can pick it.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

there is a large variance of when the guitar is tapped in a free state and when it is tapped in a playing position. I am beginning to believe that it is more about air volume and how that is reacting with the top. I am not discounting the back but I think taping on a guitar if it is not in a playing position will give a false reading

just my 2cents

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John Hall
blues creek guitars
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