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Hello everyone,
Does anyone know of a reference list of typical internal damping values for various tone woods? I’m’ interested in north American woods in particular.
If not, how about a practical set up for measuring it in the shop?

Daniel Haines' study on tone wood properties has damping values for a number of woods. It appeared in the Catgut 'Journal'(now defunct, alas!), the 'Journal of Guitar Acoustics' (available in reprint from Tim White at tpwhiteco@aol.com the last I knew), and I seem to remember seeing it someplace else (American Lutherie?).

There is an easy way to get at least an approximate value for damping using your computer, if you've got a sound card and mic. Set up a sample of the wood in question as if it were a glockenspiel bar, with supports about 1/5 of the way in from each end (or side, if your'e looking for the crosswise damping). Place the mic in the center of the end, facing the surface of the plate, and tap in it the middle. You should hear a fairly well defined pitch, which will be pretty low in a lot of cases.

You'll need to record the sound, and use one or another of the software packages that can do a Fast Fourier Transform (FFT) on the file to extract the pitch of the lengthwise (or crosswise) bending mode, which _should_ be the lowest frequency peak in the transform. I use a quirky old freeware DOS program, FFT4WAV3 to do this. You'll need another program with this to record: I use an old version of 'Cool Wave'. Another free program, that has it's own recording setup, is 'Wavesurfer', available from:
http://www.speech.kth.se/software/

The big trick here is to figure out a sample rate and sample length that will give you useful information. The higher the sample rate the larger the frequency range you can look at. The rule is that your upper limit frequency will be half the sample rate; so that if you record at 16,000 samples/second, the highest frequency you can look at is 8000 Hz. We don't really need to lok at high frequencies here, so a high sample rate is not going to help much.

The sample length sets the frequency resolution, and that's the real issue here. FFT works by figuring out how much energy there is in the sound within a range of frequencies: a 'bin'. The size of the bin depends on how long the sample is: if you recorded one second of sound, then each bin will be 1 Hz wide. A two second sample gives you 1/2 Hz bins. Looking at my data I find lengthwise resonant pitches are often at around 70 Hz, with bandwidths (see below) around 1/2 Hz, which suggests you will need at least a 4-5 second signal to resolve the peak well enough, and 8-10 seconds would be better. It might be possible with some programs to use multiple taps to get a long enough signal, although that will also introduce some noise.

At any rate, once you've got the data, you can graph out the peak for the lowest bending mode. Find the peak frequency (Fpeak) and also the frequencies on either side of that (Fhi and Flow) where the amplitude is 3 dB down (or 70.7% in linear terms) of the peak height. At that amplitude the energy in the object is 1/2 what it is at the peak, so the difference between the high and low 3dB down points gives you the 'half power band width'. The Q value, a measure of damping, is found from:
Q= Fpeak/(Fhi-Flow)
A high Q, or narrow band width, equate to low losses.

If you have a signal generator, you can drive the wood sample (I use a speaker) and 'read' the vibration somehow (I use a smal piece of iron on the sample and a magnetic pickup). A DVM, set for AC/MV on the output will give you the amplitude, and you get the frequency from the sig-gen. A sprinkle of glitter on the sample will tell you whether, in fact, you're looking at the bending modes or not.

Al, I know you've looked at this type of thing a lot and was wondering if you can hear enough with your ear to determine if a piece of wood has high or low damping? It seems like you could pick out a "lively" piece of wood pretty easily...?

I have a hard time figuring out how you'd use the damping info in building a guitar. It seems like minimizing mass of the soundboard is probably the main factor in determining the responsiveness of the guitar(?).

Nice summary though Al, thanks for responding!

Trev

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http://www.PeakeGuitars.com

Thank you for Your reply Alan,
Man, I feel like I should buy you a beer, or wash your car, or.......something. ?
I would love to be able to utilize what you've written, but it sounds like quite a project from where I stand. Hopefully someone else can use it sooner than I.
I asked the question because fairly frequently I hear descriptions of this or that wood having," low(or high) damping properties". I assumed people were drawing on some fairly accessible source for this information. but clearly either they are relying on the knuckle rap by the ear method or there are more scientifically skilled folks here than I thought.
Like Trevor, I wonder how well the ring by the ear test correlates to measurements. It certainly requires less testing gear! when I listen to a piece of wood ring and think ,”oooh, aaaah”, am I hearing what I think I am? was my pinch not quite on the node? Is my memory of the wood in my last guitar accurate? how do I compare this billet to that thinned side?
Strength to weight can be manipulated with weight, thickness, and brace design. but as has been pointed out many times, efficiency is not the whole story of sweet sound. If it were, we would all be making banjos right? The earth has not been stripped bare of Brazilian rosewood for it's amazing strength to weight ratio has it? I can compare strength and weight to Brazilian with a scale, calipers, and a dial gauge. I was simply wondering if damping figures were as obtainable.
This discussion has me now wondering , Strength , weight, dimension ,internal damping, is there some other overlooked quality of wood? Or just the interplay of these commonly tested and manipulated items? Not that those are not enough variables
Thanks again for your reply. I will look for the references you gave.

They're more than enough to make it a horrible problem

Strength/stiffness is non-uniform over the piece (lengthwise/crosswise are different)
'Dimension' is actually a multi-dimensional variable comprised on the three Cartesian measurements as well as the shape
Weight and internal damping are just an extra two variables on top...

But the most fun part is this: they're not linearly related. If there were really four straight numbers there (rather than the minimum of seven for a relatively homogeneous piece of wood cut into a rectangular prism), the graph of their interrelations would still look like a five-dimensional haystack

That's not to say that we can't make better guitars by taking good measurements and trying to make the best correlations we can. I think Al and a number of other people putting numbers on things are proving the viability of going about it quantitatively, and every little piece we figure out starts making that ball of yarn look a little more like a hyperbolic paraboloid

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Generally speaking you can get a pretty good feel for the damping factor of wood by tapping and listening, but...

Several years ago I got some persimmon wood, and from tapping on it it seemed to have reasonably low damping: it tended to ring for a pretty long time. When I actually masured it, it turned out that the damping was fairly high. It rang for a long time beause it was so dense that you had to hit it pretty hard just to get it going, and then it took it a while to die out.

Balsa is just the opposite. The thin slices that you get at the hobby shop die out almost instantly when tapped, but a thicker piece I measured turned out to have about the same damping as spruce. Those thin sheets have to move a lot of air to move themselves, and since they don't weigh much the air sucks the energy right out of the wood.

So, if you have two pieces of wod that are the same size, and have about the same mass, you can pretty easily hear which one has the lower damping; it will have a clearer pitch, and ring longer. The further you get from 'average' density, the harder it is to be sure.

As for why it matters: high damping tends to 'eat' high frequency sound. A top with low damping, such as cedar or redwood, will probably end with more upper overtones, and that may be why the sound tends to be 'fuller'. In the case of a back I think it's the narrow band width of the low damping woods, like BRW, that makes the sound 'richer' than, say, maple. THe low damping wood does not move over such a wide frequency band, and thus may absorb less energy.

Of course, it's distressingly easy to make a lousy guitar from good wood, but, at least with good wood you have a chance.

Al,

In regard to damping, I know that Acoustic Bass (AB) guitars tend to be weak or soft sounding without amplification. Most of the one's I've heard in the past were built with low damping woods. This question is out of my own lack of understanding regarding AB guitars, but I wonder in light of what you're saying, for the sake of a natural increase in the Bass volume, if using back and side woods that are a bit more damping would be more beneficial? Any thoughts on this? I am currently in the process of trying to complete a 5 string AB that I built from some very old Myrtle with a sitka top, a Gaboon Ebony fretboard and bridge. Haven't had a chance to hear it yet since it's not completed but the box taps nicely. I have your chladni DVD but don't have a tone generator so I have no chladni readings to reference.

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Bill Hodge


One does not simply, own enough guitars!

Damping is, IMO, not the problem with ABGs. Wavelength is. The lowest notes have wave lengths that are so much longer than the size of the box that it simply can't radiate those sounds effectively. You can get some sound out of the 'main air' mode, but it would probably require that the hole be so small as to negate the effectiveness. The acoustically scaled large bass in the Violin Octet, which is tuned to the same pitches, I believe, has a body that is 51" long. That works.

Interesting indeed looks like I'm going to have to put my R&D cap on. Thanks Al, I appreciate your time!

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Bill Hodge


One does not simply, own enough guitars!