Official Luthiers Forum!

I am looking for a reference book or a website that lists the physical characteristics of different woods. (density, stiffness, hardness, damping, etc). Do any of you have any recommendations? Thanks!

I don't have it but the Guild of American Luthiers has a book on Luthier Woods

Lutherie Woods and Steel String Guitars
http://www.luth.org/books/book.htm#wood

Sondre, the most comprehensive online database that I have found is "The Wood Explorer."

http://www.thewoodexplorer.com/

Select or search for a specific wood type by scientific or common name and you will be presented with a data sheet that is about 3 to 4 pages deep with graphical representations of properties and at the bottom of the data, all of the numerical properties for that type.

There are a number of publications available that have much of the same data available and the the US Dept of Agriculture, Forest Service has a variety of publications available for download at their site: http://www.fpl.fs.fed.us/document-lists ... -list.html

The "Wood Handbook" available there has a wealth of engineering and technical data. The information you seek is all out there you just have to be willing to spend a bit of time and dig for it. YMMV

_________________
Peter

Just remember that wood is a natural material, that varies widely in it's properties. You could certainly use the sort of homogonized numbers you'll get on a site like that in design work; to figure out if a certain size of brace will be in the ballpark, for example. However, since the lengthwise Young's modulus (E) of softwoods can vary by +/- 20% or more within a given species, and he crosswise E values by much more than that, you really need to do your own testing if you're going to try tp push the structural and acoustic limits. It's not difficult to do some basic tests, but it does take some time.

Thanks folks! I'll have a look at those books. That website also looks really useful.

Alan, thanks, that's a good point. I was thinking just to get a basic understanding of the tendencies.

It seems that it is particularly difficult to find information on the internal damping properties of woods. It would be really convenient if there was a book or a website that had information about that too. Can damping measured in numbers? Or is it more subjective, like "what you hear" when you tap the wood?

Damping is usually measured in 'Q value'. There are a couple of ways to do that, but the one I've used is the 'bandwidth' method. It's based on the notion that any internal energy dissipation will only happen when the thing is actually moving. A material with low losses will show sharply defined resonant frequency peaks, because it will be very active on resonance and not very acrtive otherwise. With more damping the peak height is cut down, so that the peaks look more rounded. Thus measurung how 'rounded' the peak is gives a good indication of damping.

When I'm testing materials, I use a signal generator to drive the piece of wood with a constant input power. The frequency that shows the maximum activity for that power input is the resonant frequency, by definition. If you go higher or lower in pitch the activity drops off. The energy in the system is proportional to the square of the amplitude, so if you find the frequencies on either side of the peak where the amplitude is 70.7% of the maximum (3 dB down), the energy at those frequencies is 1/2 the maximum. We usually call the peak frequency Fp, the 3 dB down point on the high frequency side Fhi, and the 3 dB down point on the low frequency side Flo.

Q= Fp/(Fhi-Flo).

If you have a computer with a sound card and a program that can do Fast Fourier Transforms (FFT), you can sometimes find the Q value of a material from a tap test. Record the tap, run the FFT, and find the peak frequency and 3dB down points graphically from the plot. The big problem here s getting enough frequency resolution to find the points accurately. OTOH, all measurement methods have problems.

One interesting thing about Q value is that it tells you directly how much energy the system loses per cycle of vibration. If the Q value is 50, then 1/50th of the energy in the system is dissipated per cycle. It amounts to less and less loss per cycle, of course; in the above example, the first cycle loses 1/50th of the original allotment of energy, and the next cycle looses 1/50th of the 49/50ths that are left. The amplitude drops off over time, and drops much faster when the losses are higher. You can see that something like Brazilian Rosewod, that can have a Q value of around 200 in some cases, will 'ring' a lot longer when it's tapped than, say, oak, with a Q value of more like 50.

A high Q system is easy to drive at a frequency very near to it's resonance, and relatively hard to drive at any other frequency. It only really wants to vibrate at one pitch. Thus a quartz crystal, with a Q value of a couple of million, might be very easy to drive at 10,000,000 Hz, and take twice as much power to drive at 10,000,002.5 Hz. It makes a good clock, since the 'ticks' are all the same. An aluminum tuning fork, with a Q value in the high thousands, sounds pretty musical (if boring), since it makes only that one pitch. The BRW sample cited above will also give a much stronger impression of pitch than the oak one.

The 'ring' and 'pitch' tests are the ones most people use to tell if something has low losses. They are not infallible, but not bad.

Alan, this is great reading! Thanks! Do you know if there is a book or website that lists the approximate ranges in Q values of various wood species?

Most wood testing is done from the perspective of cabinet makers or building or aircraft construction; they're not as interested in the acoustic properties as we are. Daniel Haines did a lot of testing of tone woods, mostly directed at violin family stuff, under the aegis of the old Catgut Acoustical Society. His results wre published in their 'Journal', which might be hard to find now. It was also re-printed in a number of places. Again, most of the woods he tested were violin woods, although he did look at a few samples of mahogany, rosewood, and so on.

Brian Burns did a lot of testing of top woods, and did publish the results, I think.

Until a few more of us get around to publishing you're just going to have to do some of your own testing. Then you can tell US!

Thanks. Maybe I'll try some testing on my own. I just need to buy me a stock of tonewoods first

I appreciate you people taking the time to answer my newbie questions