Re: Adirondack Spruce
- From: "alcarruth" <alcarruth@xxxxxxx>
- Date: 31 Mar 2007 14:59:29 -0700
Dave Schramm wrote:
"I've been using Cedar with a density of 16-17 lbs/cuft. Have you
found any
lighter than that?
What is the lightest Sitka you have found? Anything in the 16-19 lbs/
cuft
range? "
If I did my conversions correctly, all of the wood I've been testing
is denser than that, although some of the cedar I have approaches the
19 lb/f^3 density. It's always hard to get exact readings on thin
pieces of wood, some of which are still rough cut, and I'm generally
more interested in the relative readings than an absolute number.
Tashi asked:
" Is this information subjective or do you have data , methods of
obtaining this data etc. ?I would be particularly interested to know
how you tested the samples of spruce and came to your conclusions
concerning Damping of each wood."
This is based on data. I use my signal generator and a speaker to
drive a top or back blank half, and find the resonant frequencies in
lengthwise and crosswise bending, as well as the 3dB down points. I
get the levels by taping a small piece of iron 'metglass' to the edge
of the plate, and reading the amplitude with a magnetic pickup. I have
a little program that I wrote that calculates the lengthwise and
crosswise Young's modulus, damping factors, and density, from the
frequency information along with the dimensions and masses of the
pieces. This is similar to the procedure that Daniel Haines used,
except he tested smaller strips and hade a better balance. Brian Burns
has also used this method. Haines' paper has been reprinted a number
of times in the Catgut Acoustical Society 'Journal', and the 'Journal
of Guitar Acoustics', available from Tim White. I learned this method
from Morton Hutchins, Carleen's husband, who worked with Haines and
published some seperate papers based on such testing.
This is not perfect, of course. In particular, depending on the aspect
ratio of the pieces, there can be torsion or other modes close in
pitch to the ones you are trying to measure, and that will throw
everything off. In that respect, the strips are better, but to get
accurate weights on them you need a lab balance. Besides, once you've
cut a top into strips 1/2" wide, you can't very well make a guitar out
of it. As I said above, I know I'm not getting 'lab' results with
this; it's more a 'shop' technique that allows me to rank my wood
relatively.
Olegario:
Get a copy of Howard Wright's 1996 thesis, "The Acoustics and
Pschycoacoustics of the Guitar" (I hope I remembered the title
right!), given at the Universtiy of Wales in Cardiff under Richardson.
You should be able to download a copy of it in .pdf format from the
university web site. He developed a computer model of the guitar,
modeled the strings, and using that as a driving force, modeled the
sound the 'instrument' would generate at some distance in front. He
was then able to make changes in the 'instrument', and have people
compare the sounds to see what effect various changes had. His model
has its limits, of course, and I don't agree with all of his
conclusions, but there's a lot to learn in it. He describes his
methods quite well, I believe.
Alan Carruth / Luthier
.
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