Official Luthiers Forum!

Yes, we do like that curly wood...mostly for sides and backs where we know wood to be the spice rather than the main ingredient of tone.   

In our uke building here, I have found that as pretty as really curly koa may be, it does not make tops that sound as good as the boring straight grained stuff, and this doesn't surprise me one bit.   The stuff is all runout.   That said, my "go-to" uke at night is my totally blinged out curly koa one, though I suspect that will change when I get my spruce topped one finished up.

BTW, myrtle is a fantastic wood for backs and sides...

I think I'm going to make a Lucchi meter my indulgence of the year...

[QUOTE=Jim_W]I didn't check out the link yet, but i believe the Lucci
meter is measuring the speed of sound? If thats true, then the density
and modulus determine that. [/QUOTE]



Yep...assuming we can all measure density with the same degree of
accuracy, if you know Young's Modulus you can calculate the Speed of
Sound, and vice versa.



The real question is, with all these setups, which (YM or SoS) can be measured more precisely?  More accurately?



Anyone know if the Lucci meter can measure SoS in air?  In
water?  This would go some way toward testing its accuracy as
these are well known quantities if you know the temperature (+ humidity
for air).

Methinks one could possibly do the same test as the Lucchi meter by
using a piezo element from a signal generator to excite the wood at one
end, and a pair of piezo elements at each end fed into a high-speed
oscilloscope with recording capability.



Excite one end with a pulse, and record the pulses received on the
piezos at each end.  The length of the wood, divided by the time
difference in pulse arrivals, is your SoS.



There are some very good high-speed oscilloscopes (like 100e+8 to 1e+9
samples/second) with recording capabilities for under $1500.  The
time resolution of these scopes is about 100,000 times faster than
speed of sound in air over 1 meter.

[QUOTE=erikbojerik]...like 100e+8 to 1e+9
samples/second...


[/QUOTE]



Make that 1e+8 to 1e+9.



Expensive ones can give you 4e+9 samples per second.

Most of the time I've seen vibrations testing it is done with accellerometers and oscilloscopes. Most of my experience is with very large metall things where a few extra ounces (for the accellerometer) doesn't affect anything one way or the other.    You basically figure out the best way to support the thing, and then litterally bonk it with a hammer. The thing vibrates, the accellerometer reads it, and you use the FFT (Fast Fourier Transform) function to ID the dominant modes.

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I think that's essentially what the Lucci does. It sends an impulse into the end of the bow blank with a small piezo element, and then reads the reflection with the same element. That's why runnout is a problem; the sound doesn't run straight along the stick, and rather than hitting a nice flat end to bounce off, it just sort of peters out. At any rate, the ellapsed time is a reading of the velocity of a compression wave in the material, and from the length and density you can derive the Young's modulus. In the bow business I think they often rely on starting out with blanks of a uniform length, and just worry about the time, which will be shorter for lower density or higher E values. There ought to be a low-tech way of doing the same thing, using a computer with an FFT program.    

Lucchi has a probe at each end, one a send (and maybe also receive), the other a receive.   You punch in the path length and ping away.   Yes, this could be hacked, but then you'd probably want to calibrate it to give Lucchi numbers as they have become one sort of standard, at least in the violin biz.   You'd still need a laptop plus transducers plus appropriate signal generator, send amplifier, receive preamp, etc.   Might just be cheaper to buy a Lucchi meter, especially for portability.