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Technical Discussions
Sandro Cocco
Sandro Cocco
06:54 Sep-30-1999
woodtest for musical instruments

Hi everybody.I am a violin maker who is trying to find a reliable method to test the quality of spruce and maple.It would be useful for me to measure the longitudinal velocity of sound .
How should I proceed? I experimented with piezo transducers located at the extremities,driven by an impulse(a small hammer!)and connected to an oscilloscope.But after just 19 inches the delay is visible but hardly measurable. What advantage would I have in using an ultrasonic frequency? And how could I measure this very short time in a numeric form? Could an impulse counter be of any use? Thanks for your attention.Sandro Cocco.


    
 
 
Tom Nelligan
Engineering,
retired, USA, Joined Nov 1998, 390

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
05:35 Sep-30-1999
Re: woodtest for musical instruments

: Hi everybody.I am a violin maker who is trying to find a reliable method to test the quality of spruce and maple.It would be useful for me to measure the longitudinal velocity of sound .
: How should I proceed? I experimented with piezo transducers located at the extremities,driven by an impulse(a small hammer!)and connected to an oscilloscope.But after just 19 inches the delay is visible but hardly measurable. What advantage would I have in using an ultrasonic frequency? And how could I measure this very short time in a numeric form? Could an impulse counter be of any use? Thanks for your attention.Sandro Cocco.

I've run into this sort of musical instrument test a few times over the years. Wood is highly scattering at ultrasonic frequencies, but it is indeed possible to obtain a useable longitudinal wave signal through distances greater than your 19 inch requirement using commercial instruments in a through transmission (pitch/catch) mode at low frequency (500 KHz or below).

You should consider purchasing a pair of commercial transducers in the 100 KHz/250 KHz range. (These are available from my company as well as various other sources around the world.) There are a number of available ultrasonic instruments that you can use. The best choice is a pulser/receiver optimized for low frequency/high gain operation, used with an oscilloscope (or digital processing) to measure the pulse transit times. Some flaw detectors will also work, but not all have the necessary low end bandwidth.
From your description, the main problem with your current approach is that you are neither driving the transducers optimally, nor amplifying the received signal by the 50-60dB that's typically required.

You will find that wood is highly anisotropic -- sound velocity and attention are very different depending on whether you're going with or across the grain.

--Tom Nelligan




    
 
 
Greg Smith
Greg Smith
06:23 Sep-30-1999
Re: woodtest for musical instruments
: Hi everybody.I am a violin maker who is trying to find a reliable method to test the quality of spruce and maple.It would be useful for me to measure the longitudinal velocity of sound .
: How should I proceed? I experimented with piezo transducers located at the extremities,driven by an impulse(a small hammer!)and connected to an oscilloscope.But after just 19 inches the delay is visible but hardly measurable. What advantage would I have in using an ultrasonic frequency? And how could I measure this very short time in a numeric form? Could an impulse counter be of any use? Thanks for your attention.Sandro Cocco.


Sandro, We manufacture an airborne flaw detector which may be well suited for your requirements. It incorporates the use of a 50KHz transmitter & receiver. We have tested several wood components including Phone poles & conventional building material. If you would like I can send you an application note on the inspection of wood and some of the results we have found.



    
 
 
Thomas Adunka, Am Lorberkogel 8 A-9330-Althofen, Austria
Thomas Adunka, Am Lorberkogel 8 A-9330-Althofen, Austria
03:10 Jun-04-2002
Re: woodtest for musical instruments
Dear Sandro,
I just did find your adress in the internet.
How are you? Where are you living and working at
the time?
When we met at Oberlin, you told me, you would
visit Italy and also Austria. Please, let me know, if you have any plans to do so.
For my own, I'm mostly repairing and restoring instru-
ments. Playing a lot and always remembering your
famous interpretation of Bachs C-major Sonata.
Now in summer I want to make some new violins and
also do some varnishing.
It would be nice hearing from you soon.
My new adress: Am Lorberkogel 8
A-9330 Althofen, Austria
Tel & Fax 0043 4262 4314
You can also use my fathers e-mail adress.
Best wishes Thomas


    
 
 

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