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Measuring the Velocity of Sound in wood - directly


catnip

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I was following another thread on Wood properties and I thought I would ask this question as a new topic.

Are the any simpler ways to directly measure the velocity of sound in wood that do not require a huge investment $$$?

I looked up the cost of a Lucchi Meter and it was too costly for my budget. I know that Vernier Scientific sells affordable measuring devices for high school physics teachers but I did not find anything that measures the velocity of sound in a solid material. I am guessing that I would need two microphones, one for each end of the wood sample and super accurate recording device to measure the difference in time ( delta t ) and/or some computer software. I know that the velocity of sound in wood can be measured indirectly ( see one of the SCAVM articles on wood measurements) but I would prefer to measure it directly.

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I was following another thread on Wood properties and I thought I would ask this question as a new topic.

Are the any simpler ways to directly measure the velocity of sound in wood that do not require a huge investment $$$?

I looked up the cost of a Lucchi Meter and it was too costly for my budget. I know that Vernier Scientific sells affordable measuring devices for high school physics teachers but I did not find anything that measures the velocity of sound in a solid material. I am guessing that I would need two microphones, one for each end of the wood sample and super accurate recording device to measure the difference in time ( delta t ) and/or some computer software. I know that the velocity of sound in wood can be measured indirectly ( see one of the SCAVM articles on wood measurements) but I would prefer to measure it directly.

If you are talking about compressional waves, you need to explain why this is important. The bending stiffness of a plate or a curved shell may not be directly related to in-plane stiffness. In a plane plate, there may be no relationship. There would be a coupling in curved shells, and that interests me. It does not interest anyone else that I know of. I have tried to raise the topic in other threads and it gets no response.

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In theory, you only need a frequency counter with timer, a tone source, and two transducers to act as a trigger and signal receptor. I tried making one one time, but the results were not satisfactory due to the difficulties of generating a suitable tone burst (which should be very short and high frequency).

I think such a device could be constructed for less than $500, but it would require some effort, and proper selection of components. The parts I tried were not up to the task.

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For this need to directly measure the velocity of sound in wood,

I offer a remedy.

All that's required is a strong bench vise [preferably metal],

some of your prized tonewood collection, and the following

"Three Step Procedure":

Three Step Procedure:

1. Prepare several tonewood samples, all the same dimension and

handplaned smoother than a baby's bottom.

2. Place a tonewood sample in your vise and slowly crank

the jaws closed ... Don't Stop !!! ... keep turning 'til the wood fibers

are crushed and/or your muscles/tendons/ligaments can bear no

more.

3. Repeat Step 2 on as many samples as it takes to cure this

need to measure the velocity of sound in wood.

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If you are talking about compressional waves, you need to explain why this is important. The bending stiffness of a plate or a curved shell may not be directly related to in-plane stiffness. In a plane plate, there may be no relationship. There would be a coupling in curved shells, and that interests me. It does not interest anyone else that I know of. I have tried to raise the topic in other threads and it gets no response.

I think the bending wave velocity is dependant on the in "plane stiffness", that is the compressional sound speed (also in a flat plate) will influence how fast a bending wave will travel along a plate. The thickness does too.

I think there is a fair amount of interest in the subject you are talking about, it is not easy to think or deal with. Bissingers measurements of 3D vibrations in the VSA Strad project deals with the topic empirically. His later articles deal with it, but not on a fundamental level.

Can you see the In Plane extensional mode of the bridge island area in your violin box FEA model?

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I think the bending wave velocity is dependant on the in "plane stiffness", that is the compressional sound speed (also in a flat plate) will influence how fast a bending wave will travel along a plate. The thickness does too.

I think there is a fair amount of interest in the subject you are talking about, it is not easy to think or deal with. Bissingers measurements of 3D vibrations in the VSA Strad project deals with the topic empirically. His later articles deal with it, but not on a fundamental level.

Can you see the In Plane extensional mode of the bridge island area in your violin box FEA model?

John, I think there would be some interest among those makers and reesearchers at the VSA Oberlin Violin Acoustics workshop. Would you consider to attend the 2009 workshop?

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Now Bill, I empathize with your situation. In fact, I too have been known to introduce controversy from time to time for attempting to inject humour. The general solution to this ongoing problem is merely to be more subtle in the choice of words, rather than abort the mission.

For example, had you instead said “Is this a Freudian slip, or has an abundance of fatty tissue accumulated within the vicinity of your distal and proximal interphalangeal joints?” I am sure everything would have been just fine.

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Many years ago, I was contemplating to put one together but had to give up due to the time involved. The major electronics components are available in IC form. The piezo-materials such as PZT (lead zirconium titanate) could be cheaply obtained. Most of the ultrasound devices operate in the range 1-3 MegaHertz. I think it would be better that several people pool together $2500 and buy one Lucchi meter.

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Many years ago, I was contemplating to put one together but had to give up due to the time involved. The major electronics components are available in IC form. The piezo-materials such as PZT (lead zirconium titanate) could be cheaply obtained. Most of the ultrasound devices operate in the range 1-3 MegaHertz. I think it would be better that several people pool together $2500 and buy one Lucchi meter.

The signal is a 40kHz or 50kHz pulse. Jonh McLennan from Australia has investigated how it works and published the results on his website. I think the electronics is very simple, at least to an expert in the field.

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I attended a woodshow today and asked several wood (experts) dealers if they knew anything about measuring the velocity of sound in wood, and was greeted with the same response "Why do you want to measure it?" Without going into detail why, I just answered "If you don't ask questions you won't find out the answer".

I suppose you can measure it with an oscilloscope, a microphone and a hammer. Twice the length divided by the time difference between the initial pulse and the reflected pulse should give a rough estimate of the velocity... I guess.

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I discussed this design issue with my son, an EE at Lincoln Labs, and we came up with a couple of designs. The simplest approach is to get a dual-trace oscilloscope from eBay and use your computer or a function generator to generate the tone. You measure the time delay between the input and output signals.

The more advanced idea uses a PLL (phase locked loop chip) to measure the phase shift which gives you the time delay. Yes, you can make a cheap device, but there is your time invested in development which can be non-trivial.

Anyhow, I have some questions about the practical nature of this.

(1) Aren't there two velocities due to the different stiffness in cross-grain and parallel-grain? Which is important and why?

(2) So you go through your wood pile to find the "best" pieces. What do you do with the rest of the wood you bought? Alternatively, you could take your tester to a tonewood supplier - maybe. How about at a VSA vendor exhibit?

:)

Mike

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I think the practical benefit of making certain measurements would not be for selecting wood (at least early on), but rather for gaining information.

If one had a table of the tonal characteristics of their violins coupled with the various properties of the wood, SOS, MOE, MOUSE, (O.K. the last one was a joke, but for Canadian makers it could stand for Modulus Of Useless Stuff Eh!) certain insight might be had when correlating the results back to the wood’s properties.

Of course it could never remotely be a science, especially with so many other factors at play, but the utility of knowing such things is that it adds to the collective body of information one possesses, that later manifests itself as intuition. :)

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(1) Aren't there two velocities due to the different stiffness in cross-grain and parallel-grain? Which is important and why?

(2) So you go through your wood pile to find the "best" pieces. What do you do with the rest of the wood you bought? Alternatively, you could take your tester to a tonewood supplier - maybe. How about at a VSA vendor exhibit?

:)

Mike

(1) They are both important. One engineer/maker I know of multiplies the cross and parallel radiation ratios and takes the square root of the product to come up with a single "quality number". I came up with the same idea independently. Conceptually, the reason is that a radiation ratio only tells one dimension of "goodness", but to radiate sound, AREA is needed... hence the multiplication of cross and parallel parameters. There could be other secondary influences making one more important than the other, but until more is known, this looks like a good start.

(2) I agree partly with GMM22 about gaining information and trying to correlate numbers with the resulting tonal qualities (however you manage to do THAT). However, I think that there is theoretical and possibly some experiencial evidence that higher radiation ratio is generally desirable. I am not clear about the capabilities of ultrasonic testing... can it only measure speed of sound, or density too? SOS is just part of the information that is needed; density is the other. If both can be measured accurately with the device, then I could envision sorting thru a supplier's stockpile for the "good" stuff... if they let you. The guy I mentioned in (1) tests a supplier's log (actually just a sample thereof) and gets only billets cut from that particular log. As for sorting through one's own stockpile, you'd have to make your own decision what to do with "the rest". Experiments perhaps, or a lifetime supply of endblocks and corner blocks. Or build a violin, put some Italian-sounding label in it, and list it on eBay. :)

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  • 1 year later...

I have now come to the point that I want to know some property of wood I use. This interest arises mainly from reading Don Noon's posts about wood processing. There are 2 ways I can proceed: (1) Spend $3,000 to buy a Lucchi meter. When I go to tonewood supplier, I would bring Lucchi meter, an electronic scale, a measuring tape and a finger plane. I would measure the volume and weight of the spruce wedge to get the density. Then I would plane a small area on the end grain smooth and put on the transducers to get the speed of sound. In this way, I only buy the wood deemed desirable. All the suppliers I dealt with allow me the pick through their stock, but have to pile them back in the original way. (2) Continue the old way: buy only lighter wood. When I am home, cut a piece of wood from the wedge to make bassbar, and measure the xylophone frequency and density.

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