Ridiculously easy way to measure speed of sound?

[David Burgess]

That looks awfully low... C=4682 is mighty low for spruce, unless it's soaking wet.

Nurse me along here. I've had too many irons in the fire for the last couple of weeks, and haven't digested everything which has gone on.

"Low" means speed of sound, or frequency, or both?

The wood is about 20 years old, stored under controlled conditions, so it's not wet. My next prominent peak occurs at 7347 with a mic, but the piezo strip didn't really notice this.

Here's a link you may find useful:

How the block is held should be important. A xylophone has resonators (blocks, pipes, whatever) which lie in from the ends. The rest points become nodes for vibration. I think that to measure the speed through the piece of wood, you need to support it the ends (only.)

Doug

Doug, that may be helpful to reduce the clutter. My presumption was that a piezo strip would pick up less non-significant vibration than a mic, but I'm always learning.

[Salve Håkedal]

This worked very well for me! Thanks Don!

My hammer is a dice of ebony mounted on a nylon tail piece loop. It weights about 2 grams.

I planed the end of the billet before I knocked it lightly. The mic close to the other end, but not touching.

The billet is one of a two-piece.

The peak in the specter plot is very prononced at frequency 6785Hz.

It's 423 mm long so speed of sound must be 5740

[NewNewbie]

Here is the link to Whack-a-Stick

[Don Noon]

Here's a link you may find useful:

Sylvie

How the block is held should be important. A xylophone has resonators (blocks, pipes, whatever) which lie in from the ends. The rest points become nodes for vibration. I think that to measure the speed through the piece of wood, you need to support it the ends (only.)

But what do I know...

Doug

Doug,

You (and the link) are describing bending modes, which is the "old" way I used to measure the speed of sound. That DOES need support at the nodal points, .224L in from the ends. This thread is about compression waves that travel along the fiber. No bending.

Nurse me along here. I've had too many irons in the fire for the last couple of weeks, and haven't digested everything which has gone on.

"Low" means speed of sound, or frequency, or both?

The wood is about 20 years old, stored under controlled conditions, so it's not wet. My next prominent peak occurs at 7347 with a mic, but the piezo strip didn't really notice this.

David,

Speed of sound is proportional to frequency in this measurement method. They go together.

Your second peak at 7347 looks more reasonable than the other one, giving C = 6362 m/s, which looks very high... but I assume you have really good wood. They both could be wrong, though.

Slave's planing of the impact zone should also give a better measurement, as it would help keep the impact as sharp and short as possible. A fuzzy end grain would give a whomp, not a smack, and with the high-frequency measurements we are looking for, we need the smack.

[David Burgess]

Hah, Newb, you come up with some good stuff.

My knee-jerk reaction to "whack a stick" might be different from that of others, but I think its clarified sufficiently.

[NewNewbie]

Here is the simpler version.

http://cte.napier.ac.uk/firrs/whack-a-stick.php

http://cte.napier.ac.uk/firrs/videos.php

3rd one down.

"Measuring stiffness with sound download the video in flash format (flv)

There is also a pdf information sheet for this video"

Measuring sitiffness with sound.pdf

All this came from Doug Rice's 'Sylvie' link in post #25.

Thanks Doug!!!!

[Don Noon]

What was the 'old way' calculation found to be in m/s?

Was it 2.5 % higher or lower?

Thanks

The old way (bending mode) measured 2.5% higher. I haven't done enough research to determine what could cause the difference, but I have some ideas: "surface hardening" of the lumber (it' at least 25 years old) causing a higher bending frequency, or something related to the point impact of the new method causing a slight lateral component. In any case, I'm pretty happy with less than 3% disagreement.

It would be great if someone who has a Lucchi meter would send it to Don, or measure the same samples Don has measured.

Who has a Lucchi meter?

I know Anders has one. Probably the best idea is for those with the meter to do the smack test themselves, and compare to their Lucchi meter readings. I wouldn't mind if someone wants to send me one, though.

[Melvin Goldsmith]

Is it good to have wood that sound waves travel through fast?

[Anders Buen]

Is it good to have wood that sound waves travel through fast?

I think so. It should be equivalent to stiff wood. If the density is low, then some think it is even better.

[Melvin Goldsmith]

I think so. It should be equivalent to stiff wood. If the density is low, then some think it is even better.

Anders

Thanks for your reply...Would low density stiff wood always have a fast speed for sound or could it not have other factors introducing damping as an unrelated variable?

[stradofear]

Aw, Melvin, I thought you'd next ask "is stiff wood better"? Because I want to know that, with specific evidence to prove it.

[Anders Buen]

The old way (bending mode) measured 2.5% higher. I haven't done enough research to determine what could cause the difference, but I have some ideas: "surface hardening" of the lumber (it' at least 25 years old) causing a higher bending frequency, or something related to the point impact of the new method causing a slight lateral component. In any case, I'm pretty happy with less than 3% disagreement.

I know Anders has one. Probably the best idea is for those with the meter to do the smack test themselves, and compare to their Lucchi meter readings. I wouldn't mind if someone wants to send me one, though.

3% is a very good agreement. I do have a Lucci meter and some spruce blanks with the numbers on. So will try it now.

Have anybody tried to get the crossgrain numbers? Maybe not as important, but still worth measuring.

[go_oa]

You can get a small device that plugs into a printer port

and (after installing software) turns your computer

nto an osciliscope. With FFT in the software.

It was about $50 when I got it.

[Doug Rice]

Is it good to have wood that sound waves travel through fast?

That's a very good question Melvin.

Now, since sound travels faster in brass than in spruce....

[Don Noon]

Anders

Thanks for your reply...Would low density stiff wood always have a fast speed for sound or could it not have other factors introducing damping as an unrelated variable?

Aw, Melvin, I thought you'd next ask "is stiff wood better"? Because I want to know that, with specific evidence to prove it.

The often-repeated theory is that speed of sound divided by density is a "figure of merit" for wood, as a higher number will allow you to get the target tap tones or modes while using a lighter weight plate... and ligher weight supposedly equals more responsive and louder. Speed of sound itself is a measure of stiffness divided by density, so you can see density enters the "figure of merit" twice. In other words, lighter is gooder. Stiffness too.

Damping is a separate consideration, although from my measurements I have seen some correlation between damping and stiffness (inversely).

Of course, there seems to be no proof that any of this relates to how good the instrument sounds. Yet. But, is there any proof of ANYTHING related to quality of sound?

[Melvin Goldsmith]

That's a very good question Melvin.

Now, since sound travels faster in brass than in spruce....

.......................

I have experienced free plates on good sounding instruments that rang like bells....I have also experienced free plates that were as dead as cardboard to tap tones but were parts of very good and maybe the best playing/sounding instruments.....I'm pro science

[Don Noon]

That's a very good question Melvin.

Now, since sound travels faster in brass than in spruce....

That's either fancy brass or crappy spruce. Brass is under 5000 m/s, spruce is generally 5000 - 6000 m/s.

Now, brass at 892 Kg/m^3 and 4700 m/s gives a radiation ratio of 5.3 compared to spruce at 13 - 16. One reason folks don't make fiddles out of brass.

[Anders Buen]

I tried the whacking experiment on three blanks here and measured the sound speed using a Lucci meter.

It was not easy to see which resonance to look at, by no means my curves looked as nice as Salves or Dons plots. But it turned up a resonace in the 7,5kHz range and it was split for two of the blanks I measured, of some reason. I wacked using a peg. I think the peg itself had a resonace in the 8kHz range. I tried a somewhat heavy knife too whacking in the stiffest direction, but I think it did ring too much. The peg worked best. Here are the numbers:

Blank No 6: Lucci, Whack: 5454m/s, 5381m/s

Blank No 7: 5710m/s, 5818m/s

No 8: 6000m/s, 6005m/s

So the "error" was less than 2%. The numbers one get with the Lucci meter does also wary. I choose the highest value I read out. The moisture content will have an effect so I have some different readings on the blanks, but not by large numbers.

[Janito]

It was not easy to see which resonance to look at, by no means my curves looked as nice as Salves or Dons plots.

If the peaks are not easy to see, is there a selection bias towards resonance numbers that 'make sense'.

Frankly, I don't think anyone can replicate Don's or Salve's data because the experimental set-ups have been so inadequately standardised.

I am guessing someone out there is hitting a piece of wood with an old shoe and not being able to match the 'expectations'.

[Don Noon]

Maybe I'll have to design up an automatic precision wood whacker, which I'll sell for only $2999.95.

Yes, one trades off cheap and quick for some other difficulties.

[Janito]

Maybe I'll have to design up an automatic precision wood whacker, which I'll sell for only $2999.95.

There will be some who will pay this for a standardised procedure that guarantees Goodness.

--------------

I agree that simplicity often has it's trade-offs, but that is not to disparage the value of the Noon-Whacker (perhaps the 21st century's alternative to the 'orgasmotron')

[David Tseng]

After the supper, I decided to make the impact hammer: 1/8" rod with piano wire/sound post handle. I tested on a spruce wedge 44 cm long. The 1st peak is at 6158 Hz, C=5419 m/s. The old way 5432. Density 343 (mks) RR=15.8. The 3rd peak is more than 3f, I don't know why. As the frequency increases, sound wave tends to travel in a straight line. Therefore, a Lucchi meter can detect the variation in the radial direction of along the grain velocity of sound. Also wood is a dispersive material, that means velocity of sound varies with frequency. We must interpret the sound velocity with this effect in mind.

[Anders Buen]

If the peaks are not easy to see, is there a selection bias towards resonance numbers that 'make sense'.

Frankly, I don't think anyone can replicate Don's or Salve's data because the experimental set-ups have been so inadequately standardised.

I am guessing someone out there is hitting a piece of wood with an old shoe and not being able to match the 'expectations'.

I rested my blanks on four foam rubber pieces (Syolmer gray, the stiffest brand, but in small pieces) and damped the out of plane modes using a ruler or my hands lying on top of it. I spent a very short time on this as I am midst in an intense process writing an article on room acoustics matters with a near deadline..

I think it is important not to fool peaople to believe it is a simple matter to do such measurements. Quite frankly I think we may expect a lot of confusion. But with a good training, techique and some knwledge what to look for, this might work.

[Anders Buen]

After the supper, I decided to make the impact hammer: 1/8" rod with piano wire/sound post handle. I tested on a spruce wedge 44 cm long. The 1st peak is at 6158 Hz, C=5419 m/s. The old way 5432. Density 343 (mks) RR=15.8. The 3rd peak is more than 3f, I don't know why. As the frequency increases, sound wave tends to travel in a straight line. Therefore, a Lucchi meter can detect the variation in the radial direction of along the grain velocity of sound. Also wood is a dispersive material, that means velocity of sound varies with frequency. We must interpret the sound velocity with this effect in mind.

Good. I think the dispersion effect is limited to the bending waves and not the compressional waves.

I like the look of your hammer.

[Anders Buen]

Are you guys letting the mic be in direct contact with the wood?