Acoustic Guitar Terminology"DAMPING"=Luthiers Please chime in

[charles Tauber]

Originally Posted by Knives&Guitars

And I like you added opinion about weight being a bigger factor with necks. Would love to have you expound more on this.

The point that I was trying to make is that each individual guitar is an integrated system of inter-related variables. One can obtain "similar" responses by altering the value of one variable while countering that result by altering a different variable. For example, some achieve the response they want by using a heavy neck, while other achieve the response they want using a lighter neck. In each case, it is part of a whole, of what else has been done as part of the use of that neck weight. Simply using a lighter-weight neck, for example, doesn't guarantee the response you want. Ditto for choosing a fingerboard based on the damping properties of its material.

Playing at it, conjecturing what component contributes what to the sound of the final instrument, can be fun, but it doesn't really take you any closer to achieving a guitar that has the sound you want. If you really want to begin to understand how it works, re-read varve's (Dave Olson's) post.

Keep in mind that most current guitar design is based on empirical, rather than analytic, methods. Through trial and error, makers have arrived at what "works" and what doesn't. Only relatively recently, has there been much hardcore science applied to guitar making.

[AcousticDreams]

Originally Posted by runamuck

This is anecdotal, but my experience with various woods and their damping characteristics, have to do with high resin and mineral content such as silica. Take teak or lignum vitae as two examples, 2 woods that are terribly dead acoustically.

Actually I really like this observation-Theory.

I pioneered a straightening technique for Port Orford Cedar arrows, some 25 years ago. Port Orford Cedar Was(still is) the Number#1 choice for those whom still use wood for their arrows in traditional archery. Since the dawn of time, Heat has been used to help straighten arrows. Sometimes the heat was created through compression-rubbing techniques, and other times through fire.

I experimented using a variation of this technique that more closely related to tempering of Steels. The idea here is to harden. Tempering of wood for the laminates in Limbs of the bow has been done since the nineteen fifties. Specifically with Bamboo(which of course is actually a grass).

Wood arrows are rather precious, and extremely expensive at this point. Even by the king of manufacturers of Port Orford Cedar arrows, often there are arrows in the batch that will not hold their straightness as compared to the rest. I surmised this was due to Pitch pockets. Using the best of my abilities I would determine the weak part of the arrow. I would then target this area using a low heat source using rapid movement, in order not to burn that specific area. If one is patient enough, a small puff of smoke will release and the result is that area becomes stiffer. This can turn what would be a secondary arrow, into Number one arrow quality.

Water curing - drying was also done for centuries by some, with the woods used to make the bow itself. As we all know, there are some who believe that water drying of wood makes for great tonewoods. Some theories include the concept that the resins are washed out, instead of baked in.

[AcousticDreams]

Originally Posted by charles Tauber

Keep in mind that most current guitar design is based on empirical, rather than analytic, methods. Through trial and error, makers have arrived at what "works" and what doesn't. Only relatively recently, has there been much hardcore science applied to guitar making.

Personally I place the highest value on Empirical data over science & their corresponding numbers. I do not disregard science and numbers, they are a wonderful starting point. But there is no substitute for trial and error - experimentation.

As the years progress, our viewpoints on science and numbers are sometimes is reevaluated, and thus a new reality is disclosed. Sometimes Logic trumps science. But the king of realities is most often proves to be simple trial and error.

Learning what I have just learned from you all, has helped me to reconsider where I would like focus my attentions on for changes in guitar tone.

[charles Tauber]

Originally Posted by Knives&Guitars

Sometimes Logic trumps science.

Science is based on logic, without which there is no science. If logic trumps science, then it wasn't science but something masquerading as science.

[runamuck]

Originally Posted by charles Tauber

Science is based on logic, without which there is no science. If logic trumps science, then it wasn't science but something masquerading as science.

Exactly. It's only when empirical data isn't possible, or there isn't a large amount of it, that theory is regarded in science. And even then, in Science, a theory is as good as you get, the culmination of reasoning, logic, observation and experiment.

When talking about the properties of wood, there's plenty of empirical data that's been amassed and spending 1/2 an hour googling any number of details will show that. The anecdotal experience I gave has some value but not much on its' own.

[AcousticDreams]

Originally Posted by charles Tauber

Science is based on logic, without which there is no science. If logic trumps science, then it wasn't science but something masquerading as science.

Originally Posted by Knives&Guitars

Sometimes Logic trumps science.

Yes you are absolutely correct. The key to my statement about Logic Trumps Science is my preface word used. "sometimes".

Masquerading as science, as you said...or it was that we at that time in history, they did not use the most accurate methods for collecting the data.

I have seen too many times throughout might lifetime where Medical conclusions, and scientific data has taken a turn in its basic premise.
Empirical combined with anecdotal notation always has my fullest attention.
Of which all of you have given me plenty of. Again Thank you.

[rick-slo]

The body of "science" is a collection of theories that can be tested by the scientific method. Confidence in a theory is
obtained by having what it predicts about the world agree with the results of testing. Many "theories" out there are not
testable and therefore philosophical, not scientific. Also you can never prove a negative (the source on many
discussions on the forum unfortunately) and any such proposition is not part of science.

[Alan Carruth]

I pretty much just skimmed the posts so far; if I repeat something I apologize.

Damping is, as has been said, a measure of how quickly the energy of vibration is dissipated. So far as I can tell there does not seem to be any strong correlation between the other properties of wood and damping. For example, it is often thought that high density and low damping go together. However, it's not uncommon for Western red cedar to have damping as low as BRW, while the African Blackwood I have is denser than most BRW but has higher damping. As with all wood properties it can vary a lot, and all you can do is measure it.

One way damping is often characterized is by a measure of 'Q value' or 'Quality factor'. It's a number that tells you what proportion of energy is being dissipated per complete cycle of vibration at the measurement frequency. A Q=100 means that 1% of the energy is being dissipated per cycle, while a Q= 30 says it's 3.333%. This works like compound interest, so a Q of 100 doesn't mean the energy in the system is zero after 100 cycles: if I'm using the 'rule of 72' right, it's more like half the starting energy after 72 cycles, and 1/4 after 144. Damping is most easily measured at a resonance.

One way, used in a study by Haines, is to drive the piece at resonance, and then stop the driving signal. The amplitude will diminish, and you count the number of cycles that are needed to get the amplitude down to a given percentage of the initial amplitude (you can also record a tap, and look at the wave form) This gives a number called the 'log decrement'. I'm not totally clear on this, since I've never used it, but Haines' study has been published a number of times (in the Catgut Acoustical Society 'Journal' and, I believe, elsewhere; possibly 'American Lutherie' or 'Guitarmaker').

Note that, in general, 1/Q=log decrement, with the caveat that some people use frequency in Hz for log dec and some use radians/second. If you see a 'pi' in there they used radians. Q is dimensionless.

The method I use is based on resonant bandwidth. The peak frequency of the resonance is determined, and also the frequencies on either side of the peak where the amplitude is .707 times that (1/2 the square root of two), or, equivalently, 3 dB down. At those points the energy in the system is 1/2 that at the peak, since the energy goes as the square of the amplitude. The numbers are then plugged into a simple equation:
Q= Fpeak/(Fhigh-Flow), where
Fpeak is the peak frequency,
Fhigh is the frequency above the peak where the amplitude is 3 dB lower, and,
Flow is the frequency below the peak where the amplitude is 3 dB lower.

There are a couple of ways to get those numbers. I like to drive the piece of wood using a signal generator at the frequency of the fundamental bending mode. Once the peak frequency is found the signal generator is re-tuned to lower and higher frequencies without changing the power output to find the 3 dB down points.

Another way is to use a computer to record a tap tone, and then run it through a spectrum analyzer to find the resonant peak. The 3 dB down points are read off graphically.

You need to keep in mind that there is no 'the' damping factor for a piece of wood. For one thing, each type of deformation will have it's own damping factor. Thus there are three different damping factors associated with bending (longitudinal, radial, and tangential), plus six associated with shearing deformations, and six more for the Poisson's ratios. And that's a minimum.

Haines found that the lengthwise (longitudinal) and cross wise (usually radial) bending modes he measured were not only different from each other, but also seemed to vary with frequency. He was not at all sure why this would be the case. In his tests most softwoods tended to have low damping at lower frequencies, and it rose slowly up to 2000 Hz, after which it rose more quickly, with a pronounced 'dog leg' in the plot. One that worked differently was Sitka spruce, which seemed to have higher damping at low frequencies, which fell off until it dog legged up at 2000 Hz.

Since longitudinal and radial damping are different, it's important when testing for damping to insure that what you're seeing are 'pure' bending modes. Normally this is done by testing narrow strips of wood (as Haines did) so that any radial bending modes in a longitudinal strip would be much higher pitched and thus not couple. If you're testing tops and backs that you intend to use it's harder to be sure you're not looking at mixed modes unless you actually see the Chladni patterns; curved node lines or ones that run diagonally across/along a plate instead of parallel to the edges (of a rectangle, right?) are giveaways there. In the case of 'closed' 'X' and 'O' modes they are totally mixed, and tend to be close to the average between lengthwise and crosswise damping.

It's very hard to know what the actual effect of material damping is on a guitar, at least in the normal range of damping that we see in wood. For one thing, when you're testing a guitar you're looking at the whole system. In some sense, I believe the material damping sets a sort of upper limit on what can be accomplished in the structure, but I suspect we almost never even approach that limit. It is distressingly easy to make a bad guitar form good wood. Poorly chosen thicknesses for the parts, out-of whack bracing, poor build quality and glue joints, and so on, can knock the damping right down.

It would be nice if we could isolate damping as a variable. I have a WRC top and a Red spruce top that are, for all intents and purposes, 'identical' in terms of density and stiffness, but have very different damping. In theory, one could make two guitars with these two tops and know what the damping does by the difference in the way they sound. The problem is that you first have to be able to make two guitars from 'identical' wood that sound the same, and do that a few times so you know it's not a fluke. So far I have not even been able to do that once, and I wonder if it's even possible using wood.

Wright used a computer model back in '96 to isolate variables on a guitar in his PhD thesis (still available on line as a .pdf, so far as I know, from the University of Wales in Cardiff). He was looking at system damping, of course, rather than material damping. He found that even fairly large changes in damping made no discernible difference in sound. His was, by modern standards, a fairly crude model, although the sound it generated were quite guitar-like.

Given that damping tends to decrease response faster at high frequencies (thirty cycles goes by faster at 1000 Hz than 100, and much faster at 6000 Hz). you'd expect that low damping would tend to favor high frequencies in the sound. This surely holds true in the case of high damping nylon strings as opposed to their low damping steel cousins. However, guitars made of low damping wood, such as BRW and WRC, are usually said to be 'darker' sounding than ones made of higher damping woods, such as spruce and mahogany or maple. This is likely one of those places where what the thing actually does is not what it 'sounds like', but, again, with only very limited ability to isolate variables, who can say?

IMO, damping in glue is a particularly vexing question. I'm not happy with any of the experiments I've seen so far that have tried to measure the differences between, say, hide glue and Titebond. In some cases they seem to be looking more at impedance matching than damping. In any case, given that the damping factor of wood is so high, and variable, it seems hard to sort out what's what. I've argued that using something like glass as the substrate would be better, but the argument is that glue bonds differently to wood and glass (does it?) and that would negate the results.

In short, although we know some stuff about the subject of damping, it seems to be far from settled science.

[rick-slo]

"Damping is, as has been said, a measure of how quickly the energy of vibration is dissipated."


I would rather think of it as the percentage of the string's energy that is converted to sound,
and not directly a temporal thing. Some guitar's have a louder sound but a faster decay. The
total sound output may equal that of another guitar that is not as loud but that sustains longer.

[coldfingers]

Sound radiation is actually just another form of damping, though it's not usually thought of that way. The best example of a guitar with minimal total damping would be Les Paul's famous log.

[Alan Carruth]

ric'slo wrote:
"I would rather think of it as the percentage of the string's energy that is converted to sound, and not directly a temporal thing. Some guitar's have a louder sound but a faster decay."

That's more a function of the impedance match between the strings and the bridge-top system. Strings have fairly low impedance. When you tie them to a soundboard that has low mass and stiffness, such as a banjo top, the impedance match between the top and the strings is close. The energy 'leaks' out of the string in a hurry, and gets converted into sound, so the note is loud with little sustain. On a Les Paul the bridge and top are heavy and stiff, and reflect most of the energy back into the strings, so they ring for a long time without producing much acoustic sound.

As far as the string is concerned, the sound coming off the top is damping, but there are lot of other things that can dissipate energy. Nylon strings, for example, lose a lot of energy to heat via internal damping, and a similar amount in pushing the air aside as they vibrate, according to Fletcher and Rossing.

For most instruments sound production is actually pretty far down the list of things that dissipate the energy. Wind and brass instruments have high wall losses, due, in part, to the very high sound pressure levels in the bore. Strings, as has been said, can lose a lot before any energy even reaches the top. The efficiencies of musical instruments tend to be abysmal. They only make any useful sound because so much can be pumped in. It's said that the output limit of the French horn is established by the blood pressure of the player: play too hard and blood flow to the brain is cut off, and you pass out.

Acoustic guitars seem to be more efficient than most other instruments; the generally accepted figure is about 5%. It's hard to measure, in part because there is so little energy in a plucked string to begin with. Violins are about half as efficient as that, apparently, but the bow dumps in so much more energy that they can get away with it.

Of course, you're allowed the think of it any way you like. However, it's a lot easier to talk about this stuff with other people if everybody agrees on what the words mean.

[rick-slo]

Originally Posted by Alan Carruth

Damping is, as has been said, a measure of how quickly the energy of vibration is dissipated.

Yes, I was just going off what you said here earlier. I agree the impedance match effect is an important factor.

[AcousticDreams]

Originally Posted by Alan Carruth

ric'slo wrote:
"I would rather think of it as the percentage of the string's energy that is converted to sound, and not directly a temporal thing. Some guitar's have a louder sound but a faster decay."

That's more a function of the impedance match between the strings and the bridge-top system. Strings have fairly low impedance. When you tie them to a soundboard that has low mass and stiffness, such as a banjo top, the impedance match between the top and the strings is close. The energy 'leaks' out of the string in a hurry, and gets converted into sound, so the note is loud with little sustain. On a Les Paul the bridge and top are heavy and stiff, and reflect most of the energy back into the strings, so they ring for a long time without producing much acoustic sound.

As far as the string is concerned, the sound coming off the top is damping, but there are lot of other things that can dissipate energy. Nylon strings, for example, lose a lot of energy to heat via internal damping, and a similar amount in pushing the air aside as they vibrate, according to Fletcher and Rossing.

For most instruments sound production is actually pretty far down the list of things that dissipate the energy. Wind and brass instruments have high wall losses, due, in part, to the very high sound pressure levels in the bore. Strings, as has been said, can lose a lot before any energy even reaches the top. The efficiencies of musical instruments tend to be abysmal. They only make any useful sound because so much can be pumped in. It's said that the output limit of the French horn is established by the blood pressure of the player: play too hard and blood flow to the brain is cut off, and you pass out.

Acoustic guitars seem to be more efficient than most other instruments; the generally accepted figure is about 5%. It's hard to measure, in part because there is so little energy in a plucked string to begin with. Violins are about half as efficient as that, apparently, but the bow dumps in so much more energy that they can get away with it.

Of course, you're allowed the think of it any way you like. However, it's a lot easier to talk about this stuff with other people if everybody agrees on what the words mean.

Very much enjoying my learning experience from this conversation. I have been doing lots of thinking on this subject matter.

If you don't might me expounding a little bit more on this subject, but in a different context now; acoustic top damping.
I understand that wood damping characteristics are quite different. And it was a pleasure to learn how one wood could be Stiffer & harder, but have a lower damping values than a less stiff, less hard species. And even more interesting at your mention that some of the frequencies effected have an opposite effect as what the numbers of damping theoretically should direct. Not sure I am translating what you said in the best manner, but in your example of Indian Rosewood and Ebony having the same Damping numerical factor and yet Rosewoods are thought of as darker sounding.

I have also heard you mention from previous discussions how as little as two Grams(about 30 grains?) can make a difference in the Top. I realize it is not exactly damping. But stiffness of a top might have a Type of Damping effect? In this regards then being that Indian Rosewood and Ebony have the same damping factors. And that Indian Rosewood is lighter than Ebony. What effects are created by the woods- specifically Bridges? Is it only in response of the top? Because of one bridge being lighter than the other. Thus creating a slightly more open sound as the top can move easier or possibly farther? Or does this difference in weight create a Damping like effect, by being more or less stiff, and alter how the frequencies behave?

[printer2]

I would say that the dampening of the fretboard material will be overshadowed by the neck wood. As said with the slots cut into the fretboard you have effectively twenty individual pieces rather than a solid mass and these little pieces resonate individually at too high a frequency to matter. Also the neck wood would dampen out any fretboard resonance if there was one. The mass of the fretboard would have some influence on the total mass of the neck but with hardness being loosely associated with density most fretboard woods are not too far from each other in density. From my limited data set of one guitar with a dense neck wood I found it gave a less dampened and stronger bottom end than you would expect from the size of guitar.

[Bruce Sexauer]

I have strong ideas about the nature of damping, and have spent a lifetime applying them. Unfortunately, a few years ago Al took me to task re my concept of the meaning of the term, and I have had to wrestle with the possibility that I have no idea what I am doing. Somehow, my guitars continue to improve.