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A question was asked about this on another thread, which has now been closed. I didn't want to leave it floating.

First, I have to point out that when I use the term 'damping', it is a 'term of art': that is, it has a specific meaning that may not be exactly the same as the common one. In general usage, anything that cuts down on the amplitude of a vibration is said to 'damp' it. Thus, for example, adding mass to the bridge, say by using depleted uranium bridge pins, would 'damp' the vibrations, since the heavier top would be harder to move. In acoustics, 'damping' refers to the various mechanisms that dissipate energy from the system. These 'losses' can be of many kinds; internal friction within wood, poor design or construction, and even the sound that is radiated from the guitar are all dissipative. They are all 'losses' so far as the system of the guitar is concerned, but the added mass of those DU bridge pins is not. Thus, in acoustic usage, the heavy bridge pins don't add to 'damping', but an increase in sound output from the top probably does. Some types of damping are good!

One characteristic of acoustic damping is that it depends on how much the system is moving. It's like a rusty bearing that might keep a swing from swinging; so long as nobody's using it, the rust doesn't matter. It's only when you're trying to swing on the thing that the rust gets to be a problem. The higher you try to swing the louder the thing squeeks; you're putting more energy into damping, and it's coming out as sound, rather than getting the swing higher in the air.

One outcome of this property of acoustic damping is that the loss tends to be a certain proportion of the signal per cycle. That is, if you look at the amplitude of the swing you'll see that after, say, twelve cycles, it will have dropped by half, twelve cycles later it will be half of the half that remains, and so on. It's a logarhythmic scale, like frets.

So, if the loss is proportional to the number of cycles, it's going to tend to kill off high frequencies faster. If the amplitude drops by one half every 1000 cycles, then at 100 Hertz (cycles per second), it's going to take ten seconds to drop that much, whereas at 1000 Hz it will only take one second, and 1/10 second at 10,000 Hz.

Low damping in wood thus tends to yeild a guitar with more high frequency sustain. This can manifest itself in several ways in terms of tone, and certainly different people hear things differently. Because of the way our ears work (which is a whole other huge subject!) low damping often comes across as a more 'rich' or 'full' tone, and not necessarilly as 'bright'. 'Clarity' is another characteristic that is often associated with low acoustic damping. In some cases low damping in particular ranges can lead to a 'forward' or 'harsh' timbre.

Given the limited energy we have available in strings it's usually a good idea not to 'waste' any of it in acoustic damping, at least, not in the wrong sorts! But low damping is not always a good thing either, nor is higher damping always bad. As usual it seems to me that the more you understand about it the more likely you are to be able to get what you want from what you've got.

I apologize for taking so long to answer the question, and hope this helps.    

Although it wasn't my question, this was terrific, as always...I just never get tired of reading your posts Alan. Thank you.

Bill

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Eagles may soar, but weasels don't get sucked into jet engines.

Thanks Al,

Your posts are compulsory reading here.

Cheers Martin

I believe what you are saying confirms my usage and perception of the
word "damping", but I'm not entirely sure. Specifically in relation to
mass, inertia, impedance, etc., of the bridge - I'll give an example in my
words, you tell me if it lines up with your thinking.

Adding mass to the bridge could be said to dampen the top by limiting
it's response and movement, in general usage of the term. As a trade
specific term however, the description would not be entirely adequate
because when considering the instrument in it's entirety there is no
energy being lost. Rather, more energy is simply being reflected back in
to the string and kept within the system. Does this agree with you? If so it
seems this description could be a bit at odds with others, as I would think
adding a bridge mute to a violin could be reasonable described as
damping. Or perhaps it wouldn't disagree, as you could say you are
damping the bridge and soundboard, but not the system as a whole.

Second, when referring to damping do you differentiate between a
vibrations lost from the system in a desirable way (setting air in motion),
and a non-desirable loss such as friction and heat? I believe you
answered that above, but I just want to be clear.

I'm not touching on discussion of the physics or practical results of
damping, but rather just making sure we're putting the same meaning to
these words as we use them.

When is someone going to write a dictionary/thesaurus of lutherie terms?
I suppose as soon as someone is cocky enough to believe they have
ultimate authority to do so.

_________________
Eschew obfuscation, espouse elucidation.

David Collins wrote:
"Adding mass to the bridge could be said to dampen the top by limiting it's response and movement, in general usage of the term. As a trade specific term however, the description would not be entirely adequate because when considering the instrument in it's entirety there is no
energy being lost. Rather, more energy is simply being reflected back in to the string and kept within the system."

Right; by adding mass you've changed the impedance of the bridge, so that there is a greater mismatch between the string and the top. More energy is reflected back into the string at the saddle, particularly at higher frequencies.

"I would think adding a bridge mute to a violin could be reasonable described as damping."

In common usage, yes, but in physical terms, no. That's the point, at least in part; we must be clear about how we're using the terms. This particular sort of confusion was only recently pointed out to me in another context.

"Second, when referring to damping do you differentiate between a vibrations lost from the system in a desirable way (setting air in motion), and a non-desirable loss such as friction and heat?"

Anything that tends to dissipate energy adds to damping in the sense that I'm using the term. Friction in the wood, 'losses' through flow in and out of the soundhole, and sound radiation are all 'damping' as far as the system of the guitar is concerned, as each of them dissipates energy. Our task is in part to maximize the sorts of damping we like, and minimize the others. Some precision in terminology helps in this.

As usual, there can be secondary effects. For example, because the DU bridge pins keep the top from moving as much the damping is probably lower with the added mass than it would be otherwise; just the opposite of the usual meaning based on the fact that the guitar is qieter. This is not quite as contradictory to common sense as quantum mechanics, but there's enough to it that it pays to at least try to be exact.    

In the strictest sense of the word, damping refers to that property which determines how long a vibration will sustain. Materials which exhibit a high amount of damping will not vibrate for long periods of time...while materials that have lower damping coefficients will sustain much longer.

Here's a couple more comments regarding the physics behind Alan's example above with the depleted uranium bridge pins...this is really an example of tuning down the top. Resonant Frequency is always determined through the equation F = (k/m)^.5 where F = Frequency, K = stiffness, and M = mass. So if you increase the mass without changing the stiffness then "F" decreases (i.e. it resonates at a lower note). The other thing that happens (as Alan mentions), is that there is a fixed amount of energy being input into the guitar from the strings. This energy just doesn't go as far since the top now weighs substantially more. I think that the point to take note of is that there is a difference between adding mass, and adding damping. Both will serve to deaden the top albeit through different mechanisms. This is how I understand the physics behind these situations, hopefully this helps clarify the example.

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http://www.PeakeGuitars.com

Thanks Alan. That pretty much agrees with my thoughts on usage and
definitions, though it never hurts to clarify.

I'm sure that damping is most often considered in relation to the
soundboard and box, but over the past few years I've spent more time
considering the importance of the role of the neck. I got some papers years
years back that Schneider did for Gibson related to increasing mass and
inertia toward the end of the neck, and it's something I've always wanted to
spend more time playing with. Aside from theoretical differences, I would
love to do some relatively controlled builds, perhaps pushing low mass/high
stiffness, high mass/low stiffness, or various combinations to their outer
reasonable ranges. I have ideas as to what I may like, though there is no
substitute for a tangible comparison to decide what direction you prefer.

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Eschew obfuscation, espouse elucidation.

The neck is hugely important to tone and sustain, and this is an area where electric guitar and bass players are miles ahead of most acoustic purists. Electric guitar players are especially sensitive to the differences in neck wood and fingerboard material, and bass...well, that's where carbon fiber composite necks started in 1976.

Just look at it this way...one end of the string is attached to the top.   Where's the other end?   Might that not be important?   Duh!

I've done experiments in changing from one neck material to another, and it makes a huge difference.

Rick,
Any chance you could cite some examples/results of what you've found in your experiments or day to day builds in the case of necks?
-j.

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“If God dwells inside us like some people say, I sure hope He likes enchiladas, because that's what He's getting”
-jack handy

I know you were asking Rick, but I'll toss in my 2 or 3 cents anyway.

I use a number of simple examples to emphasize the importance of the
neck in regard to tone. First is to place a vibrating tuning fork on the
saddle of an acoustic, then on the nut or first fret. On many instruments,
with the fork at the nut you will hear the soundboard resonate at what
sounds as much as 1/2 to 2/3 the volume of the fork at the saddle. The
influence of a string is different of course, but it's still a good example of
sound transmission through the neck.

To emphasize the effect of damping I ask people to imagine a neck made
of rubber or a plastic spring, and what effects they would expect the tone
and sustain. When thinking of it at such an extreme perspective it quickly
becomes obvious that the neck can play a very important role.

The best way to think about it is to consider the motion of a wave in a
string. Even a standing wave is not really a stationary shape. The wave
travels back and forth from end to end. Each time the wave hits an end
point (saddle, nut, or fret) a portion of the wave's energy is absorbed at
that point (damping), and a portion is reflected in to the string. When the
wave ends at the nut or fret on a very floppy neck, less of that energy is
going to be reflected back, and the energy that is absorbed in to the neck
will not make it back to the box very efficiently. More of it will be
absorbed in the flexing of the neck, moving a rather useless bit of air,
turned in to heat in the neck, and in the shock absorber of your palm. On
a more stiff or massive neck more energy will be reflected back toward
the bridge, plus more of the energy that is absorbed at the neck will be
more efficiently transfered to the sound box.

Now it seems that the effects of mass and stiffness would have many
similar effects, though far from identical. First, the damping effect from
an increase in mass vs. an increase in stiffness seem as though they
would have distinctly different curves in relation to frequency, though I'm
not entirely sure where and by how much. Second, of the energy that is
absorbed at the nut or fret, mass and stiffness changes would certainly
have different effects on how efficiently those vibrations are transfered
back to the box, or absorbed along the way. In any case, the neck is far
more influential in shaping tone in my opinion, than most people give
credit. I suppose it all relates to damping in a variety of ways.

I work on a an awful lot of banjos, and something that has always struck
me is that the most favorable necks are generally not so stiff, and with a
relatively massive headstock. There are even makers such as Geoff
Stelling who specifically prefer epoxy fretted boards with no compression.
Bluegrass resonator banjo players like their necks rather floppy. Many like
a ton of relief on an already floppy neck, with a big fat headstock and
heavy tuners. Even when the action is kept so high as to rule out fret
interference, a noticeable difference can often be heard when a neck is
stiffened simply by it's straightness and tightened truss rod. Low
stiffness, high mass. I'm not advocating this for a guitar, but for banjos
the preference seems rather consistent.

Of course banjos are a different beast all together, but they are an
excellent tool for demonstrating the influence of changes in neck
stiffness on tone. I think considering the influence of the necks are
almost entirely a consideration of damping.

_________________
Eschew obfuscation, espouse elucidation.

Al Carrith said--

   "Given the limited energy we have available in strings it's usually a good
idea not to 'waste' any of it in acoustic damping, at least, not in the wrong
sorts! But low damping is not always a good thing either, nor is higher
damping always bad. As usual it seems to me that the more you
understand about it the more likely you are to be able to get what you
want from what you've got."


    Nice informative post Al. I'm really happy that you included this
statement. A little while ago I was having a discussion concerning mass
and is effects on the vibration system that the guitar is when it comes to
damping.

   The builder that I was talking to said that he has come to the place
where he enjoys no damping of the top's vibration. I quickly explained
that it was impossible to have NO damping since every bracing
component, the bridge and fingerboard tingue and the finish all
contribute to the level of damping that is present.

    Our goal is to take advantage of the damping that we will always have
to deal with by coaxing it to do what we like or need it to do as we build.
Hey, if it's going to be there, we need to learn to tame and control it.

Good stuff,
Kevin Gallagher/Omega Guitars



    

There is an interesting article in the recent Fall 2007 GAL Journal...#91...by R.M.Mottola, "Sustain and Electric Guitar Neck Joint Type".

The purpose of his experiment was to compare sustain on 3 neck types:
-Neck-Through
-Bolt on
-Glued

In order to eliminate other variables such as hardware, scale length,wood species, etc. he made a test bed with necks of identical materials that isolated the joints as the only variable.

His conclusion found the following ranking according to spectrographic analysis and timing the length of sustain. Those methods found the following ranking from most to least sustain:

1) Bolt on
2) Glued
3) Neck through

And while it's interesting to have this info, it was also interesting to know that 5 human pair of ears could not distinguish one from the other.

As Rick states, many players have the sensitivity to draw subjective conclusions while playing different instruments as a system...but to have the variables isolated, measured and ranked objectively is something that would be useful for builders to know.

It seems like one could adapt the same test bed and evaluate different neck woods and stiffnesses to answer some of the questions that have arisen in this post.

Good thread, Alan...thanks for starting this!

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JJ
Napa, CA
http://www.DonohueGuitars.com

Another thread that's being printed and filed as I type. This is great stuff. Thankyou.

[QUOTE=JJ Donohue]
His conclusion found the following ranking according to spectrographic analysis and timing the length of sustain. Those methods found the following ranking from most to least sustain:

1) Bolt on
2) Glued
3) Neck through
[/QUOTE]

This is strange, I would expect just the opposite.
Any theory as to why that happened?

I've not read that particular article so I'll try to refrain from direct
criticism, but I'll say this much.

Studies like this can be both good and bad. Good, because it can at least
partly indicate the relative insignificance of this one factor on overall
tone. Bad, because from what you described of the test, it should not in
any way be considered final and conclusive, though it will undoubtedly be
quoted as such on many players forums.

If you wanted more definitive results, the scale of the study would have
to be a great deal larger. For example, I would think a minimum of
perhaps three guitars of each style would have to be built. This would
allow you to establish at least some range of error. Testing several of
identical models would still inevitably give at least slightly different
results. This would demonstrate either the consistency of testing
methods while helping establish a range of error, or if results varied
widely enough could help to reveal flaws or inconsistencies to help refine
the testing procedures. Without doing something like this it is very
difficult to guess how far results have to vary before they could be
considered statistically significant.

There are many things I don't know about the study. Things like how
were results measured, by microphone, transducer, magnetic pickup?
How was the string driven, magnetically, by pneumatic or mechanically
driven plectrum, or simply by hand? If by plectrum, how close of
tolerances were the angle, depth, and force of the plectrum controlled?
How many times were the tests repeated on each instrument, and were
the mean or the average of results compared? Was there any real attempt
to establish a range of results within each instrument to help guess what
constitutes a statistically significant difference?

I'll have to read the article, and I have to say I don't doubt the legitimacy
of the results at this point. Still, if the three guitars were labeled A, B, and
C, and all made with the same neck through construction, I can't help but
wonder if he could have found a similar range of difference between
those. From the little you've described or the study, it's just not
something that I feel should be quoted as definitive.

All this said, I'm still glad people like him are doing this. Unless you're
receiving research grants or are working with a research budget for a
larger company or investor, it can be unrealistic to expect any luthier to
devote tons of money for pure research. Building nine, twelve, or however
many guitars to prove a point that you may already believe would be a
little extravagant. It sounds like an interesting read though.

_________________
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David...The fact that 5 people could not detect differences after multiple attempts was the significance of the test to me. Still, the test bed shows some promise as to an objective method of evaluating the effect of neck woods, stiffness, reinforcement, etc. on sustain, volume and possibly even tone.

Briefly, the test bed consisted of both an e and E strings clamped at one end (bridge/saddle) and over a nut and on to tuning pegs simulating a 25.5" scale length. The strings were plucked using a known caliber of thin wire that pulled the string horizontally at the same location until it broke. This wire was described by the author as being consistent with typical plucking tests. A single pickup was used to transmit the signal to an amp and speaker system. The signal was analyzed spectrographically and averaged over 15 trials. From my perspective, this seems to be a reasonably well designed test...but I'm certainly no acoustic expert.

I mention the article to elicit the opinions of experts such as Alan, you and others...not to proclaim the results as a definitive answer. We all look forward to your more experienced reaction and comments.

_________________
JJ
Napa, CA
http://www.DonohueGuitars.com

JJ: were the differences in sustain statistically significant, though? They weren't real-world significant, far as I can tell (5 people, presumably musicians, who 'know' what to listen for, can't tell the difference), but were they significant at all? I'm assuming different pieces of wood were used for each 'neck', which introduces variance in and of itself.

Mattia...not only was the neck the same species...it was the same neck! So the variance that you suspected is really cancelled. To me, that was the beauty of the manner in which the protocol wes designed.

1)The testing started with the "guitar" designed for neck through...15 sample recordings were made.
2)The neck was then separated and bolted on and another 15 sample recordings made.
3)The bolts were then removed and the 2 sections were glued together and the final 15 samples recorded.

As far as statistical significance is concerned...I don't have the article with me, but I suspect that the sample number of 15 would get it pretty close...as well as a reasonably moderate Standard Deviation as I recall.

I'm really hoping that Alan will chime in on this for a more credible critique.

_________________
JJ
Napa, CA
http://www.DonohueGuitars.com

EDIT...EDIT...EDIT

The author concludes that there is "little practical difference in sustain of notes based solely on the construction type of neck joint."

_________________
JJ
Napa, CA
http://www.DonohueGuitars.com

Iirc, Motolla also did the same experiment three times with different 'guitars'. He's a darn good experimentalist.

Neck stiffness and mass sure can make a difference in the sound of an acoustic guitar. The lowest vibration mode of the whole instrument is a 'bar' type mode, and there is a lot of bending in the neck. The nodes, the stationary points, are generally right about at the first fret and the widest point of the lower bout. On many guitars this mode is well below the pitch of the 'main air' resonance, which is the lowest resosnace that can radiate any appereciable amount of sound. However, if the neck is stiff and light, the headstock is also light, and the neck is short, the pitch of this 'neck mode' can be as high as, or higher than, the main air resonance. In that case the 'air' and 'neck' resonances can couple, and there will be a very noticable effect on the tone.

Think about the guitar vibrating in this first 'neck' mode. As the headstock and tailblock go 'up' (away from the player and toward the room) the rest of the guitar is going 'down'(toward the player). At this point the bending forces on the guitar are compressing the top along its length. Since most guitars have a top that is more or less domed (whether they were made that way or not) this lengthwise compression force is also causing the top to pop outward, which sucks a little air in through the soundhole. As the vibration continues the action is reversed at some point, and the top flattens out, pushing the air out of the soundhole. This 'breathing' action is just what happens in the 'main air' resonance. Thus, if the two are close in pitch they will 'couple': the neck motion will drive air flow, and the air flow will push on the neck.

Any time you get modes that couple you will see activity at two (or more, if there are more modes) different frequencies. In this case there will be two output peaks in the spectrum of the guitar in the low range, rather than the one you usually see. Usually the area under the spectrum curve will be greater when there is coupling: the 'total availalbe horsepower' will be greater. The heights of the two peaks will not be as tall as the usual one, but the response will be spread out over a wider frequency range. Thus the sound is more even, with less of a tendancy to a low 'wolf note' at the air more pitch. It is often also particularly 'rich', 'full', or 'dark', depending on who you talk to.

This is most common on Classical and Flamenco guitars with tapered 12-fret necks made of something light and stiff, like cedro. Tuner weight is an important variable: sometimes simply swapping out metal buttons on seald gear machines for wood ones (which saves the weight of about two machines) will raise the 'neck' mode pitch enough to get it to couple.

It's pretty easy to hear this lowest 'neck' mode. Hold the guitar up by pinching the neck at the first fret, damping the strings. With the box hanging freely tap on the back of the headstock and listen with your ear close to the top of the head. To find the 'air' mode; lay to guitar down in your lap with the top up. Pinch the low E string between your thumb and finger down near the nut, so that it can vibrate, but is also damped. luck the string as you move the pinch point up and down the length. The 'thump' will be loudest at the 'main air' resonant pitch: generally around G on the low E string +/- about three semitones.

There are maybe a couple of higher order bending modes of the whole body that fall within the pitch trange of the fundamentals of the strings, but so far I haven't been able to trace any effect they might have on the sound.    

I'm not an expert on solid body guitars, but I have noticed a few things. They also have 'bar' type modes of vibration, of course, and usually the lowest ones are lower in pitch than those on acoustics. A solid body guitar is a tapered bar, wider at the tail than the head end, and this moves all the node lines of the vibrating modes down toward the tail. One result of this is that the lower node lines of the lowest order modes tend to 'bunch up' in one place. On the guitars that I've checked that's usually right near the bridge. Since you can't drive a mode at the node line, this helps with sustain on solidbody electrics. Using a neck wood that is more or less stiff or massive should move those node lines a bit, and it might not take much to have an effect.

Solidbody basses are poorly designed in that respect: the bridge is at the lower end, which is the most active place on the body for every mode. This makes it easy for the strings to drive the body vibrations, and really cuts into the sustain of those notes. More mass or more stiffness helps, but not nearly as much as a better design would, I think.

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Yes, it will suck less out of the string and reflect more back.   

Check out the locking nuts on Floyd Rose equipped solid body guitars. That helps restore some of the sustain lost to the whammy bar setup.   It's not the reason, but it's an unintended side benefit.

What I ultimately found with all carbon fiber necks was that they tended to sound "cold" and "sterile"...that is they were so even in response that they got boring.   

My current fave neck construction (when money is no object) is a laminated mahogany neck with center stripes of whatever and two 1/8" x 1/2" graphite bars dadoed up into the fingerboard and down into the neck. This makes the fingerboard incredibly stable, and that's where consistent playability comes from.   The neck is stiff, there is some broadband damping from the neck laminates (there's another whole study...the true effects of laminating necks...), there's good reflectivity back into the strings.   For my purposes, it's what I've always wanted...good sustain, interesting tone, really good stability under varying climactic conditions, and a wood neck shaft that I can carve to suit the customer.

I personally like the wood/phenolic "Dymond Wood" material for fingerboards, too. It adds great compressive strength right where you want it...the fingerboard, and it is good and dense and reflective. It survives anything the guitar will survive, too.   That's what I used on Henry Kaiser's Ms. Antarctica guitar, and that guitar sounds wonderful and has never needed any neck adjustment from the time it left my shop through three months in Antarctica, and up to this day.

So for me, that's a neck design that just works. Good sound, can be carved by machine or hand, excellent dimensional stability under wide conditions...what else is there?

For about the last year my main interest has been pure slide(raised nut(No not me)) instruments and the implications of lighter/heavier necks & impedance match/mismatch of such to the box.

It sort of sucks that this is one of the days that I have brain that rebels and refuses to focus because I feel this applies best to such instruments(the variable damping of the players hand is absent). They also lack the sting to fret contact(contributes OVERTONES?).

Over simplified
well matched and light = loud edging on brash?
well matched and slightly heavier = loud but rounded?

then the various combinations of mismatches?

Then built with coupled back and reflective back methodology?

Weiss style instrument are generally accepted(I think) to have surprising volume. Could that partially be due to the fact that the neck is impedance matched to the body as it is part of the body?

This is not as clear and detailed as I would like but I think the meat of my curiosity is shown.

If I could build them all....

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Rick...Interesting concept of rabbeting both the FB as well as the neck to accommodate CF. I routinely laminate Mahogany acoustic necks with Maple and IRW or Bloodwood as well as including CF but only in the neck portion. A few clarifying questions please:

1) Assuming a 1/4" thick FB, how deep is the rabbet in the FB?
2) Are you using West Systems epoxy to hold it all together?
3) Have you tried this neck design on acoustics as well as electrics.
4) Do you fret before or after gluing the FB to your neck?

TIA

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Napa, CA
http://www.DonohueGuitars.com