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Post by simes on Jun 14, 2012 16:48:46 GMT -5
I had an interesting conversation today with a friend who is an engineer, musician, and occasional luthier, and has been involved in a number of research projects regarding vibrations and resonance.
We were discussing dead spots (which, it must be said, I've never had the misfortune to encounter on any guitar of mine), and he was of the opinion that they don't actually exist as an inherent property of a guitar neck, but rather are induced.
The short version of what he said was that all bodies will have a certain resonant frequency at which they will tend to vibrate, and that in industrial machinery the idea of a dead spot is actually desirable, in the sense that you want the whole machine to be a dead spot. This is routinely done by identifying the resonant frequency and attaching some kind of appendage that will resonate at that same frequency, thereby cancelling it.
His conclusion is that if a guitar neck - whose whole purpose is to resonate at many frequencies - does not do so at one or more of them, it is because it has such an appendage attached whose resonance coincides with the neck itself and cancels certain frequencies.
What would this appendage be? The truss rod, presumably. So, by avoiding vibration in the truss rod (or as a remote possibility a nut or a machine head), one would avoid dead spots.
Perhaps this is why Melvyn Hiscock recommends in his book that a traditional-type truss rod should be encased in PVC tubing or similar.
Any thoughts?
Cheers,
Simes
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Post by reTrEaD on Jun 14, 2012 18:31:22 GMT -5
His conclusion is that if a guitar neck - whose whole purpose is to resonate at many frequencies - I don't agree with that premise. The purpose of a neck is to provide access for fretting. I think ideally a neck would be rigid and non-flexible. If a neck were highly resonant, it would transfer energy from the end of the string into movement. Your hand would dampen that movement. In turn, this would reduce sustain. A vibrating string makes little sound on it's own. So we do need to use resonance to make it audible (sonically). But at the body, not so much at the neck. |
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Post by KIIMH on Jun 14, 2012 19:18:30 GMT -5
His conclusion is that if a guitar neck - whose whole purpose is to resonate at many frequencies - does not do so at one or more of them, it is because it has such an appendage attached whose resonance coincides with the neck itself and cancels certain frequencies. What would this appendage be? i d'nt kno waht a thery is, but tihs remidn me of teh usless apendge on top of my sholders ... ... culd teh usless apndage attached to teh gutar's neck be its body I am wonder? tihs is why i dont tehorize or get out much ... tahnk yuos for lisning to me, ky;e |
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Post by 4real on Jun 15, 2012 0:43:41 GMT -5
Agree. Any vibration will be dampening the vibrations of the strings. Ultimately you could go like a steinberger and have a carbon fibre neck that is really 'stiff'. Many people find such guitars a little 'sterile', but certainly no 'dead' spots.
On acoustics it is trickier, but for the neck the same applies, you want all the vibration to occur at the bridge end, the body will because it is meant to vibrate accentuate other frequencies over others with phase cancellations and such...hence the art of bracing and materials.
Strats can often have 'dead spots' and these things are a part of a character of guitar overall...you'd only hear them clean typically, some guitars also have 'live' spots too. Truss rods, nuts and tuners should all be secure and not part of the equation. But, it may be that 'sustain' is not the paramount issue anyway.
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Post by simes on Jun 15, 2012 1:27:37 GMT -5
His conclusion is that if a guitar neck - whose whole purpose is to resonate at many frequencies - I don't agree with that premise. The purpose of a neck is to provide access for fretting. I think ideally a neck would be rigid and non-flexible. OK, that was perhaps an erroneous interpretation of mine, so let's ignore that. Let me re-phrase. Could a vibrating truss rod be the cause of dampening when a string is made to vibrate at the same frequency? |
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Post by sumgai on Jun 15, 2012 1:53:31 GMT -5
simes, His conclusion is that if a guitar neck - whose whole purpose is to resonate at many frequencies - does not do so at one or more of them, it is because it has such an appendage attached whose resonance coincides with the neck itself and cancels certain frequencies. Usually I'm not one to disparage a person whom I've never met, and particularly when I can't do it face-to-face, but in this case I'll make an exception - I'm astounded that your friend, presumably a Mechanical Engineer, made such a blatantly false statement. Here, let me shortcut this by cutting to the chase right now. The following link is gonna open some eyes, I'm sure: mosesgraphite.com/technical-info/regarding-dead-spots/When's he digested that, report back here on his reactions, please. HTH sumgai (the non-Mechanical Engineer who's ended up working in that field more than once or twice) |
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Post by sumgai on Jun 15, 2012 2:30:50 GMT -5
 Let me re-phrase. Could a vibrating truss rod be the cause of dampening when a string is made to vibrate at the same frequency? Short answer: Mr. Murphy will gladly make it happen, just because he likes you - really! ;D I suppose that there have been cases where truss rods have caused this problem, but I'd lay long odds that in every one of them, it was due to either improper installation, or else a poor specimen of wood chosen for the neck (or the fingerboard). Call it bad craftsmanship or bad materials. If the problem didn't come about until well after the owner purchased it, then we might blame bad maintainance or abuse (posers repeatedly banging the axe on various hard surfaces, etc.), in theory those could cause the truss rod to "act up". But in general, no, one shouldn't even contemplate the addition of a PVC tube around the truss rod during installation. For one thing, it certainly willl change the tonal characteristics, and I for one don't want to speculate on how those changes will be perceived by the ear - what if the Mojo™ goes out the window along with the "dead spots" you were hoping to prevent, eh? And I'd bet the stock price of Rogaine would skyrocket if you actually induced even more dead spots, eh? But of course, how could you tell, unless you take the neck back apart and pull out the pipe, eh? * Oh, wait.... you just routed the channel big enough for the pipe around the rod, and besides taking out enough wood to also change the tonal characteristics in and of itself, you can no longer mount a truss rod in the standard manner. Can you say "firewood". boys and girls?  My take? Unless someone does a lot of research, and releases it for peer review, I'd steer clear of "let's try this and see what happens" ideas for building a neck. But when it comes to fixing a guitar that's now showing a dead spot, and if said dead spot is known for 100% sure to be in the neck, then the best solution is to simply replace said neck! Shocker, I know, but there it is, free of charge.  HTH sumgai * No Canadians were abused in the making of this post (but a few had fun poked at them!).
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Post by simes on Jun 15, 2012 2:53:37 GMT -5
As indicated in my previous post, that "blatantly false statement" was mine, not the engineer's. I wouldn't recognise a frequency response model if it bit me on the backside, to be honest. I stand corrected. So: ... he was of the opinion that they don't actually exist as an inherent property of a guitar neck, but rather are induced. ... in industrial machinery the idea of a dead spot is actually desirable, in the sense that you want the whole machine to be a dead spot. This is routinely done by identifying the resonant frequency and attaching some kind of appendage that will resonate at that same frequency, thereby cancelling it. His conclusion is that if a guitar neck string - whose whole purpose is to resonate at many frequencies - does not do so at one or more of them, it is because the neck has such an appendage attached whose resonance coincides with the neck string itself and cancels certain frequencies. So, would a vibrating truss rod not be a major candidate as a cause for dead spots? Note: Any other crimes against engineering are most likely due to my paraphrasing, and should not be jumped upon with any great vigour . |
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Post by simes on Jun 15, 2012 2:55:07 GMT -5
Last post posted before reading SG's.
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Post by reTrEaD on Jun 15, 2012 8:54:26 GMT -5
Note: Any other crimes against engineering are most likely due to my paraphrasing, and should not be jumped upon with any great vigour . Okay. I don't think you were directing the 'great vigor' part at me. But on the off chance you were, I think my earlier post was appropriately restrained. I just didn't agree with what I read and offered my own opinion. Don't be put off by Kyle. He really does try. He just has a different way of expressing himself.
So, would a vibrating truss rod not be a major candidate as a cause for dead spots?
Well I suppose it depends on the resonant frequency of the truss rod and how it interacts with the environment in which it dwells. More on that later. His conclusion is that if a guitar neck string - whose whole purpose is to resonate at many frequencies - does not do so at one or more of them, it is because the neck has such an appendage attached whose resonance coincides with the neck string itself and cancels certain frequencies. Okay so now we're talking about the string. The first thing we should recognize is that the string doesn't exactly resonate at 'many frequencies'. It can resonate at one frequency (or any integral multiples of that frequency) at a time. But the frequency at which it resonates will change according to where we fret the string. The other point worth mentioning is the way a string resonates. Unlike a tuning fork, we have two termini and tension on the string. I suppose you could consider the entire guitar as a system, as the 'attached appendage'. This appears to be a rather different circumstance than most industrial applications for vibration damping. Most of them are related more closely to a tuning fork. But there may be some parts of the process that are similar. ... in industrial machinery the idea of a dead spot is actually desirable, in the sense that you want the whole machine to be a dead spot. This is routinely done by identifying the resonant frequency and attaching some kind of appendage that will resonate at that same frequency, thereby cancelling it. I don't think that's exactly right. This sounds like you're partially describing a 'tuned mass damper'. What's missing here is a way to convert the vibrational energy into something else. Usually heat. We need to have the added mass interact with something that will dissipate the energy. Perhaps a fluid. Or a substance that allows for elastic deformation, with a heat as a byproduct. If we simply attached an 'appendage', it would either change the frequency, or sympathetically vibrate at the same frequency. Air isn't viscous enough as a fluid to do enough damping. And the sound waves produced would likely be undesirable if we were looking to eliminate vibration. Let's take some of those ideas back to the guitar string example. If the 'attached appendage' has a very high resonant frequency, it won't be able to resonate and steal energy from the string. But if it has an extremely low resonant frequency, it could resonate at most all frequencies that might be produced by the string. But simply resonating doesn't do us all that much harm. And converting the energy from the resonant 'appendage' into sound will have a very limited effect on damping the string vibration. While it's true that a Les Paul will have greater sustain than an acoustic guitar, we still see a reasonable amount of sustain in an acoustic guitar. I think the greatest potential for damping and dead spots would be if discrete parts of the 'appendage' system were able to resonate at different frequencies and not be locked together as a unified system. But again, the separate resonances wouldn't be a problem if they were completely decoupled. It's when the motion of one component (relative to the other) is converted into heat, where the greatest amount of energy can be leeched from the vibrating string. Long story short, most of this is over my head. I don't think I'm qualified to offer anything close to an explanation. The process of 'damping' in a guitar seems very complicated. I don't think there are easy answers. "avoiding vibration in the truss rod" doesn't seem any less complicated. How do you "avoid vibrations"? By using an elastic coupling? I reckon that could make things worse. But I could be wrong. |
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Post by cynical1 on Jun 15, 2012 9:30:45 GMT -5
Time is a bit short here, but I think the point simes initially made, while perhaps phrased to allow misinterpretation, has merit. Maybe it's me, but if I pick up a guitar or bass and DON'T feel just the strings vibrating through the neck I put it back. You feel a truss rod vibrating and odds are the rod is out all the way to compensate for a wicked bow...and probably still not doing it...or it's just plain busted at the threaded end. Read SG's link and you can see where I'm going with this. Anything that can dissipate the energy from the string vibration can contribute to "dead spots" or reduction in "sustain". lpf3 posted a bit about a truss rod available from StewMac, similar to the Martin style, that intrigued me:  Is this a magic bullet? Probably not, but it strikes me as being more solid and having less effect on the energy transmission of the vibrating string. I will say that every guitar I've repaired that had a broken, stripped or bad truss rod, "bad" as in too bowed to adjust out so the fingerboard had to be removed and the neck shaved flat, then truss rod re-installed\replaced, there was a noticeable improvement in sustain and "feel" on the guitar or bass. Granted, these are some subjective terms, and I wish I had more time to describe this scenario, but I wouldn't blow it off completely. It is one factor in a series of factors that can all contribute to the overall "performance" of a guitar or bass. HTC1 |
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Post by sumgai on Jun 15, 2012 12:33:31 GMT -5
Don't be put off by Kyle. He really does try. He just has a different way of expressing himself. Sadly, he's supposed to use a separate and dedicated room to 'express' himself, but like his avatar suggests, he refuses to become potty-trained, which is source of continuing embarrassment for the rest of us...... |
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Post by 4real on Jun 15, 2012 17:15:31 GMT -5
Well, no...you don't want anything, let alone a truss rod, able to resonate inside the guitar. Best practice has always been to dampen trusses so they can't vibrate in various ways. EDIT: Generally an adjusted truss rod is making contact in the neck and under pressure so the amount of evne one that is not 'dampened' inside is limited I imagine. Some of the effect of putting the modern supported trusses like that is that it is adding more 'mass' to the guitar as well. It is in fact that things like the old 1970's 'fathead' which attached a brass plate to the back of the headstock to increase mass and sustain. I have noticed that this kind of thing too makes a surprising difference, I think largely because the momentum of the strings is less than the energy it takes to 'vibrate' the head perhaps. Certainly I noticed the effect when adding good solid tuners or even this aluminium 3mm plate to my LP...  Even the 'dampening' of the strings behind the nut that these kinds of string tree's makes a difference. A simple way to 'test' this for yourself is to attach a clamp or something to the headstock changing the dynamics. It also helps with 'dead spots' as well as sustain. Of course, you got to watch you don't make the head so heavy the whole guitar is out of balance, but demonstrates any effect there. Normally some stout tuners, like the locking tuners I tend to now adds that kind of mass at the head. Fender style heads are also long and can do with string trees often to dampen things back there and cut back on flex...  My strat perhaps does not strictly 'need' string trees (staggered locking tuners), but they do add more pressure on the nut on high strings and cut back on sympathetic vibrations, and add mass too...they made a noticeable difference.  My tele has the same tuners, no trees, but the LSR nut includes small string dampers (tiny black foam behind the rollers) to stop sympathetic string vibrations. ... The neck is of course a long thin thing that is prone to potential vibration loss (you can get dead spots a lot in a bass it seems), but even the bridge can make a big difference. I certainly noticed dramatic changes in individual note separation and definition on my Khaler guitars (LP and Tele)...very solid devices. Similarly when I fitted a 'tremsetter' into my strat which has an exaggerated 'floating' conventional trem system. But hey, that kind of effect may not be what you are after and there is a cost of balance and weight with some of these strategies that could be addressed in the design and materials (graphite support bars in the neck for instance). Part of the sound of the strat is the neck pocket and sprung bridge and all those 'losses' in the system. An example of going the 'other way' I suppose is my latest guitar which uses just short of the entire new string length on the high E with a lot of 'free' string behind the nut and bridge and a semi acoustic body of sorts. Occasionally you can get some 'dead spots' on it though not too bad, sometimes 'reinforcement' of other notes, especially in the bass...but many of these can be compensated for with the tone controls it seems. On that guitar, the idea was not to have a massive sustain, but a more acoustic and percussive nature and to bring out some of these character quirks, not so much 'dead spots' but certainly designing for a different 'character'. This too might be the thing one is looking for in a strat or some kind of quirky 'dano' like guitar or where the playing is more percussive or you want to hear a quicker 'decay' and perhaps a bit less 'bloom' as one might find in a heavily built Les Paul on the attack but something 'sharper' as is typical with fenders and a lot ot 'cheap' guitars that don't accentuate such qualities. In short (ish) the envelope (the attack and decay) of the guitars sound is far more important in most cases than any 'trick wiring' or 'pickup' qualities of other aspects that tend to dominate and sometimes expected to 'sound like an X guitar' as many think they are reaching for. This can be 'adjusted' a little, dead spots attenuated, and qualities enhanced but these kinds of things are 'structural' and generally 'subjective' as to what you want out of a guitar and what you intend to play on it. A loose truss rod, probably universally bad, but sustain may not be your particular 'holy grail' either. There is no 'best. I tend to disagree with the original suggestion... Necks, far more than bodies, tend to have resonant frequencies, but one can adjust this by adding stiffness and mass and limit it in various ways, as illustrated in some of my 'practical' experiences and experiments, many of which you can test for yourself and see what you think are the effects of such things. If there are 'dead spots' this will often change or eliminate them by altering the necks resonant frequency and allowing less losses (so sustain and harmonic damping) to occur. Guitar bodies, at least in solid bodies, resonate far less than any neck will do. |
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Post by simes on Jun 20, 2012 3:04:18 GMT -5
Note: Any other crimes against engineering are most likely due to my paraphrasing, and should not be jumped upon with any great vigour . Okay. I don't think you were directing the 'great vigor' part at me. But on the off chance you were, I think my earlier post was appropriately restrained. I just didn't agree with what I read and offered my own opinion. No, not at all. Don't be put off by Kyle. He really does try. He just has a different way of expressing himself. So I see. Well, there is certainly some interesting discussion here. Thanks everybody. Just as a brief off-topic excursion, and one which probably doesn’t justify a new thread … I have to buy a truss rod for a new build with the neck joint at the 20th fret (25½” scale), so about 444mm from the nut. I have two domestically available options with 410mm and 440mm respectively. The 440mm rod is much more expensive and would involve extra shipping costs. It occurs to me that it might be a good idea for the truss rod to extend from the nut (it is to be adjusted behind the nut) to as close to the neck joint as possible. Is this correct, or would the 410mm rod suffice? |
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