A "parasite" tone

[Alan Carruth]

How high are the strings off the top at the bridge saddle? It looks very high to me. If so, that would help explain the problem.

[Massimo Milan]

If I measured well, action at 12th fret is .120 and strings are .627 inch off the top at the saddle. Why would this help explain the problerm?

[Doug Shaker]

Does adding weight to the headstock change anything?

[Massimo Milan]

Does adding weight to the headstock change anything?

no; the only thing that changed something was twisting on itself the wounded strings, ten-twelve twists as to tighten the woundings, and while keeping firmly the string not to let it un-twist, fixing it to the post and tensioning to pitch. This makes the parasite tone to disappear on D string and almost disappear from A string. But, after some warming up while playing and re-tensioning to pitch, the parasite tone comes back. A steel D string was experimentally tried, and sounded perfectly. So, it seems that the parasite tone has somethnig to do with the strings....

[Doug Shaker]

If the idea is that we are getting a longitudinal wave along the strings that is somehow resonating with the rest of the guitar and becoming audible, AND, as others have said, it seems to be present with a vengeance on some guitars and not on others, then my thought is that you need to change something about the neck-string system to get out of the loop.

If adding a weight to the headstock doesn't help, perhaps tightening or loosening the truss rod a little would help.

[Brad Heinzen]

If we're talking about the same problem, I've found it to be remarkably difficult to eliminate by messing around with the neck. 7 or 8 years ago, I had a bad one, and decided I'd do whatever it took to get rid of what I assumed was a zip. I started with the easy stuff - c-clamps on the head stock, resurfacing the frets, new nut, new saddle, etc. I then upped the ante to refretting, changing bridges, and eventually replaced the entire fingerboard. None of that had any effect at all.

After all this, I concluded that it must be something very fundamental in the basic geometry, and that fixing it wasn't going to involve less than significantly changing one of the main resonances of the guitar. Since it was a guitar that played and sounded great otherwise, I just gave up, and left it to be handled by string selection. If you give the plain old D'Addarios a good twist, you'd never know it was an issue at all. It also grew out of it a bit over time. I have no doubt that you could never run Hannabachs on that guitar though.

[Massimo Milan]

.......perhaps tightening or loosening the truss rod a little would help.

there's no truss rod (no need of it)

[Alan Carruth]

"....627 inch off the top at the saddle. Why would this help explain the problem?"

Because the longitudinal wave in the string pulls the top of the bridge toward the nut. This causes the bridge to rock fore and aft, which pulls on the soundboard and produces sound. The higher the strings are off the top the more of this you're going to hear. I got some good measurements of this several years ago. I've compared the sound of a guitar with a 'normal' saddle that put the strings 11mm off the top, and a 'tall' that made them 18mm off the top. The tall saddle had more output on the second partial (from the twice per cycle tension change of the string) as well as at the pitch of the 'zip' tone. Since the zip tone is normally dissonant making it stronger would tend to make the sound harsher.

I can't think offhand of any way a tall bridge could contribute to coupling between the longitudinal and transverse vibrations of the string, but I suppose it's possible. I didn't look at that in my experiment. If such a couple exists it could make it more audible, though.

I'll note here that, contrary to widespread opinion, a tall saddle doesn't give more power, nor does an abnormally high break angle. Looking at that was the point of the experiment with the two saddles. The primary driving force on the bridge is from the transverse movement of the string, which pushes the top in and out like a loudspeaker when the string is moving 'vertically' with respect to the soundboard. I summarize the string forces in a little article on my web site called "String Theory" (I couldn't resist....).

[Massimo Milan]

".... The tall saddle had more output...as well as at the pitch of the 'zip' tone. Since the zip tone is normally dissonant making it stronger would tend to make the sound harsher.....a tall bridge could contribute to coupling between the longitudinal and transverse vibrations of the string.... If such a couple exists it could make it more audible....

Which are the variables determining the zip tone? Where can we try to make a change?
Say we can act just from the strings side, what would you suggest to try? veeery veery hard tension strings ?
hybrid stringing with low tension-low gauge steel wounded or silk&steel wounded and nylon plains? or....?

( BTW I tried to dig your "String Theory" article but I must admit I sunk at half way. I promise I will try again )

[Alan Carruth]

The 'zip' tone is a longitudinal wave in the string, analogous to a pressure wave in a long thin pipe. The pitch is determined by the effective Young's modulus of the string, the effective density, and the length. So far so simple.

Young's modulus is a measure of how much force it takes to stretch or compress a given size sample of something by a certain amount. The 'effective' Young's modulus of the string is pretty much the Young's modulus of the core material. For steel strings this is more or less invariant with tension: the zip tones of the E and B strings on most steel sets are pretty much the same. Nylon is more complicated. Because of it's chemical structure the Young's modulus goes up as you put on more tension. The change is relative to the ultimate breaking tension of the string. The high E string tends to be under a bit more tension than the G in the D'Addario sets that I have a listing for, so all else equal you'd expect the high E to be closer to it's breaking stress, and have a somewhat higher Young's modulus. Of course, the G is thicker, so it can carry more tension than the E, and that puts it even further from breaking, so the Young's modulus is even lower. Thus the G has a lower zip tone than the E on the classical sets I've checked, and the B is in between. I don't remember the exact numbers, but the table is in the article. That's the plain strings. With wound strings you can more or less ignore the wrap in terms of Young's modulus, but otherwise the relationship between Young's modulus and tension will be similar.

For plain strings the effective density is just the density of the nylon. Wrapped strings are more complicated. The wrap is there to add mass without adding stiffness, so that you get a lower note at the given tension. It also drops the pitch of the zip tone, since the wrap has to move along with the core to at least some extent. In fact, I imagine there's no appreciable 'slippage', but I don't have any data on that.

Since the Young's modulus is only slightly related to the tension on the string, there is no set relationship between the tuning of the string and the zip tone. As it works out, for satisfactory strings the zip tone is usually somewhere between the 7th and 8th partials of the transverse fundamental of the string; the note the tuner says you've got. Thus the zip tone is generally more or less dissonant.

All else equal the longer the scale length you use the lower the pitch of the zip tone. You don't have much choice about the zip tone of the plain strings once you've selected a scale length and string.

Wound strings give you some leeway because you can change the effective density of the winding by twisting or untwisting the string. It's not much but it can be useful.

The reason it's useful is that the zip tone is driven in the vibrating string by the transverse motion and the resulting tension change in the string. It's complicated, but basically when you pluck a string somewhere near one end you actually pull the string a little bit in that direction. That is, when you pluck near the bridge it's as if you grabbed the string and stretched it toward the bridge a little. The closer you play to the bridge the greater the effect. The only way to avoid driving the zip tone is to pluck exactly in the middle of the vibrating length; over the 12th fret for an open string. Nobody does that because it sounds lousy, among other things.

What happens with the zip tone once you've plucked the string gets complicated too. Remember, the zip tone is at a much higher frequency than the fundamental of the string. You can envision it getting a little 'kick' from the string vibration twice for every cycle of the fundamental; once toward the bridge and once toward the nut. If the zip tone pitch is an exact multiple of the frequency you tuned the string to it can build up in amplitude. The more exact the frequency match the greater the effect; the more the amplitude of the zip tone builds. Classical D strings seem to be made this way, for some reason, so they're a problem.

Again, it's complicated, but basically when the zip pitch amplitude is high the two ends of the string are under slightly different tensions at that matching partial. The string wants to make two different pitches near that frequency, and they tend to beat. What you get for sound depends on a whole raft of different variables.

The zip pitch is acting through string tension, and pulling on the bridge top. The taller the bridge the more leverage the string has to rock it, so the guitar produces more of that zip sound, including any problems. So there's one variable for reducing the zip tone. Another, as we've seen, is altering the density on wound strings by tightening or loosening the wrap. That only works for the wound strings, of course. With plain strings your choices are more limited. You can try some different strings to see which ones might work. I suspect that 'carbon' strings might have lower zip pitches, since they're made of a denser material, if I understand it correctly. Otherwise about all you can do is alter the scale length.

One bright spot: since the zip toner is only given a kick every once in a while it ha to match up pretty closely in order to build up enough to be a problem. That means the pitch match has to be really exact. Anything you can do to add some damping to the string will help kill the zip tone too. That may be why playing right on the fret kills it; your finger damps it out.

[Alan Carruth]

I forgot to mention:

Since the zip tone is driven by playing the string off center in it's length, the closer you play to the bridge the greater the effect.

It's possible that a top resonance could couple with this if the pitches lined up right. Or wrong, depending on how you look at it...

[Clay Schaeffer]

" I suspect that 'carbon' strings might have lower zip pitches, since they're made of a denser material"

A couple more choices might be gut or nylgut (also denser materials). They should be slightly thinner or will increase tension to reach pitch. It would be a good idea to lower the saddle if possible.

[Massimo Milan]

A couple more choices might be gut or nylgut (also denser materials). They should be slightly thinner or will increase tension to reach pitch. It would be a good idea to lower the saddle if possible.

"Aquila" strings are nylgut, I tried them: no change if not transitory, provided to twist on itself the string before fixing it to the post, after some "warm up" the parasite tone comes back.
As to the saddle, the action at 12th fret is now 2.5 mm, wouldn't it give some more buzzing lowering the saddle?

[Brian Evans]

I was curious so I looked for typical action with a nylon string guitar, and it seems to be 3mm high E and 4mm low E at the 12th fret. Concert guitars that will be played by highly competent players loudly will often have higher action than that, so low action is not a particularly desirable characteristic in classical nylon string guitars. Except apparently flamenco, where string buzz is part of the sound, and people like me who are lousy classical style guitarists who can't play with power in the first place and are used to 1/16th inch action on a steel string. Saddle height would seem (per Alan) to do more than just set action height, it is also critical in coupling the string energy to the top. A typical string height over the top on a nylon string instrument is 11 - 12 mm, while yours is 16mm, or 50% greater. If I understand Alan's comment, excessive string height over the top can change the way the string energy works and increase the zip tone. String height over the top is set by neck angle, and I have read that some classical guitars actually have negative neck angle to put the string height where the luthier wants the bridge and saddle to be, with a resulting correct action height.

Just my musings as I try to absorb the information in this thread...

[Massimo Milan]

More news.
I changed strings to those Thomastick "John Pearse Folk".
I fixed the B A D (isn't it funny?) strings after twisting clockwise several times as to tighten the woundings, and the sound of A and D was not only bad as usual, but more than ever.
So I untwisted the strings and twisted them again counter-clockwise, as to loosen the woundings, and this ameliorated very much the tone, more than any other trial before. Was it a consequence of some "artificial ageing" of the strings by twisting-untiwisting-twisting? Or it it the last one the right way to alter the density of the strings? Anyway, I'll let you know if this is a stable result or not.

[Alan Carruth]

Changing the density of the string either way would change the zip pitch, so you wan to go in the direction that will move it away from an exact multiple of the fundamental you've tuned to, if that's indeed the problem.

You can check the zip pitch of wound strings by rubbing your finger lightly along the string. You might actually be able to pick it up with a tuner, or else record it on your computer and use spectrum analysis software to find the exact frequency. On plain strings you can rub along the string with some violinist's rosin, either just the cake itself or by rubbing some on a rag.

[Massimo Milan]

You can check the zip pitch of wound strings by rubbing your finger lightly along the string. ...

I could hear the zip pitch of 4th-5th-6th strings

6th E --> zip Eb
5th A --> zip Ab
4th D --> zip C#

then?

[Alan Carruth]

"6th E --> zip Eb
5th A --> zip Ab
4th D --> zip C#"

You need to covert those pitches into frequencies, and see how they relate to the fundamental tone of the string. Since the zip tone is usually between the 7th and 8th partials, we can start by looking there since we don't have the sort of frequency information a spectrum analyzer would give.

The low E fundamental is 82.4 Hz. There's an Eb at 622.2 Hz, and if you divide that by 82.4 you get 7.55: that zip tone seems to be almost exactly between the 7th and 8th partials of the low E. Of course, it could be at 1244.4 Hz: the tuner would also see that as Eb, which is almost exactly the 15th partial, but from what I know of strings that's unlikely. The A string zip tone seems to be in almost exactly the same relationship, and the same holds for the D string.

Hmmm...

At this point I'd love to get some string data from the test rig I have, but at the moment I'm pretty swamped in the shop with two projects on tight deadlines. I'm not sure I can be of much help for the next month or so.

If you could get some spectrum software on this, you could look at the sound output of a string when it does this, and compare that with the same string when it doesn't. Tuning the string a bit above or below it's normal pitch will change the relationship between the fundamental and the zip tone, since the zip won't move nearly as much as the fundamental does with a change in tension. See if there's a 'split' in one of the higher partials. These should show up as pretty clean single peaks in the output; if they show up as a pair of closely spaced peaks that's a sign that something is wrong.

[Massimo Milan]

Meanwhile:
I tested those Thomastik "John Pearse Folk" strings lowering the tuning by 2 semitones: the zip tone has completely gone. And I can hear that the zip tone became consonant to the pitch of the string, i.e. now the situation is:

6th D --> zip D
5th G --> zip G
4th C --> zip C

Which conclusions can be drawn from this finding?

[Alan Carruth]

"I tested those Thomastik "John Pearse Folk" strings lowering the tuning by 2 semitones: the zip tone has completely gone."
By that I'm assumin you mean that the problem is gone.

"And I can hear that the zip tone became consonant to the pitch of the string,"

So lowering the tuning pitch a whole tone dropped the 'zip' pitch by about a semitone, confirming that it is not strongly related to the string tension, but there is some effect. In this tuning the zip tone is at eight times the fundamental pitch, and the interaction with the strings is, at least, not harmful, if not beneficial.

"Which conclusions can be drawn from this finding?"

As I said in the string paper, having the zip tone at an exact odd multiple of the strings fundamental is a problem. It looks as though having it at 7.5 times the frequency, half of another exact odd multiple (15) could also be an issue. When the multiple is even the driving force of the string is out of phase with the zip tone, so it tends to cancel out fairly quickly.

As far as this guitar is concerned, there probably is no quick fix beyond either finding strings that don't misbehave, or tuning down enough to skirt it that way. On future instruments you'd want to avoid using a really tall saddle, and possibly adjusting the scale length to shift the relationship between the zip pitch and the fundamental.