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A "parasite" tone

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A "parasite" tone

Postby Massimo Milan » Fri Aug 26, 2016 6:46 am

Hi all,
I came to this forum trying to get rid of a problem affecting my brand new acoustic guitar. My trusted luthier and friend cannot yet understand the source of the problem.
We are dealing of a brand new nylon stringed crossover guitar: cedar top, indian rosewood back and sides, walnut neck, ebony fretboard, maple bridge, spanish heel neck, no truss rod.
True oil and french polished shellac finishing, white MOP rosette and binding: a masterpiece.

Now this is the problem. When fretted in the middle of the fret, all the strings but mostly 4th and 5th sound like if it were two guitars sounding, being one of them just slightly flat. When fretted just near the fret bar, tones come off perfectly clear. Fret bars were checked and slightly recrowned to make the top sharper; the fretboard is perfectly straight and flat; the saddle is made of bone and it was checked, it's ok.
We put on the fretboard surface a taller fret to which the tang was filed of, and fretted the string to that "new" fretbar: no way, actually the doubled tone was more evident, all along the fretboard, fretting any of the strings. Trying with taller and taller frets, the resonance becomes more and more evident!

I'd say it is quite clear that the secondary flat tone comes from the "secondary" diapason between the saddle and the point where the finger press the string; this parasite tone can be heard at the attack and peak, while it fades away at the beginning of decay so in the decay phase you can hear only the main tone.
Any ideas about what else to check/do/try?
Please help me, I longed for this guitar for one year and still now can't play it. I will share your opinion with my friend luthier who built this guitar.
Thank you in advance for your time.

Massimo
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Re: A "parasite" tone

Postby Mike Conner » Fri Aug 26, 2016 9:21 am

Perhaps you have already checked ths, but maybe the nut slots are a little higher than necessary? If so, that can leave a length of string free to vibrate behind the fretted position (between the nut and the fingered position). I encountered an issue like this with a steel string some years ago, and it would have a sitar type resonance at certain notes when played with a capo. After hunting for the ghost vibrations in the usual way (tightened tuner bushings, checked for loose braces, etc) the owner happened to stick a flat pick under the strings behind the capo and the ghost was gone! Quick touch up of the nut slot and good to go. Your taller fret experiments suggest this might be worth checking into.

You could also try damping the strings from the nut to tuning machines with some cloth and see if that solves it. Same thing if you have a tailpiece - damp the strings between the saddle and string anchor.

Hope this helps, and good luck!
//mike
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Re: A "parasite" tone

Postby Mark Swanson » Fri Aug 26, 2016 9:54 am

I would suggest the same thing. Neck relief can have an effect too but you don't have a trussrod so you can't adjust that. Anyway, string vibration behind the fretting finger is something that isn't usually thought of, and it can be missed quite easily, so check it by somehow dampening the string between the nut and your finger to see if it goes away.
    Mark Swanson, guitarist, MIMForum Staff
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Re: A "parasite" tone

Postby Jason Rodgers » Fri Aug 26, 2016 10:25 am

Put a hair tie around the neck at the nut, or tie a sock on there, and then play. If the sound goes away, you've found it.

If it is that length vibrating behind the fretted pitch, do you only notice it when you bring fingers down with some force (hammer-on), or just any finger pressure? Is it any particular pitch or octave that has this effect? I wonder if the neck as a unit is vibrating sympathetically at some frequencies and putting motion to that afterlength. Another thing to try is to attach a clamp - like a spring clamp - to the headstock and see if that stops it.
-Ruining perfectly good wood, one day at a time.
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Re: A "parasite" tone

Postby Massimo Milan » Fri Aug 26, 2016 11:20 am

Jason Rodgers wrote:Put a hair tie around the neck at the nut, or tie a sock on there, and then play. If the sound goes away, you've found it.

If it is that length vibrating behind the fretted pitch, do you only notice it when you bring fingers down with some force (hammer-on), or just any finger pressure? Is it any particular pitch or octave that has this effect? I wonder if the neck as a unit is vibrating sympathetically at some frequencies and putting motion to that afterlength. Another thing to try is to attach a clamp - like a spring clamp - to the headstock and see if that stops it.


Well it is not a back fret buzz, it is a parasite tone which sounds on every fret all along the fretboard; it's a real doubled tone, the second originating from the string segment between the fretting finger and the saddle, that's why it sounds slightly flat (due to longer diapason). I tried also the clamp to the headstock, no way.
I am quite sure I have realized what is it; the problem is to find why and get rid of it.
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Re: A "parasite" tone

Postby Brian Evans » Fri Aug 26, 2016 12:23 pm

Are the frets a normal guitar size and height? When you fret "in the middle of the fret" do you mean in the middle of the space between the frets, and are you pressing the string all the way down to the finger board? Have you tried different make and tension of strings? Is it the same with all right hand technique - thumb, rest stroke, normal stroke, using a pick? All I can think of...
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Re: A "parasite" tone

Postby Alan Carruth » Fri Aug 26, 2016 12:28 pm

I've already stated my opinion on another forum that this sounds like a case of the strings 'rolling' on the fret tops when they are fretted further back. The string then has two different vibrating lengths; the normal one when it's moving vertically with respect to the soundboard, and another when it's moving 'horizontally'. The vertical polarization stops at the fret, while the horizontal one stops a bit further back, and makes a lower pitch. Generally the difference in pitches is small, and usually only shows up as a 'beat' tone, if it is audible at all.

Normally you would expect a taller fret to fix this, so the experiment with the taller frets is puzzling. On the other hand it's very difficult to think of anything else that would show those symptoms: it happens on every fret, but only when you depress the string in between frets, and doesn't happen when the string is fretted on the fret.

Since it happens on all notes it's not a 'wolf' caused by a resonance. One loose fret might cause this on a single note, but not all the way up the neck. It's just possible that there is some loose part someplace: I had a set of Schaller tuners once that had a loose collar on the tuner button shaft that made an awful racket, but it wasn't by any stretch a 'tone', and didn't track the played pitch.

This is a puzzler for sure. I wish I had it on the bench....
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Re: A "parasite" tone

Postby Massimo Milan » Fri Aug 26, 2016 12:29 pm

Brian Evans wrote:Are the frets a normal guitar size and height?

Yes

When you fret "in the middle of the fret" do you mean in the middle of the space between the frets,

yes

and are you pressing the string all the way down to the finger board?

no

Have you tried different make and tension of strings?

yes

Is it the same with all right hand technique - thumb, rest stroke, normal stroke, using a pick?

yes

thank you
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Re: A "parasite" tone

Postby Massimo Milan » Fri Aug 26, 2016 12:33 pm

Alan Carruth wrote: This is a puzzler for sure. I wish I had it on the bench....


Just received a message from my friend luthier saying he is probably near to the solution If true I will let you know for sure, for now thank you so much for your contribution.
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Re: A "parasite" tone

Postby Brian Evans » Fri Aug 26, 2016 2:48 pm

I will say that I don't think it's string or fret related, but is a harmonic sympathetic vibration somewhere else. dibs on that until we know what the fix is... :)
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Re: A "parasite" tone

Postby Bryan Bear » Fri Aug 26, 2016 3:33 pm

How is the break-angle of the saddle? Could the string be moving side to side there?
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Re: A "parasite" tone

Postby Massimo Milan » Fri Aug 26, 2016 5:18 pm

Bryan Bear wrote:How is the break-angle of the saddle? Could the string be moving side to side there?


The break angle is about 45°
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Re: A "parasite" tone

Postby Massimo Milan » Sun Aug 28, 2016 2:10 pm

Update.
Assumed, as absurd as it may sound, that the guitar is too much responsive, and that an exceeding amount of energy is driven back from the saddle to the fret, the bone saddle was substituted with an ebony saddle. Well, no change in the generation of the parasite tones, just a (little) fall of high frequencies as it could be expected. So, is this a too much responsive guitar ??
Yet I'm not a luthier, but i must confess that this issue is shaking much of my knowledge about physics and reality.
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Re: A "parasite" tone

Postby Bob Gramann » Sun Aug 28, 2016 8:47 pm

I generally build (and play) steel string guitars, but I have built 5 classicals including the one that I kept and play. Every one of them has exhibited this behavior--a note fretted halfway back or more from the fret generates a secondary note as you describe. It gets to be less and less of a feature moving up the neck to where the distances between frets are shorter. My conclusion was that this was a feature of nylon strings and I had to fret carefully when I play. I never notice it when I play. One of my classicals is owned by a very good player who plays it constantly--I think he's had it for 9 years. He has never said anything but compliments about the instrument. I used the LMI fw74 fretwire with a crown height of .043". As you described, the secondary note seems a bit flatter than the fretted note. It seems to me that the string is just jumping above the fret. I haven't checked out classicals by other makers for this feature. If I do remember when I have someone else's in my hands, and if it doesn't have this feature, I will be sure to analyze the differences in the guitars.
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Re: A "parasite" tone

Postby Massimo Milan » Mon Aug 29, 2016 3:11 am

Yesterday I found on the web an article by the luthier Paul McGill in which seems to be described exactly this roblem.
He says:
"I had a classical guitar made by a notable maker in my shop once that would not stop buzzing. The frets were accurately aligned, and still it buzzed. This guitar was very lively and had great oscillation of its strings. After much time spent studying the problem it became clear that the noise disappeared if the note was played with the finger depressing the string right on top of the fret. If the finger was moved back even an 1/8th of an inch, the buzz reappeared. The strings had so much amplitude that the strings would accelerate right off the fret, causing a buzz even though everything was properly aligned. I raised the action, the buzz got worse. The owner said he liked bass wire on his guitars, so I installed huge bass wire in an attempt to eliminate the problem. I was skeptical, but after installing the larger wire, the noise was greatly reduced. It was still there if the player wasn't careful, but it was manageable."
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Re: A "parasite" tone

Postby Brian Evans » Mon Aug 29, 2016 8:24 am

What an interesting observation! Two luthiers observing that "the string is jumping/accelerating off the fret". I would expect the flexibility of the nylon strings is the issue. I would wonder if hard tension strings would make any difference at all. What influences the amplitude of the string, I wonder?
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Re: A "parasite" tone

Postby Massimo Milan » Mon Aug 29, 2016 8:36 am

I have just found out that in 2013 on another forum this issue has yet been discussed, and also on that occasion the article by Paul McGill was a key insight
Hope not to make something "illegal" if I quote that thread
http://www.acousticguitarforum.com/foru ... p?t=286696
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Re: A "parasite" tone

Postby Alan Carruth » Mon Aug 29, 2016 2:47 pm

Try adding some mass at the bridge. Poster adhesive is a good way to do that. You can find the smallest amount that will work, and the best place to put it pretty easily. It will, of course, impact the treble response, but with luck you can minimize that.
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Re: A "parasite" tone

Postby Massimo Milan » Mon Aug 29, 2016 2:53 pm

Alan Carruth wrote:Try adding some mass at the bridge. Poster adhesive is a good way to do that. You can find the smallest amount that will work, and the best place to put it pretty easily. It will, of course, impact the treble response, but with luck you can minimize that.


I have no news from my friend luthier since some days, guess he's working on it; I'm humbly sharing with him any advice I get here there and everywhere ;-) thank you !
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Re: A "parasite" tone

Postby Bob Gramann » Mon Aug 29, 2016 6:21 pm

I used closeup lens on a DSLR camera and made a movie of the string at the fret. It showed clearly that the string jumped off the fret after the initial pluck. I couldn't record at any more than the standard 30 fps, so I couldn't do a good slow motion video. But, I would assume that if there is enough energy at the initial pluck to lift the string obviously off the fret, there is probably enough to lift it on subsequent vibrations. I'm going with the jumping string theory.

On a steel string gutar, you don't get good tone if you depress too far back from the fret. I guess that's why this isn't an issue on the nylon string guitar when I play--I'm used to fretting right behind the fret.
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