Official Luthiers Forum!

Owned and operated by Lance Kragenbrink
It is currently Sat Aug 09, 2025 1:04 am


All times are UTC - 5 hours


Forum rules


Be nice, no cussin and enjoy!




Post new topic Reply to topic  [ 30 posts ]  Go to page Previous  1, 2
Author Message
PostPosted: Wed Sep 19, 2012 3:09 pm 
Offline
Cocobolo
Cocobolo

Joined: Sat Sep 04, 2010 9:28 pm
Posts: 303
First name: Hugh
Last Name: Evans
Country: USA
Focus: Build
Status: Professional
Do you have a tube in the port? If not, that would give you another parameter by which to shift the offending peak. The only downside is that adding a tube will only be able to lower the frequency of the peak. Also consider that before enlarging the port, if you go that route, you can test the hypothesis first by adding a flange. In both cases, there's a good chance of shifting the peak far enough down to get it out of the way. Otherwise, there's plenty of good advice to work with here already.

Going off on a slight tangent: I realize it's probably a bit frustrating at the very least... But this is a neat problem at the same time (part of why I asked about the presence of port tube, similar to a bass reflex enclosure.) Basically, what it boils down to isn't that the instrument is failing to intonate properly... Rather the string vibration is close enough to a resonant peak in the instrument response itself that it's inducing sympathetic vibrations, aka wolf tones. I'm not sure if there's a reason beyond coincidence, but I've only personally experienced them a couple of times and they were in the same frequency range as this case. Again, really interesting problem, and I look forward to seeing how it resolves.


Top
 Profile  
 
PostPosted: Thu Sep 20, 2012 1:48 pm 
Offline
Brazilian Rosewood
Brazilian Rosewood

Joined: Sat Jan 15, 2005 12:50 pm
Posts: 3933
Location: United States
hugh evans wrote:
"Rather the string vibration is close enough to a resonant peak in the instrument response itself that it's inducing sympathetic vibrations, aka wolf tones."

It's actually trickier than that, if I understand what you're driving at. In cases where there is a resonance in the instrument that causes the top to move a lot at the pitch of a played note, or a partial of one, the string resonance can actually be 'split', so that it shows up at two different frequencies close to each other. In addition, the low impedance of the top at resonance can bring it close enough to to the impedance of the string to seriously reduce sustain.

All of the math that describes what vibrating strings are supposed to do starts out by assuming that the ends are 'fixed': totally immobile. If this were the case on a guitar, you would not hear it, so we know that condition is not being met.

The main way that strings drive the top is through the 'transverse' force developed as the string vibrates. There are a couple of ways to look at this, but one way to think of it is that as the string moves 'up and down' it pulls the top along with it. The string can also move 'side to side', of course, but, except for the possibility of rolling on the top of the saddle, the bridge itself won't move that way enough to make a difference: you can consider the bridge to be a 'fixed' termination for crosswise motion over most of the frequency range of the guitar. That's not true in the 'vertical' direction, particularly at the 'main top' and 'main air' pitches (Trevor's two (1,1) monopole modes). If the end of the string is moving, the stationary point that defines the vibrating length will not be at the top of the saddle, and the string will thus 'want' to make a different pitch than its supposed to, but _only_ when it's moving 'vertically'. As Trevor points out, the effect of a strong top resonance is to 'repel' the string pitch: it will be either lower or higher than the top resonant pitch, but can't match it.

Normally, of course, when you play a string, you don't set it off in a purely 'vertical' or 'horizontal' direction; it's moving at some angle. If, say, it's the open G string, and the 'main top' resonance is just above that, at say, 196 Hz, what will happen is that the 'horizontal' part of the vibration of the string will produce the correct pitch of 195.9 Hz, while the 'vertical' motion will proceed at a lower frequency. The string will 'beat' with itself at whatever the difference frequency is. If that difference approaches seven Hz or so, it will sound a lot like a fret buzz. This is not really a 'sympathetic vibration'; nothing else needs to get involved, since the string is beating with itself.

The really extreme case of this is the 'wolf' on the 'cello. There are four major ingredients to this recipe:
1) the string is bowed, so there's a constant input of energy,
2) the bowing constrains the motion of the string to a single plane, in the direction that the top of the bridge is moving the most,
3) the bridge itself is very tall, so that a small motion of the top can induce a large travel at the top of the bridge, and
4) there is a strong resonance that causes the bass bar to move a lot vertically, while the sound post is 'nailed'.

When this happens (all too frequently) the impedance of the top of the bridge at the string end for the fundamental of the note (remember that impedance is frequency dependent) becomes very nearly equal tot he characteristic impedance of the string itself. In essence, te bridge 'looks like' a continuation of the string at that frequency, so the string loses its termination, but _only_ at that frequency. There's usually a decent impedance mismatch at the octave, so the string simply shifts upward in pitch. When this occurs, the vibration of the body at the troublesome fundamental is no longer being fed, and it does out quickly, since this is usually a strong radiator of sound. Once the body vibration dies away, the top of the bridge stops moving, and the string suddenly 'sees' a proper termination, so it shifts back down to the fundamental,and the whole thing starts over.

If you could bow the open G on a guitar in the 'vertical' direction, you might well get the same sort of response, particularly on some Classicals or Flamencos with very light bridges. As it is, I've seen guitars that had a pretty exact pitch match between the top mode and a played note where there is not much of a problem. In some cases the mass of the bridge is sufficient by itself to impose a reasonable termination on the string. In other instances, the fundamental does indeed 'come and go', but it's not very noticeable in the overall transient of the sound.

There are _lots_ of other 'wolf' notes that lurk in the guitar, and some do involve vibration of the back or air as well as the top and the string. This makes the hunt challenging, but often, once you've found out the cause, it's pretty easy to fix a wolf.


Top
 Profile  
 
PostPosted: Thu Sep 20, 2012 1:59 pm 
Offline
Contributing Member
Contributing Member
User avatar

Joined: Fri Mar 24, 2006 12:42 pm
Posts: 2360
Location: Windsor Ontario Canada
First name: Fred
Last Name: Tellier
City: Windsor
State: Ontario
Zip/Postal Code: N8T2C6
Country: Canada
Focus: Build
Status: Amateur
I was hopping this thread would get some discussion going that might help me find a cure to my problem, but the great thing about this forum is often a thread becomes more interesting with each response and adds to our knowledge base

I will have the guitar back this weekend and will keep everyone informed what is happening with it. I hope we can keep this great discussion going.

_________________
Fred Tellier
http://www.fetellierguitars.com
Facebook page http://www.facebook.com/pages/FE-Tellier-Guitars/163451547003866


Top
 Profile  
 
PostPosted: Fri Oct 05, 2012 3:46 pm 
Offline
Contributing Member
Contributing Member
User avatar

Joined: Fri Mar 24, 2006 12:42 pm
Posts: 2360
Location: Windsor Ontario Canada
First name: Fred
Last Name: Tellier
City: Windsor
State: Ontario
Zip/Postal Code: N8T2C6
Country: Canada
Focus: Build
Status: Amateur
I thought I should add an update on the progress. I have been able to raise the body resonance to F# + 40 cents and the problem has been reduced to a tolerable amount, around +4 cents on G2 and -3 cents on F. The sound of the guitar has not changed to my ear so I will stop and get the owner to play it for a while and decide if he needs it closer.

The change was enlarging the sound port which got me to 93hz and added a brace to the back which got me to 94hz. If we need to go farther I will add material to the back brace I added.

Thanks for the input guys, it always helps to see another persons ideas.
Fred

_________________
Fred Tellier
http://www.fetellierguitars.com
Facebook page http://www.facebook.com/pages/FE-Tellier-Guitars/163451547003866


Top
 Profile  
 
PostPosted: Fri Oct 05, 2012 7:51 pm 
Offline
Koa
Koa
User avatar

Joined: Mon Sep 05, 2011 10:45 pm
Posts: 1484
First name: Trevor
Last Name: Gore
City: Sydney
Country: Australia
Focus: Build
Status: Professional
Well done, Fred!

If the owner wants closer, you have to start asking if he's listening for Just temperament (which he's not going to get) rather than equal. Equal temperament thirds are ~14 cents sharp of Just, which puts your small residual error into perspective. You also have to hope that for the keys he plays mostly in, the residual errors close the gap toward Just intervals rather than widen them. If he plays all sorts of stuff in all sorts of keys, best just to get as close as possible to accurate equal temperament.

There's always the Sect. 4.7.2.3 solution... (custom empirical nut and saddle intonation).

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 30 posts ]  Go to page Previous  1, 2

All times are UTC - 5 hours


Who is online

Users browsing this forum: No registered users and 38 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
Powered by phpBB® Forum Software © phpBB Group
phpBB customization services by 2by2host.com