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Resonance - Curious Observation
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Author:  George L [ Sat Apr 27, 2013 6:55 pm ]
Post subject:  Resonance - Curious Observation
Friends,
I've been playing my most recently completed guitar (#4) quite a bit lately and I'm very happy with the way it turned out. The guitar has a pleasing tone, strong projection and sounds pretty darned good overall. However, there is something going on with certain notes that I don't understand and I suspect gaining some insight into this phenomenon may help me inch closer to a "left-brain" epiphany of some sort--at least I hope this is the case. :-)

From the beginning I've heard something that seemed out of balance and after much tinkering today I've determined that playing any B or A note results in the guitar ringing out with what I would describe as a noticeably strong harmonic sustain--the strings and body seem to have excess energy at those frequencies. It is more pronounced with B notes than A notes, but they both ring out. I don't hear anything unusual when I pluck a C or any other notes.

So, could this be the result of my top, back and body resonances being too close? I read about this all the time and try hard to keep up, but I'm much better at learning by doing than by reading--I'm just that way. If I understand this correctly and my plates and/or body are "coupling" then what I have here is a fairly clear example of a wolf note, right? I'm actually really excited by this as I think it could propel me along the learning curve. I have a new microphone and recently downloaded Audacity, so I may be able to learn a good deal by experimenting with this guitar.

Right or wrong, I'm looking to you all for clarification as to what might be happening and for insight into what I might do to explore and remedy the issue. If you have ideas you'd like to share I'd be very happy to read them.

Thanks in advance,
Author:  klooker [ Sat Apr 27, 2013 7:11 pm ]
Post subject:  Re: Resonance - Curious Observation
I can't offer any insight but I'm sure Alan & Trevor will be along soon.

In the meantime, I'd record some taps with Audacity. I made a mallet with a super ball & a dowel. I'm sure you could fashion something similar or buy something from a music store. I tap on the bridge, with strings on, but I thread a thin rag through them to prevent them from vibrating.

Record a series of taps, select the taps by clicking & dragging , then click ANALYZE - PLOT SPECTRUM. Be sure SIZE is set to 16384.

Also Google frequencies of musical notes which should show you what frequencies correspond to what notes.

Hope this helps (or gives you something to do) for now.

Kevin Looker
Author:  meddlingfool [ Sat Apr 27, 2013 8:47 pm ]
Post subject:  Re: Resonance - Curious Observation
It's all coupling anyway, unless it's a total piece of junk. But you're saying that it sounds good and I believe you. I'm just saying its not coupling itself that is the problem. In fact, coupling is a goal.
The question is, at what frequencies are your main three resonances occuring? If you're getting a wolf note, chances are one of them is too close to a scale tone for its own good, and will be hunted down and shifted once located.
Since you have the technology at your fingertips, the answer shall soon be revealed!
Author:  meddlingfool [ Sat Apr 27, 2013 8:48 pm ]
Post subject:  Re: Resonance - Curious Observation
Also, your fingertip works just fine, no mallet required...
Author:  George L [ Sun Apr 28, 2013 10:57 am ]
Post subject:  Re: Resonance - Curious Observation
Looks like I'm misusing the term "coupling." Okay, good to know. Does it seem like what I'm describing is a wolf note" or something else? I've read many pages of information on wolf notes, but never really noticed any of the guitars I've owned or played exhibiting anything like that, so I'm not sure what they sound like.

I have a busy day today, but should have some time this afternoon to try and record some taps and plot the spectrum using Audacity. I'm looking forward to giving that a go. Thanks for the replies. I appreciate your help.
Author:  David Malicky [ Sun Apr 28, 2013 1:45 pm ]
Post subject:  Re: Resonance - Curious Observation
A wolf would normally sound louder a first, then quickly quiet, as the body quickly 'steals' the string's energy when the resonances line up. And I think A-B would be an unusual place for a wolf, as it's not near the usual air and top resonances. A wolf would also be in a very narrow range, like A or B but not both. But, Alan and Trevor know much more than I on wolfs.
How long has the guitar been strung up? In the first few weeks, sometimes there can be odd effects.
Author:  Alan Carruth [ Sun Apr 28, 2013 3:25 pm ]
Post subject:  Re: Resonance - Curious Observation
There are number of things that can be 'wolf' notes, with somewhat different behaviors. The essential common element is a strong resonance that can couple with the string. In some cases it extracts energy from the string very quickly and dumps it into the room, giving you a note that's twice as loud for half as long, say. This is usually noticed as a lack f sustain on that note, since our ears are less sensitive to large changes in power than they are to the presence or absence of a sound at a threshold level. The other common manifestation of a 'wolf' is a note that 'warbles', or 'buzzes' or simply has a poorly defined pitch. This hapens most commonly when the top moves so much that bridge motion confuses the string as to how long it is. Hunting these can drive you wild.

What sort of guitar is it? Some of the main resonant pitches of the top, such as the 'main top' (monopole) and 'long dipole' modes, tend to come in at similar pitches on steel and classical guitars of normal design, while others, such as the 'cross dipole' and 'cross tripole' may be much different, or even missing. Knowing what you've got might help us to suggest places to look for the problem.
Author:  George L [ Sun Apr 28, 2013 6:38 pm ]
Post subject:  Re: Resonance - Curious Observation
Thanks for all the replies. The guitar in question is based on a 00-sized 14 fret Martin. It is spruce over mahogany, with spruce braces, a maple bridge plate and rosewood bridge. It has been strung up and played fairly regularly for quite a while--probably close to a year. As I said originally, it sounds pretty good. However, building guitars has redirected my focus intently on details that I would have previously overlooked or ignored. In days gone by I would have just played whatever was handy and not worried about what was happening "under the hood."

So, this afternoon I plugged in my microphone, dampened the strings with a piece of soft fabric and used Audacity to record a series of ten taps on the bridge. I then let Audacity plot the spectrum and it came up with this:

Image

I've been digging through old threads with similar info, but I can't quite grasp what it is I'm looking at here. It feels like I'm tantalizingly close to learning something new today--or at least taking a step closer. Anyone want to share thoughts about what this is saying and how I can stat learning to read these plots on my own?

Thanks again!
Author:  Fred Tellier [ Sun Apr 28, 2013 7:45 pm ]
Post subject:  Re: Resonance - Curious Observation
The 1st tall peak is your main air frequency and the next the coupled top frequency. Your back is not very active as its peak is not real strong the one slightly above 300 is probably it. I some times will turn the guitar around and tap the back which will raise the back peak.

Fred
Author:  George L [ Sun Apr 28, 2013 9:12 pm ]
Post subject:  Re: Resonance - Curious Observation
Thanks, Fred. When I have this plot active in Audacity and use the cursor to highlight information, it shows the peak main air frequency as 117 Hz (A#2) and the coupled top frequency peak as 206 Hz (G#3). The one you say is is probably my main back frequency peaks at 301 Hz (D4). If I'm reading the old threads I've uncovered correctly, then my main air and main top are too close, being only three semi-tones apart (G# - A - A#) and my main back is smack dab on D, which I'm guessing is probably not optimal.

I'm not sure this has anything to do with the tonal characteristics that prompted me to start this thread, but working to adjust these relationships may be beneficial nonetheless and can help me learn how these frequencies interact.

So... what to do now? Corrections to my interpretation of the data and tips and tricks that may help remedy the situation would be welcomed!
Author:  Trevor Gore [ Mon Apr 29, 2013 7:18 am ]
Post subject:  Re: Resonance - Curious Observation
George,
Your plot will be easier to read if you choose "Linear" instead of Log Frequency (lower right hand box, bottom left of screen).

Your main air and main top are slightly less than an octave apart (one tone less, in fact) so nothing to worry about them being too close. They are normally ~ an octave apart, but you should avoid them being exactly an octave apart or on scale tones.

My 00 sized guitars come out around 100, 180, 226Hz, so yours is looking somewhat stiff in comparison. The high air resonance is likely due to having a fairly stiff back (my guess would be the peak ~260Hz), but better too stiff than too soft. The peak at 300Hz I would guess as being your cross dipole.

If you have a resonance problem (hard to say at this distance) strong B or A notes suggest a high order resonant peak that is a harmonic of ~Bb. The peak would be high enough to give plenty of "gain" but not so high as to create a wolf note due to over admittance. The frequencies of the higher harmonics of Bb are 233Hz, 466Hz, 699Hz etc. You have a peak ~440Hz which is right on an A harmonic. The B note harmonics are at 123.5, 246.9, 493.9Hz etc., but I don't see any peaks at those frequencies.

You could try doing another tap plot, but be a bit more random in where you tap and see if you can find a high peak on one of the harmonics of A, Bb or B. Alternatively, if you have the gear, you could do Chladni tests at those frequencies and see what "jumps". Either way, if the problem is a resonance problem, you'll need to know the mode in order to figure out what to do about it and you'll need to do Chladni tests to reveal the vibration mode.
Author:  George L [ Mon Apr 29, 2013 8:45 am ]
Post subject:  Re: Resonance - Curious Observation
Thanks for your analysis and advice, Trevor. The more I dig the deeper I go. I'll confess that all this confuses me to no end, but I like puzzles and it's fun to try and figure things out. I will definitely record more taps, not only with this guitar but with all that I have now and those I build in the future. I may never be a master builder, but searching for correlations between materials, techniques and tone can only help my efforts. Your books are making their way to the top of my reading list.
Author:  Fred Tellier [ Mon Apr 29, 2013 11:09 am ]
Post subject:  Re: Resonance - Curious Observation
George, you now are entering an interesting part of Luthiery that I really enjoy messing with. Every new tool we add to our tool box has a learning curve, it sounds like you are well on your way with the frequency evaluations.

Fred
Author:  gozierdt [ Mon Apr 29, 2013 2:17 pm ]
Post subject:  Re: Resonance - Curious Observation
+1 on Fred's comment. As a retired Mechanical Engineer, I enjoy the tap and chladni testing
as much as any part of building. I can't claim to have anywhere near the knowledge of people
like Trevor and Al, but it's sure fun trying...
Author:  George L [ Mon Apr 29, 2013 5:22 pm ]
Post subject:  Re: Resonance - Curious Observation
Thanks guys! My father was an engineer. Though he never voiced any disappointment, I know he was dismayed when I chased my muse along an opposite path. Sadly, he died a long time ago, while I was still floundering in my own private Bohemian rhapsody. I wish he were around now, not only to see that I actually turned out okay but because I think he'd enjoy helping me figure this kind of stuff out.
Author:  George L [ Tue Apr 30, 2013 12:09 pm ]
Post subject:  Re: Resonance - Curious Observation
As suggested above, I turned the guitar around and recorded a series of taps on the back plate. I put that plot below. When I read it in either linear or log format the highest peak is found at 276 Hz (C#4). Is it correct to assume this is my back plate resonance? The second highest peak is at 324 Hz (E4). I guess that's my cross dipole?

Image
Author:  David Malicky [ Tue Apr 30, 2013 1:27 pm ]
Post subject:  Re: Resonance - Curious Observation
Yes, 276 Hz is the main back. 324 Hz is likely another back mode, since it didn't show up in the top taps, and since it's the second highest peak when tapping the back. Tapping a little closer to the waist will bring out the higher back mode(s).
Author:  Fred Tellier [ Tue Apr 30, 2013 1:31 pm ]
Post subject:  Re: Resonance - Curious Observation
If you plug the sound hole you will get the uncoupled back frequency, what you are seeing in that plot is still affected by the top coupling. If you have Trevor's design book there is a huge amount of information on this stuff. I wish my middle aged brain could absorb it quicker

Fred
Author:  George L [ Tue Apr 30, 2013 2:52 pm ]
Post subject:  Re: Resonance - Curious Observation
Thanks for the help!

This is really quite fascinating. I've recorded front and back taps for the four guitars that I've built and the three others that I currently own. As I build more guitars I plan to start collecting additional data related to the plates, braces, bridge, etc. I'm not sure I'll ever fully understand what this information is telling me, or that I'll know how to apply it in the building processes, but it will certainly be interesting to consider and evaluate the relationships. Exploration is fun. So is procrastination, which is what I'm doing now! Time to get back to work...
Author:  meddlingfool [ Wed May 01, 2013 9:15 am ]
Post subject:  Re: Resonance - Curious Observation
Will laying the guitar face down on the bench accomplish the same thing as plugging the sound hole?
Author:  WaddyThomson [ Wed May 01, 2013 9:54 am ]
Post subject:  Re: Resonance - Curious Observation
no.
Author:  meddlingfool [ Sat May 04, 2013 8:14 pm ]
Post subject:  Re: Resonance - Curious Observation
Ok, thanks...
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