Measure the frequency response of an acoustic guitar body?

[mixsit]

Does playing at different positions from the bridge change it much? ...

If you mean 'tone.. Always -and to a huge extent.

 

[jae]

If you mean 'tone.. Always -and to a huge extent.

Yes of course- I was referring specifically to diagnosing the issue with the wolf tone/sustain of that one note in particular. With other stringed instruments such as cello and contrabass there are a variety of techniques musicians use (especially with older or temperamental instruments that otherwise have good properties) to tame such notes while still preserving a good or intended/original intonation, despite the plucked or bowed position being atypical. It seems such techniques may be less practical or matter less with smaller and especially fretted instruments, especially if it is not on an open string.

 

[mixsit]

Yes of course- I was referring specifically to diagnosing the issue with the wolf tone/sustain of that one note in particular. With other stringed instruments such as cello and contrabass there are a variety of techniques musicians use (especially with older or temperamental instruments that otherwise have good properties) to tame such notes while still preserving a good or intended/original intonation, despite the plucked or bowed position being atypical. It seems such techniques may be less practical or matter less with smaller and especially fretted instruments, especially if it is not on an open string.

Ah, Thank you Sir.

 

[bluefuzz]

The answer to my question is: it could be either. A bit disappointing since the benefits of a Lx56 over the models priced one rung below it (Lx36) is supposed to be that it is made entirely by one master builder.

Yes, just because a guitar is luthier-built is no guarantee that it is any 'better' than an off the shelf instrument. Conversely, I've heard excellent sounding Asian-built factory guitars that, to be honest, sound every bit as good as anything I could build, and that sell for less than I can buy the materials to make a guitar ... ;-(

Understanding and being able to recognize what I want from a particular instrument might take fair bit of learning. There's a clear analogy to the audiophile market. A lot of the high-end products are sold on pride of ownership, optics, and bafflegab. Technical understanding can help one cope with the bafflegab.

There is probably even more 'magical thinking' in the guitar world – from both players and builders – than in the audiophile world. It can take a lot of study to learn to sort the wheat from the chaff.

Choosing a luthier to build a custom instrument might be tricky because one would want to figure out if a luthier knows how to adjust each instrument's response but even if she does, she may talk about the adjustments and their effects in a different language.

While there are an increasing number of luthiers using 'scientific' methods to build guitars – measuring the material properties of their wood, mathematical modelling, 'tuning' resonances etc., ultimately it comes down to routine, good well-proven designs, and basically good joinery skills. If a luthier is actually able to make a living building bespoke guitars then they are probably doing something right! After all, a guitar is a cultural artifact not scientific apparatus. There is no 'correct' sound from a guitar, only some cultural conventions of how 'good' guitars sound. Any more-or-less guitar-shaped box with six strings tuned to the requisite pitches is going to sound more or less like a guitar ... ;-)

 

[bluefuzz]

The LJ56 with the missing fundamental between F#3 and G3, i.e. ~190 Hz, as demonstrated in the video above.

Looks like you have the main air (body) resonance at ~105 Hz then two resonances at ~190 and 203 Hz which are probably main top and back resonances. Since the 203 Hz resonance is louder I'm guessing that is the top which is generally not desirable. You usually want the main top monopole resonance to be around 4 semitones lower than the back. The 105 Hz body resonance is also quite high for a jumbo as I would expect it down below 100 Hz for such a big guitar. This would indicate the guitar is a bit 'tight' and overbuilt which is not uncommon. Since all the major resonant peaks are clustered around F# and G then that is of course the reason for your missing fundamental. Actually it's not really 'missing' but due to all the main resonances being in the same place this note gets too much energy that dissipates quickly as a 'thunk' rather than sustaining as a note.

But don't despair, there are a couple of things you can try. First you need to figure out which resonance is which. Start by covering the soundhole with piece of light foam or polystyrene taped over it. You want something light that wont dampen the top in any way. Now record some taps on the top holding the guitar in the playing position. Try some taps with your tummy damping the back too. Then flip the guitar over and tap the back both with and without your tummy damping the top. This should give you the uncoupled response of the top and back and the 'true' resonant frequency of each plate. You will hopefully see one or more of the peaks move up or down.

Now take a small walnut size lump of blu-tak (poster putty/elephant snot) and stick it on one wing of the bridge and record the results. You should see one or more of the resonances move. What you want is to separate as much as possible those three main resonances and get them off the scale tones if possible. Try moving the blu-tak around the top and try with more than one lump. Also try putting the blu-tak on the back. If I am correct that the top resonance is highest, then lowering the back resonance may help more since I doubt you will be able to move the top far enough below the back without major surgery.

If you're lucky and some putty on the bridge helps enough then your problem can be solved just by installing one or more brass bridge pins which will add a bit of weight and look nicer than blu-tak. Alternatively just install the blu-tak inside the guitar and Bob's your uncle.

Since an increase in mass in a certain place is equivalent to a decrease in stiffness in the same place and if you can identify a place on one of the braces, either on top or back, that moves a resonance enough then you can (if you're brave) go in with a finger plane and shave the brace in that place. Since the guitar seems to be a bit overbuilt this won't have any structural consequnces if you stay behind the bridge. But it'll void your warranty obviously ... ;-)

For another fun way of diagnosing what's going on try googling 'guitar Chladni patterns'. Again, Trover Gore probably has chapter and verse on that over on ANZLF ...

The FG830 with a less severe case of a similar pathology.

This looks like the main air is ~98 Hz and the top at 185 Hz which on the face of it is better since they are not exactly an octave apart but 98 Hz is almost bang on the G and 185 is almost bang on F# which may be a bit thuddy. Ideally you want them between the scale tones, say at 95 and 200 Hz or 100 and 180 Hz repectively. Since there is no obvious third or fourth peak this probably means the back is fairly stiff and not contributing much which is not necessarily a bad thing. Often a guitar with a stiff back will have better projection where a 'live' back will have a more 'complex' tone. Think bluegrass cannon vs. delicate fingerstyle instrument.

 

[bluefuzz]

A few are trying to increase the bass response (one, uh, "luthier" even added a tube to the port)

Well the theory is fine. Antonio Torres - the developer of the modern classical guitar in the mid 19th Century - routinely installed a ‘tornavoz’ on his instruments which was basically a port tube beneath the soundhole. It reportedly does improve the bass. But the devil is in the details and, all else being equal, a side port (with tube or not) wil raise the fundamental body resonance.

I feel it appropriate to point out that while everyone focuses on the bridge as a source of vibration, the nut actually moves further and it has a two foot baseball bat for leverage on the guitar body

Yes, whatever is happening at the bridge is also happening at the nut. However, the nut is attached to a half inch thick chunk of mahogany or similar with half a pound of metal tuners screwed onto it so any movement is heavily damped. Also it doesn’t have much surface area so it's not moving much air anyway. There is a bending moment on the neck but it is at a much lower frequency and amplitude. This has more effect on bass necks, I believe, where it can be the source of dead spots if the neck resonance couples with a scale tone in the wrong way.

there's a curve at the base of the neck as it widens to meet the guitar body. look at a trumpet's bell. same curve. same purpose

No, not really. The heel is an artifact of the dovetail neck joint or the Spanish heel on a classical which both require the full depth of the body to maintain stability and a nice curve is comfortable for the player. A Fender style bolt on neck works just as well on an acoustic guitar if the neck block is sized accordingly. I have an old Eko from the 60s with a bolt on neck which works fine. The guitar is crap in other respects because it’s made of plywood but there’s nothing wrong with the neck joint! However, guitarists are a conservative bunch and they expect a full depth heel. Tradition trumps technological innovation in this case ...

 

[thefsb]

However, guitarists are a conservative bunch and they expect a full depth heel.

Electric guitarists too. Bass guitarists seem more accepting of innovation.

 

[bluefuzz]

Bass guitarists seem more accepting of innovation.

Yes, and classical guitarists too. As long as the innovation is hidden such that the instrument still looks like a standard classical. Many of the more radical innovations on acoustic guitars like double tops, carbon fibre bracing, thick laminated sides and adjustable neck joints started out in the classical world.

 

[thefsb]

After all, a guitar is a cultural artifact not scientific apparatus. There is no 'correct' sound from a guitar, only some cultural conventions of how 'good' guitars sound. Any more-or-less guitar-shaped box with six strings tuned to the requisite pitches is going to sound more or less like a guitar ... ;-)

Nicely said. I'd like to be able to say it so effectively about audio gear too. But I've not made much progress so far.

 

[whyfi]

There is probably even more 'magical thinking' in the guitar world – from both players and builders – than in the audiophile world.

100%. I've been playing guitar for 40 years and the magical thinking is fantastically strong in that community. Though I will say, improvements in digital technology and a new generation of players is improving that situation in recent years.

 

[thefsb]

Acoustic -and electric player here, read through the thread a couple of times but still not clear what it is you're trying to get from this?

Two things, the specific and the general.

The specific comes from my story with this XJ56 that I bought used on spec. I spent a lot of money on it and enjoyed it very much until I hit this F#3/G3 thunk (as @bluefuzz put it, see below) that I demonstrate in the video in which the fundamental dissipates very quickly. I want to understand it enough to judge if it is a defect or normal/expected. The instrument seller (a consumer like myself) claimed it was not a defect.

The general is how to objectively measure this performance bug/feature of the instrument, and present the evidence for it and, if possible, to start to understand its etiology.

My question in the thread title is now evidently a case of The XY Problem.

 

[thefsb]

Does playing at different positions from the bridge change it much? What about when using more left hand pressure or two fingers? Different amplitudes?

Answer to all three is: not at all.

An 'eliminator' of this type will not work well with strings under higher tension, such as those of a violin, on top of the fact there is not too much distance/vibration between the bridge and cordier anyway (which seems like an issue with most guitars...). With bass guitars I have seen people add mass or different placements of clamps (including guitar "sustain clamps") at positions on the headstock which would serve to change its mass/impedance and increase the sustain that way, so that could be one experiment. Perhaps you could also experiment with some blu tack on the body, which is a common thing to do with violins.

As far as I understand things so far from demonstrations of cello wolf tones on Youtube, my F#3/G3 thunk (as I am now calling it, previously I used the word suck to suggest the fundamental energy being sucked out of the note) is a different kind of problem.

Afaict, the wolf tone arises from the coupling of two oscilators, the string, which is a driven oscillator, and the body to which the string it is coupled. Energy is moving between these oscillators at a frequency that's low relative to the note played or the affected harmonic. It sounds a bit like a rapid amplitude modulation, a bit like the tremolo effect on a Fender amp set very rapid, although not so simple.

In my F#3/G3 thunk I don't think the body is re-energizing the string. Iiuc what @bluefuzz described above, it's that some mode or combination of modes in the body absorbs the string's energy too fast and it never comes back.

I will experiment with adding weights on the body (I will get some mounting putty tomorrow). @bluefuzz has described a debugging protocol.

 

[bluefuzz]

The instrument seller (a consumer like myself) claimed it was not a defect.

Only a fairly responsive guitar will have the potential for wolf notes, so in that sense it is not a defect. A lot of good guitars have a slightly punky F#, G, G# etc. at at least one place on the neck as that is where the ’natural’ main resonances of a guitar lie. Hence a good builder will try to distribute those resonances between scale tones as far as possible and make them far enough apart that they still couple without ‘exploding’ in a thud. It’s not easy to do so many builders (and almost all factories) build to a specific dimension and hope for the best.

 

[thefsb]

First you need to figure out which resonance is which. Start by...

In these experiments, should I aim to keep the measurement mic in about the same position relative to the guitar body as before, i.e. about 50 cm away the bridge on the perpendicular to the top?

 

[thefsb]

Only a fairly responsive guitar will have the potential for wolf notes, so in that sense it is not a defect.

So you call this a wolf note, eh. Btw, I'll get back to you with the results from the debugging tests.

I have been coming to that conclusion. It is otherwise a joy to play. So I wonder, can I live with it or will I let it annoy me? And if I sell it, will I feel compelled to mention the thunk? My problems, not yours.

A lot of good guitars have a slightly punky F#, G, G# etc. at at least one place on the neck as that is where the ’natural’ main resonances of a guitar lie. Hence a good builder will try to distribute those resonances between scale tones as far as possible and make them far enough apart that they still couple without ‘exploding’ in a thud. It’s not easy to do so many builders (and almost all factories) build to a specific dimension and hope for the best.

If it is such a delicate matter I wonder if time spent in certain environments can change the behavior of the body so that a guitar that tests well in final QC at the factory could develop such a thunk.

Anyway, the obvious advice, play the guitar before buying or have a return option, I got from my teacher...

But an LJ56 is so rare that, well, I had to either buy this one on terms available or not.

And now I want to find one of those cheap aisian guitars better than ones you can make. It's GAS, I know. I have the FG830, which sounds very pleasing. Once Brant is back from vacation I'll get the nut cut a bit deeper and I can finish set up after which I'm sure its playability will be excellent.

 

[LightninBoy]

How old is this guitar?

I had the same issue with a jumbo acoustic. I was young and cobbled together money for my first really nice acoustic. I tried a bunch of new ones from usual suspects (Taylor, Martin, Gibson, etc) and decided on a Larivee J10 - a jumbo acoustic, spruce top, rosewood back and sides, very similar to yours. The sound was enormous. Got it home and during some recording discovered a certain note - an F#- just didn't ring. Otherwise, fantastic instrument. It was new, so I had a 30 day return window. I decided to keep it.

That was 20 years ago. Still have the guitar and the F# issue is gone. Not sure when or how.

 

[thefsb]

How old is this guitar?

Not sure but I think 2019.

I had the same issue with a jumbo acoustic. I was young and cobbled together money for my first really nice acoustic. I tried a bunch of new ones from usual suspects (Taylor, Martin, Gibson, etc) and decided on a Larivee J10 - a jumbo acoustic, spruce top, rosewood back and sides, very similar to yours. The sound was enormous. Got it home and during some recording discovered a certain note - an F#- just didn't ring. Otherwise, fantastic instrument. It was new, so I had a 30 day return window. I decided to keep it.

That was 20 years ago. Still have the guitar and the F# issue is gone. Not sure when or how.

That starts to answer a question I asked 33 minutes ago.

Nice story. Thanks for sharing. I'm glad you haven't traded up to a model with even higher SINAD : )

 

[bluefuzz]

In these experiments, should I aim to keep the measurement mic in about the same position relative to the guitar body as before, i.e. about 50 cm away the bridge on the perpendicular to the top?

For these types of tests it’s not really that important as you’re only interested in the relative frequencies of the peaks. Actual amplitudes etc. aren’t important. But of course having a repeatable testing regimen is never a bad idea for comparison later on. It’s a good idea to take an average of, say five ‘whacks’ which is easy to do in VA.

 

[bluefuzz]

So you call this a wolf note, eh.

Well, the violin people call it that. I think the guitar world has adopted the term although it is not exactly the same phenomenon as a cello wolf note since a cello is continuously receiving energy from the bow and that affects how resonances develop but the idea is in the same ballpark ...

 

[bluefuzz]

So I wonder, can I live with it or will I let it annoy me? And if I sell it, will I feel compelled to mention the thunk?

Let’s see what the blu-tak tells us. If you are otherwise happy with the instrument then you can probably mitigate the problem enough to live with it. There are a few other things to try if the blu-tak doesn’t help. Although the most effective remedies will involve some permanent modification to the guitar. The question is is the guitar worth more to you or anyone else in stock condition with a thunk or non-stock and thunkless … ;-)