Measure the frequency response of an acoustic guitar body?

[bluefuzz]

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.

I think it’s fairly well established that guitars can and do change with time. Why and by how much is still debatable. I wouldn’t expect the main resonances to change more than a semitone in the lifetime of a guitar but there is certainly some settling in of a new instrument. I say the wood is ‘learning to be a guitar’ but who knows what’s really happening? There is much received wisdom about ‘vintage tone’ in guitar circles but I don’t see why a guitar should necessarily get ‘better’ with age ...

That said, I built a guitar a few years ago that I wasn’t very happy with. Oh well, shit happens ... I took it out the case recently and it sounded great! Placebo? Wishful thinking? Did something actually change? I really should do some tap testing on it to see what’s going on, if anything ...

In recent years there has been a fashion for using ‘baked’ or ‘torrified’ woods in guitar making to emulate the effects of age on a guitar. There is some evidence that the volatile components in some woods oxidise with age and the lignin hardens resulting in wood that is more resistent to humidity swings and is both lighter and stiffer. Torrefaction seems to do the same (it’s a technique loaned from the hardwood flooring business). Does this result in instant vintage tone? I’m sceptical. However, I tried tried baking a top in the oven for fun a while back. It came out a bit toasty (and the house stank for days!) so I used it in a dark sunburst copy of a Gibson ‘Robert Johnson’ style L1. It’s probably my best sounding guitar. Hmm, is this due to the baked top? Consummate skill? Luck? Who knows …? ;-)

 

[thefsb]

In recent years there has been a fashion for using ‘baked’ or ‘torrified’ woods in guitar

There's a fashion in electric guitars for roasted maple. It is sold on the aesthetics and reduced expansion/shrinkage owing to atmospheric conditions. I know of no measurements for the latter. I wonder if the fashion isn't quite convenient for the lumber suppliers as an excuse for more intense kiln drying, perhaps allowing cheaper wood to be suitable for necks and fingerboards. I wonder a lot of things.

 

[jae]

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.

Could be temperature or more specifically humidity

 

[thefsb]

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.

Took a while to find time to work on this.

It turns out the 190 Hz peak was the top. I could get it down to 175 Hz with two blobs of Scotch Mounting Putty. Now the F3 thunks but not as bad as the F# and G did before. So that's some progress.

I'll try to make some recordings with sonogram video with and without the putty. And I'll order brass bridge pins.

 

[thefsb]

Does anyone know who makes bridge pins from tungsten or uranium?

Or maybe this isn't the most appropriate forum for this question.

 

[bluefuzz]

Took a while to find time to work on this.

Yes, this sort of testing is very time-consuming, which is why I rarely get round to it ... ;-)

It turns out the 190 Hz peak was the top. I could get it down to 175 Hz with two blobs of Scotch Mounting Putty. Now the F3 thunks but not as bad as the F# and G did before. So that's some progress.

Great! Do you feel it makes the guitar 'acceptable' now? There is more you could try, like making the soundhole smaller for example. Try firmly taping some thick card partly covering the soundhole. That should lower the main air resonance. If you find that helps with a small decrease then binding the soundhole would perhaps be a prettier solution. Alternatively Installing a soundhole pickup may actually kill two birds with one stone. It would add some weight to the soundboard (although not really in the best place) and reduce the size of the soundhole - and you get the bonus of being able to plug in ... Worth a try maybe.

Does anyone know who makes bridge pins from tungsten or uranium?

If you really need that much weight to get the resonances into a better place it would probably be better to consider shaving braces. Adding a lot of weight will obviously reduce the responsiveness of the guitar a lot, whereas shaving braces would more likely increase responsiveness with the same reduction in frequency. Plastic bridge pins usually weigh about 3.5 g, ebony pins 4-5 g, bone a bit more. Brass pins weigh about 25-30 g I believe (I don't have any myself).

I'll try to make some recordings with sonogram video with and without the putty.

Screen grab from VA or similar FFT analyser (average of a few 'taps' comparing before and after) is more useful. Sonogram is difficult to read.

 

[thefsb]

An update.

After careful checking, I removed the weights. I could move the thunk from F#3 down to F or even E but it wasn't really an improvement since it was just as pronounced. I think it's better on F# than F or E.

Also, I went to a retailer a few weeks ago and tried three new dreads, carefully listening to them.

1) Larrivée D-03R. This wasn't very informative because it didn't do much (unresponsive) and I found it hard to play.
2) Seagull S6. Loud, responsive, lots of fun but with a big boom on the low G2 and a distinct thunk on E3.
3) Furch Violet D-SY. Fairly responsive and well balanced but with a distinct thunk on E3.

So I'm inclined to believe that this kind of thunk is common, maybe even normal, in a responsive guitar and that I'll do better to accept it and work with/around it.

To reiterate, it's hard to even notice the effect playing on the lower frets. It's most noticeable on the 14th fret of the 6th string, but that's seldom used. So the only real problem is the 9th fret of the 5th string and this does bother me, if I let it.

Finally, thanks to what I learned in this thread, I modified my posture while playing so that the top is angled up and the back doesn't touch my body. The guitar sounds great this way and I can see what I'm doing better.

 

[bluefuzz]

So I'm inclined to believe that this kind of thunk is common, maybe even normal, in a responsive guitar and that I'll do better to accept it and work with/around it.

The 'thunk' is certainly common. If you think about it, it's unavoidable: a guitar without resonances wouldn't make any sound ... ;-)

The trick is distributing the resonances so they couple without 'thunking' too much in any one place and preferably between scale tones. But, as you say, learning to work around the thunk is part of the fun of playing (good) guitars. If you don't need to be in tune with others at concert pitch then an easy fix can be simply detuning, or tuning up, a quarter tone so the thunk isn't bang on a scale tone. It can also be worthwhile experimenting with different string weights, brands, alloys etc. I'm a big fan of Martin Retro 'Monel' strings.

I modified my posture while playing so that the top is angled up and the back doesn't touch my body.

This is always a good thing letting the back resonate.

 

[Multicore]

Hi @bluefuzz , do you remember the LJ56 mentioned here and its "F# suck"? Here's that instrument in action...

 

[bluefuzz]

Here's that instrument in action...

Sounds fine to me. Was that recorded with the soundhole pickup or were you using a mic? Does it still 'suck'? ;-)

 

[Multicore]

Sounds fine to me. Was that recorded with the soundhole pickup or were you using a mic? Does it still 'suck'? ;-)

The mag pickup you can see attached to the guitar has a small condenser mic on a gooseneck. I used that with its mixer set in the middle panned slightly to one channel. Panned slightly the other way there's an AKG D880 dynamic (handheld vocal super-cardoid) mic positioned out of the picture about 2 foot (0.7 m) away from and pointing at the neck joint. No other processing. The combo works nicely and I didn't need to add artificial reverb since the room provides enough. There's a room honk around A 110Hz but that doesn't bother this recording, in which the guitar is tuned CGCGCG.

The guitar does still suck out the fundamental around that F#. I got used to it. I like this guitar more and more with time. That's a good thing.

 

[bluefuzz]

The mag pickup you can see attached to the guitar has a small condenser mic on a gooseneck. I used that with its mixer set in the middle panned slightly to one channel. Panned slightly the other way there's an AKG D880 dynamic (handheld vocal super-cardoid) mic positioned out of the picture about 2 foot (0.7 m) away from and pointing at the neck joint.

Right. I thought it sounded like there was a mic involved. Sounds very good. I've not really experimented with pickups on any of my guitars, or even recording them much ...

the guitar is tuned CGCGCG

I see why you got an oud. ;-)

The guitar does still suck out the fundamental around that F#. I got used to it. I like this guitar more and more with time.

Yes, looks like a nice instrument, though if you're regularly tuming that low something bigger might be better - or thicker strings ...

 

[Multicore]

though if you're regularly tuming that low something bigger might be better - or thicker strings

yeah i know ... https://www.youtube.com/playlist?list=PLQrwNq6USRXjtznK9Gh8if0b1rEPH0Ej1

i'm happy with the guitar body's acoustics on the low C. a big dred or SJ wouldn't make much difference there.

i use a fairly fat bass string 0.062. because of the high G I don't want a longer scale, and i can't be bothered with fan fret for the difference it will make.

 

[bluefuzz]

a big dred or SJ wouldn't make much difference there.

That's true enough. Getting the main air resonance down below D or even E would probably mean losing a lot of power. It's all about compromise.

 

[Multicore]

It's all about compromise.

That's what I feel about the range of open string pitches too. C2 to G4 is as far as I'm willing to go on standard scale guitar. It's the same range as NST, which I don't understand. I remember reading an interview with Fripp before he published NST and he was being cagey and I was intrigued. In the end I didn't find a use for it.

 

[Multicore]

I see why you got an oud. ;-)

Modulating between different scales/modes is important in maqam. On review I see how the approach I took in the noodle I shared yesterday has some correspondence to how some soloists inproduce their raga.

 

[bluefuzz]

It's the same range as NST, which I don't understand.

I did try NST a while back and couldn't really get my head around it. It does work better on an electric though, I think. It also works better if you have Fripp's chops ... ;-)

I have played around a bit with CGCGCD beloved of folkies like Nic Jones and Dick Gaughan which is fun and obviously doesn't strain the sixth string as much. But I'm lazy and mostly just stay in standard tuning with the occasional venture into dropped D.

 

[coonmanx]

Not that this is in the original spirit of the thread but it got me thinking. What if you stuck one of those actuators on the back of an acoustic guitar and turned the entire guitar into a speaker? Would be a cool experiment but you would have to be able to remove said actuator.

 

[Multicore]

Not that this is in the original spirit of the thread but it got me thinking. What if you stuck one of those actuators on the back of an acoustic guitar and turned the entire guitar into a speaker? Would be a cool experiment but you would have to be able to remove said actuator.

I'm sure that would be a fun experiment. And it might be able to tell you some things if you're careful. The mass of the actuator would, if you attach it in the middle of the back or top, substantially affect the acoustic response of that part. It would be like adding the balls silly putty @bluefuzz talked about to help identify which peaks and troughs belong to the top and the back and the box of air.

I think the next frontier in this kind of study is the different patterns of sound hole that @bluefuzz has to choose from for my new oud One or three? Oval or round?

 

[bluefuzz]

Would be a cool experiment but you would have to be able to remove said actuator.

I have successfully used a Sustainiac model B, which clamps onto the headstock, on an acoustic guitar. Pretty quiet, but an interesting effect. Activating the body resonances with a loudspeaker to show the chladni patterns is a good way to 'see' what a guitar is doing. But as Multicore says, sticking the transducer onto the back or top will radically change the resonances of that plate.