I bought this acoustic guitar on line, used, boy did it sure look nice! Unboxed it, tuned it up and first chord, Uggh. Bright and boxy, just what I dont want to hear. I was wondering why this might be...
So I get out my Pink noise test kit and instead of driving a speaker, I drive a Dayton DAEX25FHE-4 exciter with the little USB powered class D amplifier setup to only play a Pink Noise file on an SD card.
I put a thin gasket foam pad on the exciter VC, to cover the monstrously sticky adhesive that comes with it. Turning the amp volume down to keep the exciter from jumping all over the guitar, I placed it just setting right on the bridge. Clearly driving pink noise into the same spot where the strings actuate the guitar's membrane-o-phone, or "top".
I have a couple older Yamaha G-XX models, which are reputed and to my ears, have a good sound.
It's pretty easy to see the builder's effort to hit a comb at 110, 200, 440 or the "A" note, though it may be off some.
Pretty similar response for the G130, albeit being strung to concert tuning. Apparently all that tension doesnt effect the resonances.
Now let's look at that guitar I didnt particularly care for;
It's clear that the body resonances are shifted higher in the construction of this model. One wouldnt think so by looking at it - it's got a big, deep body. I've read on line that there's pretty much nothing one can do, without risk to the guitar's mechanical integrity, to change this. Better off to sell it and just get something you like.
I have a guitar that's labeled a "parlor" model, a bit smaller in size. One would expect a higher frequency comb pattern. Let's see;
Sharper yet, with a discernable notch even at ~440. My wife and myself doesnt think this guitar sounds particularly good. Hmmm....
I also have a Kala nylon, which I picked up at a yard sale where someone had a whole pile to pick from. Its bridge had torn free, so I epoxied it back in place. It tore off again a couple few years later, so I really laid it on thick for the repair. It's a thin line body, I'd guess all mahogany. I really like the way this one sounds; it's my favorite to play; of course, this particular guitar is unobtanium - there's one steel string variation I can find anywhere.
Clearly this one has resonances flat from the "110" sub-multiple of the A440. So the "A" ends up just on the integrating side of the bandpass, versus the differentiating side. Interesting.
I also have a Yamaha "Silent" nylon string electric and an Ibanez classical, similar in shape to the Yamaha guitars. The Ibanez pulls off a pretty good copy of the Yamaha's "fingerprint";
The silent guitar has signal processing for delay, echo etc and I wondered if they did any analog modeling of a guitar body, you know, to make it sound like an acoustic at the output jack;
Nope - but how interesting!
I believe one could go shopping for guitars with a pink noise analysis kit (battery, SD card playing amp, driver, cell phone for analyzer) and while the use of such may not determine what instrument you should buy, it would certainly help you decide what instrument to bother playing. I wish I had more / access to more instruments, to see what they show for body response. Just setting the transducer upon the bridge is certainly not harmful to the guitar -
I'll try to add to this, as I go through more instruments. I believe the transducer is sensitive enough that it could be driven by a laptop's headphone output directly; it doesnt take much to make an acoustic body speak loud enough to be heard by a measurement mic. I'm excited to try a sine sweep for more precision in establishing the resonances and difference in amplitude between the peaks.
So I get out my Pink noise test kit and instead of driving a speaker, I drive a Dayton DAEX25FHE-4 exciter with the little USB powered class D amplifier setup to only play a Pink Noise file on an SD card.
I put a thin gasket foam pad on the exciter VC, to cover the monstrously sticky adhesive that comes with it. Turning the amp volume down to keep the exciter from jumping all over the guitar, I placed it just setting right on the bridge. Clearly driving pink noise into the same spot where the strings actuate the guitar's membrane-o-phone, or "top".
I have a couple older Yamaha G-XX models, which are reputed and to my ears, have a good sound.
It's pretty easy to see the builder's effort to hit a comb at 110, 200, 440 or the "A" note, though it may be off some.
Pretty similar response for the G130, albeit being strung to concert tuning. Apparently all that tension doesnt effect the resonances.
Now let's look at that guitar I didnt particularly care for;
It's clear that the body resonances are shifted higher in the construction of this model. One wouldnt think so by looking at it - it's got a big, deep body. I've read on line that there's pretty much nothing one can do, without risk to the guitar's mechanical integrity, to change this. Better off to sell it and just get something you like.
I have a guitar that's labeled a "parlor" model, a bit smaller in size. One would expect a higher frequency comb pattern. Let's see;
Sharper yet, with a discernable notch even at ~440. My wife and myself doesnt think this guitar sounds particularly good. Hmmm....
I also have a Kala nylon, which I picked up at a yard sale where someone had a whole pile to pick from. Its bridge had torn free, so I epoxied it back in place. It tore off again a couple few years later, so I really laid it on thick for the repair. It's a thin line body, I'd guess all mahogany. I really like the way this one sounds; it's my favorite to play; of course, this particular guitar is unobtanium - there's one steel string variation I can find anywhere.
Clearly this one has resonances flat from the "110" sub-multiple of the A440. So the "A" ends up just on the integrating side of the bandpass, versus the differentiating side. Interesting.
I also have a Yamaha "Silent" nylon string electric and an Ibanez classical, similar in shape to the Yamaha guitars. The Ibanez pulls off a pretty good copy of the Yamaha's "fingerprint";
The silent guitar has signal processing for delay, echo etc and I wondered if they did any analog modeling of a guitar body, you know, to make it sound like an acoustic at the output jack;
Nope - but how interesting!
I believe one could go shopping for guitars with a pink noise analysis kit (battery, SD card playing amp, driver, cell phone for analyzer) and while the use of such may not determine what instrument you should buy, it would certainly help you decide what instrument to bother playing. I wish I had more / access to more instruments, to see what they show for body response. Just setting the transducer upon the bridge is certainly not harmful to the guitar -
I'll try to add to this, as I go through more instruments. I believe the transducer is sensitive enough that it could be driven by a laptop's headphone output directly; it doesnt take much to make an acoustic body speak loud enough to be heard by a measurement mic. I'm excited to try a sine sweep for more precision in establishing the resonances and difference in amplitude between the peaks.
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Do not need a transducer to drive the top. AI used a stick of wood with an eraser tapped to the end. Record the impulse response of the guitar and let FFT figure out the response. A parlor sized guitar I built years ago.
On the guitar with little bass, shaving the lower back braces can give more lowrange output without compromising the top structure.
On the guitar with little bass, shaving the lower back braces can give more lowrange output without compromising the top structure.
That is a very interesting set of plots. 🙂
I'm guessing the peak in the 110 - 120 Hz region is the actual acoustic Helmholtz resonance of the air cavity / soundhole?
I wonder what you think about also looking for Chladni patterns at some of the bigger resonances, just to see what the guitar top is actually doing.
If it dried out either in the previous owner's hands, or during shipment to you, that could have made it sound much brighter.
If it has dried out, put it in a suitably humidity controlled environment for a week or two (45% RH, plus or minus 5%), and the tone can warm up significantly.
Earlier this year I bought an obviously mistreated Yamaha FG411SV from a Craigslist seller for $70 CAD (about $54 USD). It was faded, the finish was crazed, the wood was obviously dried out, the strings were a quarter-inch high above the fretboard, and sighting down the neck revealed that the top had bowed slightly. But it still sounded good, and for that price, I thought I'd take a gamble on it as a beater guitar to take camping with me.
Here's the relevant part: I was amazed how much the tone of the guitar changed after putting it in a humidity-controlled room for a couple of weeks. Warmer, more balanced, less bright.
Oh yeah, I was able to fix the other issues sufficiently to make the guitar play well. I adjusted the truss rod, removed and filed down the saddle to lower the strings some more, and deepened the nut-slots a little (they were too shallow, just the way they'd left the factory - this guitar had never had a set-up). Then I
put on a lighter-gauge set of strings to reduce string tension. After all that, the strings ended up at the right height above the fretboard.
The guitar remains cosmetically undesirable, which is probably a bonus for a campsite guitar. There's less chance of someone stealing out of my tent!
-Gnobuddy
I'm guessing the peak in the 110 - 120 Hz region is the actual acoustic Helmholtz resonance of the air cavity / soundhole?
I wonder what you think about also looking for Chladni patterns at some of the bigger resonances, just to see what the guitar top is actually doing.
One thought: has the guitar been sitting in a suitably humidified location for at least a week or two after you bought it?
If it dried out either in the previous owner's hands, or during shipment to you, that could have made it sound much brighter.
If it has dried out, put it in a suitably humidity controlled environment for a week or two (45% RH, plus or minus 5%), and the tone can warm up significantly.
Earlier this year I bought an obviously mistreated Yamaha FG411SV from a Craigslist seller for $70 CAD (about $54 USD). It was faded, the finish was crazed, the wood was obviously dried out, the strings were a quarter-inch high above the fretboard, and sighting down the neck revealed that the top had bowed slightly. But it still sounded good, and for that price, I thought I'd take a gamble on it as a beater guitar to take camping with me.
Here's the relevant part: I was amazed how much the tone of the guitar changed after putting it in a humidity-controlled room for a couple of weeks. Warmer, more balanced, less bright.
Oh yeah, I was able to fix the other issues sufficiently to make the guitar play well. I adjusted the truss rod, removed and filed down the saddle to lower the strings some more, and deepened the nut-slots a little (they were too shallow, just the way they'd left the factory - this guitar had never had a set-up). Then I
put on a lighter-gauge set of strings to reduce string tension. After all that, the strings ended up at the right height above the fretboard.
The guitar remains cosmetically undesirable, which is probably a bonus for a campsite guitar. There's less chance of someone stealing out of my tent!
-Gnobuddy
The resonances match up to the Chladni patterns.
Gore (https://goreguitars.com.au/the-book/) found that higher performance guitars he tested had a bridge rotation of up to two degrees between the top with no strings and when stringed and up to tension. So a top that belly's up behind the bridge and dips above it is a good sign, unless it is excessive that is.
I actually did that, but built a DIY humidifier, stuffed it inside and closed the sound hole. Admittedly, there may be the chance I didnt give it, but it's sold already; taking it to the shipper this afternoon.
Your lutherie skills are something I remember doing on a Yamaha 12 string I owned many years ago. I need to brush up on that. I got a new classical today - "Orlando" made in Korea with nice wood - that sounds different than the Yamahas. I'll soon post the plot of that one.
Someday I'm hoping to trade up to owning a Gibson again. I had 3 way back when and let them all go for a song; now they're worth thousands. They had the character then when you tuned them to concert pitch, the whole guitar came alive. The SG would just sustain notes forever. Same quality with the smallish acoustic - neck too small for my hands - that I gave my roomate for my share of the phone bill.
I wonder if that phenonema can be measured. One would think so.
Would be interersting to see your plot from a really GOOD acoustic guitar, like a 1956 Martin D-28.
...but Chladni patterns reveal other information too, such as which areas of the body are actually vibrating, and which aren't. Might be fun?
Also, how about the main Helmholtz (acoustic) resonance? The one where the air in the sound-hole "bounces" on the compliance of all the air in the body? That one may not have any Chladni pattern to go with it. I suspect it happens at a low enough frequency that the entire guitar body is basically rigid there.
There is some very interesting info on this University of British Columbia web: http://www.theory.physics.ubc.ca/341-current/pluck/pluck.html
If you load up that page in a decent 'Web browser (not Microsoft crap), you can click on individual harmonics in the guitar note frequency spectrogram, and hear them individually. You can also hear the "woody" thunk that comes from the acoustic Helmholtz resonance, and what the author calls "...the first drum mode of the wooden guitar top."
I think there's a fine line between a guitar top so thin that it sounds great, and a top so weak that it will tear itself apart after a few years.
The Yamaha acoustic guitars I've seen use what appears to be fibreglass reinforcement to strengthen the area where the braces are closest to the bridge plate. That area is very strong, and there is no visible bellying of the top in one Yamaha I've owned for 12 years (bought it new).
I've also never heard a Yamaha acoustic guitar with a bad tone. Yamaha knows how to make them sound good without making the top weak. But the Yamaha's I've tried are always less-loud than similar Martins with thinner tops (and a pricetag three times as big).
When I first got the Craigslist Yamaha FG411SV, if I sighted along the top of the fretboard from nut towards bridge, the top of the fretboard fell well below the top edge of the saddle.
The neck had a negative neck angle, in other words. 🙁
I think the problem was in the vicinity of the heel joint; I think the side walls of the body had actually warped a little, from lack of humidity combined with string tension. It's a runaway problem, because as soon as you have a little warp, the strings rise higher above the joint, and that lets them exert even more torque on it, causing even more warpage.
This guitar will never again be in prime condition, but it sounds good and plays easily now, and that's all I was expecting for the price I paid.
-Gnobuddy
Printer2 is the man with the real lutherie skills!
I have just enough courage (foolhardiness, maybe) to tinker with relatively minor fixes to relatively inexpensive guitars.
One of my music buddies (a very petite man) bought a second-hand Taylor GS Mini with a spruce top some years ago. It sounds great, he loves it, and his playing improved after he got it.
Another of my music buddies (a very petite woman) saw the first GS Mini, and ended up buying one of her own, this time with some sort of exotic figured top (Koa?). It sounds great, too.
Quite recently another woman joined our jams, saw the GS Minis and fell in love with them, and bought one for herself. She's not all that petite, but the guitar seems to suit her - she is a beginner on guitar, but progressing rapidly, and she loves her GS Mini.
I am most definitely not petite, and to me, these GS Minis feel like a ukulele. Worse, with six strings on the narrow neck compared to four on a ukelele, GS Minis are virtually unplayable for me, with my big hands and fingers. Clearly, I'm not the target demographic for this model.
-Gnobuddy
It would indeed. All we need is a very rich person willing to donate one to jjiasnew to measure. 😀
Any decent acoustic guitarist can change the tone of an instrument drastically to taste, by making slight changes in picking position, using thicker or thinner guitar picks, varying playing intensity, and/or changing the gauge of the strings.
Because of this, I don't worry much about the exact tone of a guitar. Within reason, I can get the tone I want out of any decent acoustic guitar.
IMO, what the expensive guitars bring to the party is more loudness, and sometimes, more sustain. They are definitely louder than more affordable instruments!
But since nobody actually plays true acoustic guitars outside of a studio (they're all played plugged-in now), this isn't the big deal it would have been in, say, 1950.
Plugging in also lets you change the guitar tone even more than you can change it by playing technique alone. I remember getting really nice warm guitar tone out of a $110 Kona acoustic guitar (bought at Walmart!), once I'd run it through a graphic EQ pedal, and into a good acoustic guitar amplifier.
Played unplugged, the same guitar was very "meh". It had a plywood (aka laminate) top, and a thin body, so it was very light on bass, and very quiet overall. But it did have nice mids and trebles, and a good long sustain for an acoustic. No dead spots on the fretboard, either.
-Gnobuddy
All we need is jjasniew to develop out the test process so anyone can do it with a laptop running REW - and one of those Dayton exciters...
These I've cranked to 1/48th octave. What would be really nice, would be if I could run the "flat" output of the Yamaha silent guitar through these set as filters - and see if it "sounds like" the actual acoustic. Alas, I'm sure there's more aspects of an acoustic guitar's sound not captured by this analysis. How about REWs "decay" functionality? One would think different guitars would have different properties along those lines - that of ring in the body and how long those last at different frequencies.
Nice work!
I'm somewhat surprised that Yamaha hasn't already done this. This is a company with a lot of expertise in producing good sounds from electronic doodads with severe limitations (there is tremendous EQ applied to the tiny powder-puff speakers in their electronic keyboards to make them sound bigger and better, for instance).
I've had a similar idea to yours rattling around the back of my head, wondering if I could make a solid-body electric guitar sound a bit like an acoustic guitar by running its output through a suitable filter. It would be nice not to have to lug both guitars to jams, if the electric could make passably acoustic sounds in a pinch.
You can buy FX pedals that are supposed to do this, but the ones I've heard are not convincing.
You can also buy expensive piezo saddles to mount on your solid-body electric guitar bridge - and those don't sound anything like an acoustic guitar, either. Instead, they sound like the very worst piezo pickup you ever heard on an electro-acoustic guitar. Think Glen Campbell with his Ovation decades ago, in the era when Ovation acoustic guitars were king because they were the only "acoustic" guitars with a built in piezo pickup. Never mind that they sounded bad, at least you could make them sound loud!
Too much note sustain from the electric guitar is one obvious problem, but there is also a lack of that woody warmth that decent acoustic guitars have.
I suspect that if we could add the percussive "thump" from the acoustic Helmholtz resonance, and maybe the first couple of "drum modes" of the wooden top, the effect would be a lot more convincing. My idea was to duplicate those with analogue electronics, while I think your idea is to use a digital filter, which certainly allows for more complex EQ curves.
There are various black boxes on the market that claim to take your basic piezo-equipped electro-acoustic guitar and make it sound like an expensive vintage Martin in front of an expensive vintage condenser microphone. I suspect that these might also consist mostly of carefully designed EQ curves, though perhaps some reverb (to simulate "air") is also part of the recipe.
Then there is the ToneDexter ( https://www.sweetwater.com/store/de...c-tonedexter-acoustic-instrument-preamp-pedal ). In operation, it samples the sound from your guitar repeatedly, does some computing, and attempts to make it sound like your choice of expensive guitar + expensive mic. The ad copy hints at an impulse response generated by the ToneDexter and used to transform your cheap-n-cheerful guitar into something that sounds expensive; basically marketing-speak for "We apply EQ to make your guitar sound more like the target guitar+mic".
-Gnobuddy
Good idea!
I'm somewhat surprised that Yamaha hasn't already done this. This is a company with a lot of expertise in producing good sounds from electronic doodads with severe limitations (there is tremendous EQ applied to the tiny powder-puff speakers in their electronic keyboards to make them sound bigger and better, for instance).
I've had a similar idea to yours rattling around the back of my head, wondering if I could make a solid-body electric guitar sound a bit like an acoustic guitar by running its output through a suitable filter. It would be nice not to have to lug both guitars to jams, if the electric could make passably acoustic sounds in a pinch.
You can buy FX pedals that are supposed to do this, but the ones I've heard are not convincing.
You can also buy expensive piezo saddles to mount on your solid-body electric guitar bridge - and those don't sound anything like an acoustic guitar, either. Instead, they sound like the very worst piezo pickup you ever heard on an electro-acoustic guitar. Think Glen Campbell with his Ovation decades ago, in the era when Ovation acoustic guitars were king because they were the only "acoustic" guitars with a built in piezo pickup. Never mind that they sounded bad, at least you could make them sound loud!
Too much note sustain from the electric guitar is one obvious problem, but there is also a lack of that woody warmth that decent acoustic guitars have.
I suspect that if we could add the percussive "thump" from the acoustic Helmholtz resonance, and maybe the first couple of "drum modes" of the wooden top, the effect would be a lot more convincing. My idea was to duplicate those with analogue electronics, while I think your idea is to use a digital filter, which certainly allows for more complex EQ curves.
There are various black boxes on the market that claim to take your basic piezo-equipped electro-acoustic guitar and make it sound like an expensive vintage Martin in front of an expensive vintage condenser microphone. I suspect that these might also consist mostly of carefully designed EQ curves, though perhaps some reverb (to simulate "air") is also part of the recipe.
Then there is the ToneDexter ( https://www.sweetwater.com/store/de...c-tonedexter-acoustic-instrument-preamp-pedal ). In operation, it samples the sound from your guitar repeatedly, does some computing, and attempts to make it sound like your choice of expensive guitar + expensive mic. The ad copy hints at an impulse response generated by the ToneDexter and used to transform your cheap-n-cheerful guitar into something that sounds expensive; basically marketing-speak for "We apply EQ to make your guitar sound more like the target guitar+mic".
-Gnobuddy
Chlandi patters are used by some luthiers, the problem is knowing what the tea leaves are telling you. I have played with them and will in the future but generally they are not all that useful. The Helmholtz resonance has the back and top moving with a live back. A back that is not allowed to vibrate (incorrectly called a reflective back) has one less resonance peak. Two Yamaha guitars. "two Yamaha guitars, an FG830 budget-priced dreadnought and an LJ56 top of the line jumbo"
https://www.audiosciencereview.com/...cy-response-of-an-acoustic-guitar-body.25590/
The red is the budget guitar, the resonance at 180 Hz is the top resonance and there is no corresponding peak nearby so it shows that it is a non-live back. The green at 195 and 205 Hz shows that it is an active (live) back and the back contributes to the sound. The red plot has a dip at 210 Hz but it is lower down and far enough away from the main top resonance that if it is the back resonance, it will not produce much output. It may not be the back resonance, you would have to do a series of plots to really know what is what.
In the plots the OP supplied I can easily see which are probably live backs and which are not by the two resonances being together (top and back) or not. The Yamaha G130 is not, above it the Orlando is. Hear is a plot with the naming convention of the resonances.
https://www.semanticscholar.org/pap...erry/0ce4ed2819b5701979751d21155786e333bf3a63
Gore (one of the authors of the book above) did measurements of the top tilt at the bridge of a couple dozen high performance guitars and he came up with a measurement of 2 degrees bridge rotation indicating about the maximum you would want the bridge to rotate and still have a long service life of the top.
On the straightedge across the frets hitting lower on the saddle or beneath the bridge top, time for a neck reset in order to get the action back where it should be. It is not humidity related, the weak point in the guitar is the sound hole and over time a guitar will start to fold up at the sound hole if not built to counteract the effect. With the current guitar I am building I built a 'box' around the sound hole to reinforce the area. Some use carbon fiber tubes to stop the need for a reset. The strings are higher off the sound board but that is immaterial, it is the height of the saddle that determines how much torque is produced. As the guitar folds back on itself you lower the saddle in order to have the guitar more playable. This does reduce the amount of torque, but there is a limit on how far down you can shave the saddle.
I have seen laser holograms made to reveal small-amplitude vibrations in mechanical devices. These days diode lasers are cheap and easy to use. Sticking lenses in a sand-filled box provides enough vibration suppression for DIY holograms. I think it might be an interesting thing to try, if it catches anyone's attention.
Another interesting possibility: if you have a guitar top resonant mode at, say, 300 Hz, what happens if you put the guitar in a dark room, and light it up with an LED connected to a function generator that flashes it at, say, 301 Hz? Will the strobe action reveal - in slow motion - the actual movement of the guitar top or back?
Very interesting stuff, thanks for sharing! 🙂
No doubt, but I wasn't going to pay for a neck reset on a $70 guitar...nor do I have any idea how to do a neck reset myself!
I took the strings off, tightened the truss rod (the neck had too much forward bow), and let the guitar sit in the humidified room for several days. At the end of that time the negative neck angle had mostly gone away.
In this particular case, I think poor treatment of the guitar (lack of humidity) did cause the problem, and simply removing the tension and letting the wood absorb the right amount of moisture allowed it to creep back most of the way into its original shape.
Action was still high (the Yamaha I bought brand new back in 2010 was the same way). After fine-tuning the nut slots, I carefully measured string heights with a pair of digital calipers, and then sanded away quite a bit of material from the bottom of the plastic saddle, lowering it as far as I could into the slot in the bridge plate. Action was now comfortably low, and torque on the top substantially reduced as a result.
The final step was putting on new lower-gauge strings (I prefer those anyway) to reduce string tension.
It will never be a flawless guitar, but it makes a nice beater now. 🙂
When I bought it, the same seller was also offering a no-name acoustic guitar for $50 CAD. It was in better physical condition than the Yamaha - but the Yamaha had much better tone, even in the poor condition in which I found it. For $20 more, I took a chance on it. It worked out. 🙂
-Gnobuddy
I finally dug up an amplifier that connects to an analog out so as to do sine sweeps. As understood, these give a more focused look at the response of the guitar; here's a plot of the sine actuated response versus that of the pink noise. IMHO, the pink is good enough to reveal a response that would in turn make a sound I wont like, as I experienced with the Alverez RC20SC.
When it hits that ~200 hz mark, the guitar sounds like it's going to blow apart - it really fills the room with sound at that max point. Unfortunately,
When it hits that ~200 hz mark, the guitar sounds like it's going to blow apart - it really fills the room with sound at that max point. Unfortunately,
Let's try that again; (website quit me when uploading image...)
I finally dug up an amplifier that connects to an analog out so as to do sine sweeps. As understood, these give a more focused look at the response of the guitar; here's a plot of the sine actuated response versus that of the pink noise. IMHO, the pink is good enough to reveal a body / soundboard response that would in turn make a sound character I personally wont like, as I experienced with the Alverez RC20SC.
When it hits that ~200 hz peak, the guitar sounds like it's going to blow apart - it really fills the room with sound at that max point. Unfortunately, some amplification is needed for the 4 Ohm driver; the headphone output alone just wont do it. I have yet to bust apart a headphone to see if I could couple that kind of driver to the guitar bridge in a similar way. With a quiet enough background, maybe works.
I finally dug up an amplifier that connects to an analog out so as to do sine sweeps. As understood, these give a more focused look at the response of the guitar; here's a plot of the sine actuated response versus that of the pink noise. IMHO, the pink is good enough to reveal a body / soundboard response that would in turn make a sound character I personally wont like, as I experienced with the Alverez RC20SC.
When it hits that ~200 hz peak, the guitar sounds like it's going to blow apart - it really fills the room with sound at that max point. Unfortunately, some amplification is needed for the 4 Ohm driver; the headphone output alone just wont do it. I have yet to bust apart a headphone to see if I could couple that kind of driver to the guitar bridge in a similar way. With a quiet enough background, maybe works.
I remember doing this with a drum head years ago. Using the strobe technique, I could definitely see different parts of the head in motion. In a stretched membrane, different sections are going up and down relative to one another after being struck. I imagine the soundboard of an acoustic guitar behaves similarly. Wouldnt it be cool if you could see the outline of the structure underneath in this way?
I happen to have a strobe light and if it still works, I can easily try it - at least at those resonant points, where motion may be large enough to see. Unsure of the frequency necessary to get movement about the underlying structure...
In case you get the itch.
http://yamahavintagefg.com/wp-content/uploads/2020/08/Vintage-Yamaha-Neck-Removal-NOTES-v2020.8.pdf
Or the cheap guitar method.
