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 Post subject: Resonant analysis
PostPosted: Sun Aug 23, 2015 1:07 pm 
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Koa
Koa

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Location: Traverse City Michigan
I am comparing resonances of a few different assembled guitars. These are electric guitars. Please don't get scared away as this is an interesting topic that can be carried over to other instruments.
I respect the opinions of those here and would like your criticism. My methods are very "benchtop" in style. I used a smart phone app and a drumstick. A laboratory situation with calibrated impulse testing would be nice to see but I am thinking of this as a pilot investigation.

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 Post subject: Re: Resonant analysis
PostPosted: Sun Aug 23, 2015 1:09 pm 
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Koa
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Preamble.
I believe that the resonance that a guitar has is responsible for the acoustic, room filling sound. I also believe that electronic pickups are able to pick-up this sound and amplify it to a speaker, through an amplifier.
Believe it or not, some do not believe the above statement.

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 Post subject: Re: Resonant analysis
PostPosted: Sun Aug 23, 2015 1:15 pm 
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Koa
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I have designed several guitars and double bass and violin instruments with historical information as a guide. In other words I have used existing designs to get the sound I want.

This is a comparison of three electric guitars. Seen below in the photos.

Attachment:
photo 2 (1).JPG

Attachment:
photo 1 (2).JPG

Attachment:
photo 4.JPG

Attachment:
photo 3.JPG


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 Post subject: Re: Resonant analysis
PostPosted: Sun Aug 23, 2015 1:20 pm 
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Koa
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There is a t shaped solid pine
A semi hollow t shaped I call Twilight
And a semi hollow double cut I call Spirit.

the construction methods for the semi guitars are similar except the tops are different. Twilight is laminated and Spirit is solid carved.

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 Post subject: Re: Resonant analysis
PostPosted: Sun Aug 23, 2015 1:28 pm 
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Koa
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Location: Traverse City Michigan
I used FFT analysis as the method of quantifying the resonances.

For those who are not interested in math or scientific analysis let me try to explain in simple terms for you so you can help critique my project.

FFT stands for https://en.wikipedia.org/wiki/Fast_Fourier_transform https://en.wikipedia.org/wiki/Fast_Fourier_transform

Simply put you can think of FFT as a way to visualize the entire spectrum of a sound. Try this.. sing EEEEEE and change to AHHHHHH without changing pitch. Those two sounds would have different sound spectrum. The FFT plot or graph would be different. E will have more high frequency plots and the AH will have more low. Keep in mind that the pitch did not change. Just the overall spectrum of sound known as timbre.

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 Post subject: Re: Resonant analysis
PostPosted: Sun Aug 23, 2015 1:31 pm 
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Koa
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Location: Traverse City Michigan
Comparison of guitar natural resonances through creating an impulse.

My method of creating the impulse was by tapping on the guitar top with a drum stick on the top only at the area of the bridge and around that area. I used what is known as the max or peak hold method which is similar to recording the entire session of bonks and analyzing them.
I then took a screen shot of the FFT spectrum.

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 Post subject: Re: Resonant analysis
PostPosted: Sun Aug 23, 2015 1:33 pm 
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Koa
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For easier visualization I am comparing only two guitars to each other three times. That will cover all comparisons.


Attachment:
twilight_tele_bonk_fft.JPG


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 Post subject: Re: Resonant analysis
PostPosted: Sun Aug 23, 2015 1:35 pm 
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Koa
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Attachment:
twilight_pine-tele_fft_capture.JPG

This is my favorite since the bodies are identical.


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 Post subject: Re: Resonant analysis
PostPosted: Sun Aug 23, 2015 1:36 pm 
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Koa
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Attachment:
spirit_pine_tele_fft_capture.JPG

And the final comparison is the Spirit with a maple carved top and chambers with double f holes.


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 Post subject: Re: Resonant analysis
PostPosted: Sun Aug 23, 2015 1:39 pm 
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Koa
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In the methods I should say that I did try to standardize the bonk by limiting my drum stick hit so they would be similar guitar to guitar. And the mic (iphone) was placed equal distance from the guitar being hit.

My app is called Spectropro v 3.0 with 2048 fft size and a log frequency and log amplitude scale.

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 Post subject: Re: Resonant analysis
PostPosted: Sun Aug 23, 2015 1:48 pm 
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Koa
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Location: Traverse City Michigan
I am going to avoid giving my conclusion and only give a discussion.

Discussion: The sound spectrum from the impluse of the drumstick on the tops of these three guitars should give a visual way to view how they will respond to the guitar pickups. In this case Humbucker style. Of course there are may variables that alter the sound coming out of an amplified speaker, this discussion is limited to just what the microphone (pickup) can amplify. Limiting this study to a reasonable level is essential. It is obvious(to me) that pickups not only amplify strings vibrations but body resonances.A reasonable thing to do would be limit analysis to the maximum level of amplitude plus or minus a reasonable amount. So for this I am focusing on the relative amplitude between -20 and -30 db.

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 Post subject: Re: Resonant analysis
PostPosted: Sun Aug 23, 2015 2:05 pm 
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Koa
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Location: Traverse City Michigan
Discussion continued:

Look at the T style guitar resonance plot. It's construction is a pine slab of wood, with a bolt on maple neck. Bonking the bridge area gives the flat looking and relatively low level of amplitude that is wide in resonance. The only resonance that is above the target zone is two small peaks around 3K.

Now compare the Spirit to the t style. It can be seen that there are more resonances in the target zone. It is acoustically louder. Focusing on the lower end of the spectrum there is clear resonance around 300 hz. Now focus on all the peaks of its resonance. Each of these peaks is a visualization of the body emitting sound. Sound output in the range of our focus, translating to a reasonable amount of sound for the pickups to "hear", is limited solely by the area of resonance under the peaks. Spirit has resonant peaks from 400 hz up to almost 4K. In comparison the t shape has a focused resonance from 2.5 to 4K hz. In players terms this translates to "more open" sounding. A lot of players of my guitars describe in these terms. Another is darker. And a term for a tele is "focused" In fact the ear is very sensitive to resonance in the 2..5 K range. The Opera Formant is a way to project to the back of an auditorium by changing the way a singers tongues forms the resonances.
Departing from the restriction of our target, in looking at the T style pine guitar, if the mics were turned up (roll up the volume) a lot more resonance could be picked up. But only limited the those peaks in the T spectrum. And none in the low range like in the Spirit.
I am limiting this discussion to the comparison of the Spirit to the T solid body since this is the most extreme comparison. They are really very different is shape and in spectrum.

This is how I use bench procedures to predict the difference in sound of a guitar I am making. There is a lot of material choices and shape choices, pickup placement, etc to consider, and the analysis only confirms what my ear hears from tapping and bonking on th assembled instrument. Free plate and parts are another story all together and beyond the scope of this.

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 Post subject: Re: Resonant analysis
PostPosted: Sun Aug 23, 2015 3:50 pm 
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Koa
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Location: Traverse City Michigan
So that is a little of my thinking. I am looking for input or personal experience based on similar methods. Or just criticism of where I am wrong.


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 Post subject: Re: Resonant analysis
PostPosted: Mon Aug 24, 2015 12:05 pm 
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Brazilian Rosewood
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First: how are you supporting those guitars? That's at least as important as how and where you hit them.

Second: you say the mic is held at the same distance for all tests; is the location also the same? Even a little bit of change, say, left or right, can alter what the mic picks up.

Third: did anything else in the room change from one test to the next?

You can get endless arguments about what the 'best' way is to do this sort of test, but everybody will agree that whatever yo do it has to be the same from one test to the next.

I've looked at body vibrations on a few solid bodies, and I agree that they can color the sound. What happens at the bridge is probably as important as what's going on at the pickup locations.


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 Post subject: Re: Resonant analysis
PostPosted: Mon Aug 24, 2015 2:06 pm 
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Alan Carruth wrote:
What happens at the bridge is probably as important as what's going on at the pickup locations.


I would say that what is most important is how the things that happen at the bridge are different than what happens at the pickup locations. Even if there is a large mode at a given frequency, if the shape of that mode causes the bridge and pickup to move the same (or causes neither to move at all), then there is essentially no movement of the string relative to the pickup, and thus no impact on the amplified sound.

This analysis is interesting in that it shows frequencies where body resonances exist that could influence the amplified sound. Things would get even more interesting if you were to record the amplified signal of each guitar excited by these same taps. Then you could correlate the body modes to pickup output and perhaps show specifically where the body modes did in fact color the timbre. (That might get complicated though, as other things on the guitars are not identical such as strings, pickups, necks, etc.) The completion of the study would be to photograph the mode shapes (Chladni patters) and show why it influenced the sound, say, at 400 Hz but not at 630 Hz.

Anyhow, very interesting.


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 Post subject: Re: Resonant analysis
PostPosted: Mon Aug 24, 2015 2:11 pm 
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Koa
Koa

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Location: Traverse City Michigan
Thanks for the feedback Alan.
I'm holding the guitar by the neck around the 12th fret. I am tapping with the guitar in approximately The same spot each time in relation to the microphone. My thinking was that since I am going for a maximum level , in other words I excited the top with the drumstick in an area around the bridge until I receive no more increase in residences. then my thinking was that was that was the max output.
And is my thinking correct that this is a fair qualitative analysis. I'm comparing one designed to another trying to predict how a player might perceive it.

So far everything does seem to make sense at least to me. I'm getting residences for each guitar that seem to make sense to me.

Incidentally I tried a few other small experiments like covering F holes and opening up and closing the back plates. I was able to see changes in the lower frequency peaks.




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 Post subject: Re: Resonant analysis
PostPosted: Mon Aug 24, 2015 3:24 pm 
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Koa
Koa

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Location: Traverse City Michigan
I read your post Jake thanks for the feedback. I do plan on recording the guitars after they are strung up and looking at the FFT. I had not thought of using the same drumstick though so I will do that. Hanging heavy pick ups and control knobs off of the plates has to change things also.

One thing I didn't really mention was that the reason for this is so I can describe how my different models will differ and sound. I get requests similar to something like I like a lot of mid range or I like a more open sound. I want to more compressed mid rangy type sound.
I usually know what they are asking for. They want it sound like a Les Paul or 335. Or A tele or strat. So I have to do my homework. The easiest way to achieve that is to use similar materials shapes. And for the professional luthier there's also trademark issues and ethical issues.



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 Post subject: Re: Resonant analysis
PostPosted: Tue Aug 25, 2015 2:00 pm 
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Brazilian Rosewood
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"I'm holding the guitar by the neck around the 12th fret."

The resonances you're looking for will be mostly of two sorts; 'bar type' bending modes, and twisting modes, or combinations of those. You might find it helpful to look up stuff, or do experiments, with simpler systems, such as plain pieces of bar stock or narrow plates, to help get a handle on how these things work. Then it will be easier to make sense of what's happening with your more complicated guitars.

'Bar' type modes will all have stationary 'node' lines or points near the ends of the object. The lowest one will have two of these, with the middle of the thing going 'down' as the ends go 'up'. On a plain rectangular bar the nodes will be about 1/5 of the way in from either end, and there will be a lot of bending going on in between those two node lines. There are 'higher order' modes, with three, four, five, and more nodes as well, although as you go up they get less active. The more node lines there are the closer the two end ones will be to the ends of the bar. All of this depends on the way the mass and stiffness are distributed in the bar.

On something like a solid body guitar the masses of the body, and, to a lesser extent, the headstock, will tend to dominate the 'mass' part of the equations, while most of the bending will take place in the neck. Holding the neck at the 12th fret is going to tend to kill any mode that is moving there, which would almost certainly be the lowest frequency mode, and probably the third mode. The second mode could well have a 'node' at the 12th fret location, or near it. You may well be suppressing modes you want to look at by holding at the 12th fret. OTOH, it's hard to know of any one place that won't suppress something. I've had good luck with acoustics hanging them up by the head, usually with a loop of strong string under the E string tuners. This is close to the node for the lowest order 'bar' mode, and doesn't suppress it too much.

One thing I've observed in tapping on solidbodies is that the lower node lines tend to 'bunch up' near the bridge. The nodes for the first three or so resonant modes all tend to be pretty close to that location. You can think of the nodes as the pivots of a see-saw, and just as it's hard to move a see-saw by pushing at the pivot, so it's hard to drive a mode near the node line. Having the nodes bunch up near the bridge means that they're hard for the strings to drive, so you don't lose energy from the strings at those frequencies.

The places that move a lot between the nodes are the 'antinodes' (yeah: I found the nomenclature confusing too, at first). Putting a pickup on an antinode would mean it was moving with respect to the (you hope) stationary ends of the string. That's going to color the sound. The more I think about it the more complicated it gets.

"I am tapping with the guitar in approximately The same spot each time in relation to the microphone. "

As with the bridge driving, you can't activate a mode by tapping at or near a node line. There's a good 'out' here, though: ALL of the modes will be active at the ends of the guitar. Tapping either at the headstock or the tail should get them all going.

Tapping the guitar will activate all the modes that are active at the spot where you tapped, and that have node lines near where you're holding it. Once it's going it will 'ring' for a while, radiating sound from the more active parts at whatever frequencies they're happy with. The mic will pick up whatever is coming at it. What the mic hears is not related to where it is relative to the tapping point, it's a function of what the guitar is doing. Since all the 'bending' modes will be active at the headstock that would be a good place to pick them up with a mic. It will be moving more there than the more massive lower end of the body will.

I'll note that there's a lot of disagreement about the 'best' way to hold stuff. Many folks, with reason, feel that the most useful way to hold things is the most realistic: support it where you'd be holding the neck and where it would be leaning against your body. That will show you how it's likely to work in use. There's always the issue there that different people hold them differently. I always try to find the most 'free' way to hold them I can, hoping to factor in the holding constraints later.

The important thing is to come up with some sort of test that makes sense from knowing how the thing is likely to behave, and then to do it the same way every time. Once you start down the primrose path of acoustic measurements it's very had to turn back, so think long and hard about this! ;)


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 Post subject: Re: Resonant analysis
PostPosted: Tue Aug 25, 2015 10:11 pm 
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Koa
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Location: Traverse City Michigan
OK thanks Alan for the help. I really appreciate it and all the help you have given me in the past too. I will try your method of holding and your suggested way of exciting the modes. I will get back here and post to see if things can become clearer.

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