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Resonator Guitar Lovers Online


May 17, 2024 - 11:24:01 PM
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12 posts since 4/12/2023

I'm returning after several (8?) months with the answer to a question that hooked me.  At least one or two ResoHangout readers found it mildly interesting.  (It's admittedly a pretty geeky subject.)  The question is: what is it about resonator guitars that makes them clearly distinct from others?  I answered the question first in terms of features of spectra and spectrograms.  I'm returning with an answer to what is the physical source those features.  In retrospect, it's obvious because it's the only thing all resonator guitars all have in common.  They're loud.  To be loud, the bridge has to move some sort of surface much more than it does on a regular acoustic guitar.  And that's all there is to it.  The extra reso stuff is essentially all produced at any time by the bridge motion exciting the unplucked strings .  It's faint but always there and sounds on its own rather metallic, e.g., like a gong.  My write up has an actual experiment to demonstrate just that: https://www.its.caltech.edu/~politzer/reso-partials/reso-partials.pdf .  The most important figure is on page 6, and there are links to all the earlier work.

May 18, 2024 - 10:36:40 AM

641 posts since 11/28/2012

I enjoyed reading your technical analysis - thanks for posting. (Yes, I'm a geek).

Ironic that as reso players, we spend so much effort (via muting/damping techniques) to mitigate the signature sound of the instrument, ie., the "twang" caused by inharmonic partials of the unplucked strings. A case of too much of a good thing...lol.

Were you surprised you didn't find a more direct contribution of the resonator body to the overall sound? As you've pointed out, it's mainly the unplucked strings causing the twang. At least that's my conclusion from your spectograms.

Also, the first string pluck results on the flat-top acoustic were surprisingly active on the spectogram, IMO.

Good stuff.

May 18, 2024 - 11:00:46 AM

12 posts since 4/12/2023

quote:
Originally posted by JC Dobro

I enjoyed reading your technical analysis - thanks for posting. (Yes, I'm a geek).

Ironic that as reso players, we spend so much effort (via muting/damping techniques) to mitigate the signature sound of the instrument, ie., the "twang" caused by inharmonic partials of the unplucked strings. A case of too much of a good thing...lol.

Were you surprised you didn't find a more direct contribution of the resonator body to the overall sound? As you've pointed out, it's mainly the unplucked strings causing the twang. At least that's my conclusion from your spectograms.

Also, the first string pluck results on the flat-top acoustic were surprisingly active on the spectogram, IMO.

Good stuff.


Thanks for the encouragement.

Look even more closely at the spectrograms.  I concluded that the unplucked strings "cause" those partials, but there are more inharmonic partials than those directly due to the unplucked strings.  Since they change when the string tension changes, they must be "caused" as I say.  Otherwise, they'd be independent of tension.  I try to explain (it's a mouthful) that I think they come from modes that include substantial motion both of the strings and the body.  And that's why different guitars twang differently.

Banjos get another set of inharmonic partials of a different origin, i.e., resonances of the head itself.  And there is a tribe among players who are delighted to have found a way to suppress those without noticeably reducing anything else.  (Personally, I'm not in that camp, but I was happy to help the inventor of the effective gizmo explain how it works and why it is novel -- https://banjobolster.com/ .  More details are in the links provided in the write-up I just posted here.

May 18, 2024 - 11:57:18 AM

641 posts since 11/28/2012

quote:
Originally posted by davidppp
quote:
Originally posted by JC Dobro

I enjoyed reading your technical analysis - thanks for posting. (Yes, I'm a geek).

Ironic that as reso players, we spend so much effort (via muting/damping techniques) to mitigate the signature sound of the instrument, ie., the "twang" caused by inharmonic partials of the unplucked strings. A case of too much of a good thing...lol.

Were you surprised you didn't find a more direct contribution of the resonator body to the overall sound? As you've pointed out, it's mainly the unplucked strings causing the twang. At least that's my conclusion from your spectograms.

Also, the first string pluck results on the flat-top acoustic were surprisingly active on the spectogram, IMO.

Good stuff.


Thanks for the encouragement.

Look even more closely at the spectrograms.  I concluded that the unplucked strings "cause" those partials, but there are more inharmonic partials than those directly due to the unplucked strings.  Since they change when the string tension changes, they must be "caused" as I say.  Otherwise, they'd be independent of tension.  I try to explain (it's a mouthful) that I think they come from modes that include substantial motion both of the strings and the body.  And that's why different guitars twang differently.

Banjos get another set of inharmonic partials of a different origin, i.e., resonances of the head itself.  And there is a tribe among players who are delighted to have found a way to suppress those without noticeably reducing anything else.  (Personally, I'm not in that camp, but I was happy to help the inventor of the effective gizmo explain how it works and why it is novel -- https://banjobolster.com/ .  More details are in the links provided in the write-up I just posted here.


Ah, I see now, I think. 

I was comparing Fig 4 to Fig 3.  The frequencies generated from the "taps" on the metal body in Fig 4 are not manifesting as readings in Fig 3 (frequencies generated by first string plucks).  For example, the National in Fig 3 shows a wide band between .3k and 1.0k with very little activity, relative to all the noise in Fig 4 in that same frequency range from the body taps.  I concluded (erroneously i think...) that there was therefore little to no contribution of the body based on the spectogram readings in Fig 3.

But I DO now see Fig 1 and I follow your logic on some "other" source contributing to inharmonic partials.  Thanks for clarifying!

May 21, 2024 - 9:58:06 AM

1207 posts since 9/29/2009

Any ideas as to the differences between the sound of a National biscuit cone and a Dobro spider cone?

May 21, 2024 - 2:16:51 PM

12 posts since 4/12/2023

quote:
Originally posted by SamCy

Any ideas as to the differences between the sound of a National biscuit cone and a Dobro spider cone?


I'm sure players have been discussing just that for almost 100 years.  (I found great YouTube from twelve years ago: https://www.youtube.com/watch?v=2fsTGodMyNM)  So, maybe you're asking for something physics-based.

I started with smooth biscuit cones because I imaged they'd be simplest to understand.  I learned that the vibrations of a resonator cone are in the realm of the theory of thin shells, and little can be said about cones without extensive numerical calculations.  I also learned it was irrelevant to the question I originally posed.  I don't have access now to any spider guitars, but thinking about the cone geometry, even before listening to on-line examples, I thought they'd be smoother and sweeter (explained below).  And I think that the relative bass response is determined by some cone design parameters set by the maker.

In the lowest mode of the cone under the biscuit, the cone moves as a rigid piston.  The central frequency of the resulting broad formant is determined the mass and return force.  At some point, the strings excite flexing modes.  (In the world of speakers, I've read it's called "break up.")  The steeper slope of the spider's cone makes it much stiffer for the same thickness aluminum.  So, the piston motion is unbroken until a higher frequency.  Furthermore, the big fold likely allows two piston or non-flexing modes, giving smoother and wider coverage to the low frequency part.  The fold also constricts the geometry of the flexing modes, which pushes them to yet higher frequency than you would get just by making the cone stiffer.  If the break-up modes are few and far between, they can give an uneven response to the strings.  Having more closer together smooths that out.

The bass response cares about the lowest frequency piston mode.  Weight and return force stiffness (determined by the bellow-like folds around the edge) control that.  More flexible around the edge and heavier will give stronger bass.

And there's an apple-to-oranges issue.  Given that modern spider bridge and cones have a bigger diameter that biscuits (Yes?), what really is the right comparison?

There's also the string/bridge impedance mismatch, which must be bigger with a spider.  (Yes, again?)  That gives more sustain.

May 23, 2024 - 2:44:19 PM

1207 posts since 9/29/2009

What do you think of the idea that the fold in the cone at mid radius enables the cone to vibrate in the higher frequency 02 Mode, much like the two angled tone bars on a steel string guitar enable the treble response, and that the webs at mid radius of the spider bridge likewise enable its vibration in the 02 Mode? See the page from Dr. Harry F. Olson's book.


Edited by - SamCy on 05/23/2024 14:45:33

May 23, 2024 - 2:54:39 PM

1207 posts since 9/29/2009

Dobro did make 10 1/2" biscuit cones for their metal-body guitars that would accommodate either the biscuit or spider cones with the use of a spacer on the cone shelf. Replacements are still available.

https://www.replogleresos.com/product/replogle-resonators-biscuit-cone-10-1-2-series/

Edited by - SamCy on 05/23/2024 15:08:01

May 23, 2024 - 6:12:56 PM

12 posts since 4/12/2023

quote:
Originally posted by SamCy

What do you think of the idea that the fold in the cone at mid radius enables the cone to vibrate in the higher frequency 02 Mode, much like the two angled tone bars on a steel string guitar enable the treble response, and that the webs at mid radius of the spider bridge likewise enable its vibration in the 02 Mode? See the page from Dr. Harry F. Olson's book.


Despite how the sketch of the 02 mode in the textbook figure is reminiscent of the folded cone, they have totally different motions.  In the 02 mode, the two node lines are fixed, and the other regions go positive-negative-positive-negative as a function of time at the resonant frequency.  In contrast, the folded cone center peak never pops into a pointing down configuration.

Rather, I was imagining that the cone fold line might act as a hinge and allow two piston-like modes.  Below the break-up frequency, there are no node lines on the conical surfaces.  The biscuit cone just moves up and down like a single piston.  In addition to that sort of mode, the folded cone could have a second mode in which the outer edge goes up when the center goes down (and vice versa) without extra flexing on the conical surfaces.

May 23, 2024 - 6:13:08 PM

12 posts since 4/12/2023

quote:
Originally posted by SamCy

Dobro did make 10 1/2" biscuit cones for their metal-body guitars that would accommodate either the biscuit or spider cones with the use of a spacer on the cone shelf. Replacements are still available.

https://www.replogleresos.com/product/replogle-resonators-biscuit-cone-10-1-2-series/


Ouch!  Now you've got me.  Do I buy an inexpensive spider (used?) and swap cones in and out or do carpentry on an inexpensive flat top that I bought used for the purpose but never went through with cutting it out and installing a shelf and a cone?  However, I suspect that there are still a lot of variables of the apples-to-oranges type.  E.g., Mike Replogle makes two kinds of spiders and two kinds of cones.  Understanding all of the spider variables might come before any comparison to biscuits.  Very interesting, though.   (I'm safe for the time being with a banjo rim project for which I just ordered a Nordic Shell rim and will be making a full banjo pot to compare to some thicker rims I already have.)

May 23, 2024 - 10:34:03 PM

1207 posts since 9/29/2009

Never imagined a vibrating flat circular plate took the exaggerated shapes pictured in the drawings for clarity, only that it moves very slightly. Did imagine that the fold in the cone formed a node with inner and outer sections moving opposite directions at some higher frequency, which exactly describes the 02 mode.

Edited by - SamCy on 05/23/2024 22:40:10

May 24, 2024 - 11:07:53 AM

1207 posts since 9/29/2009

Found a laser scanned picture and cross-section view of a speaker in one of its breakup modes. Looks a lot like the conical version of the 03 Mode in the drawings to me.

https://engineeryoursound.com/what-is-speaker-cone-breakup-explained/

May 24, 2024 - 11:36:20 AM

12 posts since 4/12/2023

quote:
Originally posted by SamCy

Found a laser scanned picture and cross-section view of a speaker in one of its breakup modes. Looks a lot like the conical version of the 03 Mode in the drawings to me.

https://engineeryoursound.com/what-is-speaker-cone-breakup-explained/


That link gives a great description of what's going on.  Thanks. 

The generally preferred behavior is much the same for speakers and resonator cones.  The textbook analyses are for flat drums and plates.  Bending a sheet into a cone adds substantial stiffness.  Since the stiffness increases with curvature, it decreases as you go out toward the edge.  So, quantitatively, the resonant modes will differ from flat and require serious computer efforts to calculate.  However, I'd also suspect that the mode shapes are qualitatively similar.

What I said before was that the spider bridge's cone "fold" could act like a hinge (albeit flexing only by a tiny angle) and provide two "pistonic" modes before breakup.

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