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physical origin of resonator guitar / banjo similarities

May 18, 2024 - 7:21:09 AM
like this
1684 posts since 1/9/2012

I've heard it said that resonator guitars have things in common with banjos.  Using spectra and spectrograms, I put my finger on some of it.  They both produce bits of sound with each note that aren't present with civilized instruments.  In acoustics, these are called "inharmonic partials."  They have frequencies that are absent in the ideal string or the ideal pipe of air when you play a note.  I'm convinced that they're what make banjos "ring" and resonator guitars "twang."  Both to the ear and to the computer, those inharmonic partials have a "metallic" sound.

I recently succeeded in tracking down what it is in all resonator guitars that produces that sound, having succeeded with banjos a few months ago.  It turns out to be much the same thing: louder => more bridge motion => sounding the strings you didn't pluck.  The story and fiendishly clever experiment are described in a short note: https://www.its.caltech.edu/~politzer/reso-partials/reso-partials.pdf .  The experiment's results are displayed in the figure on page 6.  There are also links to the previous work on banjos and resonator guitars.

May 26, 2024 - 7:12:20 PM

117 posts since 8/23/2022

Very interesting. Thank you for taking the time to do the research.

-Scott

May 31, 2024 - 2:51:29 AM

396 posts since 6/20/2020

quote:
Originally posted by davidppp

 

>I recently succeeded in tracking down what it is in all resonator guitars that produces that sound, having succeeded with banjos a few months ago.  It turns out to be much the same thing: louder => more bridge motion => sounding the strings you didn't pluck.<


This description caught my eye. As a player of early 6 and 7-string banjos this is a key feature of their distinctive sound and the joy of playing them. It's not simply the extra bass string(s) as they are picked, as you said it is the combined presence and resonance - the sounding - of the strings we didn't pluck.

May 31, 2024 - 6:26:37 AM

1684 posts since 1/9/2012

quote:
Originally posted by Pomeroy
quote:
Originally posted by davidppp

 

>I recently succeeded in tracking down what it is in all resonator guitars that produces that sound, having succeeded with banjos a few months ago.  It turns out to be much the same thing: louder => more bridge motion => sounding the strings you didn't pluck.<


This description caught my eye. As a player of early 6 and 7-string banjos this is a key feature of their distinctive sound and the joy of playing them. It's not simply the extra bass string(s) as they are picked, as you said it is the combined presence and resonance - the sounding - of the strings we didn't pluck.


Actually, a couple of months ago, I posted (https://www.banjohangout.org/topic/396285) a link to my banjo version of the same sort of investigation.  With banjos, you not only hear the uplucked strings, you also hear the actual pitches of the head resonant modes.  It's this latter set that some people seem to find objectionable and successfully suppress it with a gadget called the Banjo Bolster (https://www.banjohangout.org/topic/391237).

My decade of "banjo physics" has never uncovered anything that wasn't already familiar -- at least to discerning players and builders.  For example, that head tap tuning is impossible for some people and some electronic tuners or that a given bridge has different sound effects on different banjos are well known.  I just figured out some of the physics behind the phenomena.  In fact, that's all I ever did when I was a "respectable" physicist (which Google seems to think I was).

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