Cool Banjo Physics

[Denis Kearns]

I got this link in a recent issue of New Scientist: http://www.its.caltech.edu/~politzer/

There’s a professor at CalTech who’s been looking into how sound is produced and transmitted in banjos. A bunch of the articles are fun reading, although some of the text can be daunting to those folks not up on their physics or with short attention spans.

One of the best things was his quote:

“On the other hand, I can play banjo in my office; when someone walks by, I just say, "I'm working."

Lucky guy (no banjo jokes here, please).

Enjoy!

- Denis

And yes, I even own a couple of banjos, but never take them out in polite company….

[MrMoe]

I like the sound of a good mountain banjo better than the sound of a Gibson Mastertone.

[Jess L.]

http://www.its.caltech.edu/~politzer/
There's a professor at CalTech who's been looking into how sound is produced and transmitted in banjos.

Fascinating stuff! Thanks for posting that.

The nearest I ever got to investigation of the physics involved in banjo sounds, when I was like 12 or something - setting all my dad's banjos down horizontal and then covering the heads with a light dusting of somewhat evenly-distributed sand and plucking a string or making other sounds to see what, if any, nifty geometric patterns might appear as a result of the head's vibrations. I tried it on all the banjos in the house. (My dad was understanding, although secretly he was not entirely pleased with all the sand.) I wasn't doing it in a controlled scientific way - I didn't take into account rim size, rim out-of-round, head tension, whether or not the brackets were cranked down evenly, etc - I was just goofing around as one afternoon's entertainment, and I did not arrive at any earthshattering useful conclusions. I don't remember now if I even got to see any cool geometric vibration patterns. (Long time ago.)

Later on, though, unrelated to sand experiments, another banjo player at a festival told me about head tuning, for more consistent/predictable tone. I hadn't been aware that was a thing. So I was like, "Aha, gotta try that." (I was kinda seriously into banjo by that point.) So I started experimenting with which exact note I wanted all my banjo heads to be tuned to - don't remember now what the notes were, something-sharp, maybe G# and something-else-sharp, anyway the idea being to tune the head to a note that's *not* one of the commonly played notes in the key(s) that that particular banjo usually plays. You know us banjo players, at least the oldtime ones, we sometimes have multiple banjos each in their own specific tunings for playing tunes in a certain key... although that doesn't apply to the G-tuning bluegrassers who get notes in any key regardless of the tuning. Nowadays I only have one banjo which I don't play a whole lot anymore (possibly *because* there's only one of them lol, it feels limiting to have only one, although these days I'm grateful to have any at all), and I haven't got around to tuning the head, it's still the same as how it came from the factory and I've never even bothered to check it to see what note it's currently tuned to (shame, shame, terrible! I've really lost my edge lol), but hey it took me over 3 years just to get around to filing the bad factory frets to make it properly playable.

Anyway, thanks again Denis for the link, some really cool reading there.

[Simon DS]

http://www.its.caltech.edu/~politzer/clonk/clonk.pdf

-I couldn't understand having time on the y axis of Fig1.

[Simon DS]

I’ve notice this with the 20.5 inch scale octave further up the neck around the 5-7 fret, that there’s warm resonant clonk sound from the top and then the note itself. And that the clonk tone changes with pick angle into or across the top, humidity, string weight etc.

[Jess L.]

http://www.its.caltech.edu/~politzer/clonk/clonk.pdf

-I couldn't understand having time on the y axis of Fig1.

Oh now look what you've done, you're trying to make us think! (Not my specialty anymore.) And here I was hoping to get through the documents just blissfully skimming through the text and seeing the colorful squiggly lines in the pictures.

Anyway, near as I can tell, and I could be wrong...

Concerning Fig 1 on page 2 of that PDF, the *variable* (the thing they're measuring) seems to be *time*. Specifically, the "decay times" (on the y-axis) of a specific range of frequencies (on the x-axis). Appropriate, I suppose, as I vaguely recall (generally speaking) that the variable typically goes on the y-axis.

As to that graph's x-axis, which shows frequency in kHz, the author says:

"The equally spaced thin lines reflect the string's harmonics driving the head."

So, I think he's trying to show the decay time at each of those specific frequencies. In that scenario, time (decay time) is the variable, thus it goes on the y-axis.

That's what I get out of it, anyway.

If that is the correct interpretation, then it might have been better for clarity if the author had labeled the y-axis as "decay time", rather than simply "time".

[Simon DS]

Yeees! I see it now, many thanks!
So the author’s only defining one pluck on one string at a certain overall frequency/scale length.
That makes sense now, because time is usually placed on the x axis...

So maybe this is partly why my octave has very centred notes...

[Bill McCall]

Just to be clear, there’s no such thing as “cool banjo physics”. Its like the key of Db, it’s on charts but doesn’t exist in actual music