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Physics. Huh.


Here’s an article about the physics of the banjo. Specifically, why it twangs.

It starts thusly: “Today, we get an answer thanks to the work of David Politzer at the California Institute of Technology in Pasadena, who in his spare time, is a Nobel prize-winning theoretical physicist.” Call me crazy, but I’m guessing theoretical physicist is his day job and this banjo thing is something he does in his spare time, but that’s about the last thing I understood.

It’s fucking physics, man. Of course I didn’t understand it.

This I got. I think. If a sound vibration is matched with a vibration that is similar and several tens of hertz higher, it sounds plinky. You can (apparently) make this happen in Audacity by making a sound and screwing with it. It doesn’t apply to things like guitars and violins because wood tops aren’t as springy as a banjo head.

But I got all tangled up in the difference between the frequency of the sound and the frequency of the vibration. And that made me feel stupid. And that made me sad.

Don’t be sad, Weasel! It’s the weekend!


Comment from p2
Time: February 24, 2017, 10:08 pm

same kinda thing happens in a piano. if you listen closely after you hit a key, you’ll hear other notes both above and below the struck note. i wanna fifths & octaves, but i’m prob’ly wrong… vibration induced oscillation….

Comment from Uncle Al
Time: February 25, 2017, 12:17 am

Sweasel, I’m sympathetic to your sadness but I think it is all in the way the article explains things: the problem is in the words but not in the concepts.

Here, I’ll give it a frail and see if anybody hollers Yee-haw!

You may have noticed that the banjo-ness sound quality is more noticeable with higher notes than lower ones. To be sure, plucking the lowest pitch string still makes an unmistakable banjo sound but when you are actually playing a tune it is the higher pitches that ring out more than the lower ones.

Dave P. says that this is because the characteristic timbre or sound quality of any particular vibrating string is affected by two things in addition to the strings own vibrations. One is that, because the pitch of a vibrating string depends on its tension – ceteris paribus (oops – all things being equal) – and the vibrating string is a little longer and therefore a little tighter twice for each full vibration cycle, this slight difference in pitch is transmitted to the drum-like head via the bridge. Plus, all of the vibrating banjo strings are doing the same thing. So, the banjo head is a mess of all kinds of vibrations all happening on top of each other.

And that mess of vibrations are transmitted back through the bridge to the strings. Of special importance to this process is that the vibrations of each string are affecting the vibrations of every other string. This is unique to the banjo because the head, being a drum-like structure, is much more flexible than the wooden front sound boards of guitars, cellos, etc.

And finally, this imposition of the vibrations from string X onto the vibrations of string Y give rise to the characteristic banjo sound quality. The reason is that a guy named Chowning at Stanford AI Lab found back in the early 70s that imposing a very low frequence modification onto a vibrating string gives it a tremolo effect, but as that lower frequency is raised the tremolo disappears and a metallic ring effect becomes apparent.

And so it seems that the somewhat lower frequency bouncing around of the head, when transmitted back through the bridge to the higher note strings, makes them “ring like a bell,” the lovely sound of a well-made and well-played banjo.

I sure hope this helps!

Comment from Ric Fan
Time: February 25, 2017, 1:29 am

My pet peeve is that whenever they do a documentary on prisons/death penalty they always have a twangy music soundtrack, even if the prison is in Maine.

Comment from durnedyankee
Time: February 25, 2017, 1:42 am

And to think some old dude (or possibly an old dudette) figured all that out without knowing a dang thing about physics. Or, they did, but were more interested in banjo music. Given a choice twixt physics and banjo music I can understand getting more pleasure from banjo music.

However I have observed one should never listen to something like Foggy Mountain Breakdown while driving state highway 62 in Arkansas between Talequah and Prairie Grove. There seems to be some physical principle involved that causes pickup trucks loaded with bearded idiots 😀 to go much faster on curved roads than one should if one is listening to banjo music. Perhaps if one had payed more attention to the coefficient of friction than the music things might have been a little less exciting that afternoon 😮 .
Needless to say, the banjo music was determined to be the cause rather than middling grades received in physics.

Comment from ExpressoBold
Time: February 25, 2017, 2:38 am

@ durnedyankee
There’s always the tried and true explanation of “I blame the corn sqeezin’s…”

Comment from Uncle Al
Time: February 25, 2017, 5:08 am

@durnedyankee – I am bordering on dumbfounded. Here in Sweasel’s blog, in an article about the physics of banjo twang, I find you mentioning highway 62 and Prairie Grove…where my wife’s sister-in-law and two nieces live on Pearson Ranch Rd. and where I was visiting just last summer. (Her brother passed away last year, sad to say).

And to make things just as eerie as possible, we were binge watching original episodes of The Twilight Zone this evening.

I do love coincidences such as this. Thanks!

Comment from Uncle Al
Time: February 25, 2017, 5:12 am

@Stoatie – I just took another look at the banjo string diagram at the head of this item and it struck me that there is only one arrow with the legend “to nut.”

Comment from S. Weasel
Time: February 26, 2017, 11:40 am

That was an amazing explanation, Uncle Al. I got it now.

Still trying to work out if your last comment is a nut joke.

Comment from durnedyankee
Time: February 26, 2017, 1:12 pm

uncle Al.
I blame Stoatie for having an awesome site. Full of
banjos, art, history, chickens, …, delightful mustelid hostess and sidekick, Zombie Reagan of course, and slices of life in old Blighty.

Upon re-read, I replaced the 5th reference with ellipses because, well, it referred to the dead pool prize, and I just couldn’t bring myself to leave it there.

And I omitted the story, different December, same road, of inadvertently picking off a male guineafowl who broke LEFT when the rest of the smarter females went RIGHT. It was bagpipe music at the time. Reserved perhaps for when Dame Stoatie takes up squeezing cats to make music.

Comment from Uncle Al
Time: February 26, 2017, 1:56 pm

@Stoatie – It was a banjo player joke (in poor taste). (-:

Comment from S. Weasel
Time: February 26, 2017, 8:37 pm

Ohhhhh…I get it. Hurrrr.

I played the bagpipes for a while. Somebody set fire to my car and the bagpipes were in the trunk.

I’ll let you fill in the whole rest of the story as you like.

Comment from Deborah HH
Time: February 26, 2017, 9:49 pm

The only word I understand is “tremolo.”
That is all; thank you.

Comment from Subotai Bahadur
Time: February 26, 2017, 10:48 pm

1) Long ago, I studied bagpipes for a while [I know, a Chinese-German (and more it seems, a daughter has just traced part of my mother’s family to 1850’s Kentucky) piper] But I’d like the rest of the story.

2) My wife plays mountain dulcimer [also called Appalachian Dulcimer] with a dulcimer club and performing group here in Colorado. I showed her the post, and she wants me to ask you if you have ever encountered a “Banjomer”? Someone in her club has one.

Comment from durnedyankee
Time: February 27, 2017, 12:46 am

Next time tell us the bagpipe story Auntie Weasel!

Comment from ExpressoBold
Time: February 27, 2017, 12:19 pm

@ S. Weasel:
It must have something to do with rioting… being in an area where there was a riot…

Comment from Mr. Dave
Time: February 27, 2017, 5:33 pm

“It boggled.”

Comment from S. Weasel
Time: February 27, 2017, 7:15 pm

Oh, it’s a mystery, really. People used to set fire to Providence, RI on Halloween every year, but my car in the garage was the only thing in my neighborhood that was burned. I didn’t have any enemies; just a random act of violence, I guess.

The insurance company paid out, though.

Comment from durnedyankee
Time: February 28, 2017, 10:43 am

Okay, bagpipes were involved (which I love, must be genetic) in the only local fire and you say it was random.

You really are a good soul.

Oh…bwaahahahahah, just realized your copyright info is hilarious!
“The First Amendment does not authorize the fourth estate to be a fifth column.”

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