Secret of the Banjo's Unique Sound Discovered By Nobel Prize-Winning Physicist

KentuckyFC (1144503) writes The banjo is a stringed instrument that produces a distinctive metallic sound often associated with country, folk and bluegrass music. It is essentially a drum with a long neck. Strings are fixed at the end of the neck, stretched across the drum and fixed on the other side. They are supported by a bridge that sits on the drum membrane. While the instrument is straightforward in design and the metallic timbre easy to reproduce, acoustics experts have long puzzled over exactly how the instrument produces its characteristic tones. Now David Politzer, who won the Nobel prize for physics in 2004, has worked out the answer. He says the noise is the result of two different kinds of vibrations. First there is the vibration of the string, producing a certain note. However, the drum also vibrates and this pushes the bridge back and forth causing the string to stretch and relax. This modulates the frequency of the note. When frequency of this modulation is below about 20 hertz, it creates a warbling effect. Guitar players can do the same thing by pushing a string back and forth after it is plucked. But when the modulating frequency is higher, the ear experiences it as a kind of metallic crash. And it is this that gives the banjo its characteristic twang. If you're in any doubt, try replacing the drum membrane with a piece of wood and the twang goes away. That's because the wood is stiffer and so does not vibrate to the same extent. Interesting what Nobel prize-winning physicists do in their spare time.

Comments

what a Happy instrument

[turkeydance]

You just can't sing a depressing song when you're playing the banjo. You can't go-- "Oh, murder and death and grief and sorrow!" --Steve Martin http://snltranscripts.jt.org/7...

dont need to replace the drumhead....

[Lumpy]

Just hold a thumb against it. a Lot of us players do that to adjust the sound for different "expression"

Re:dont need to replace the drumhead....

[sribe]

Right... this seems like more of "A scientist that doesn't know anything about music explains something everyone who plays already knew" type of thing.

No, more like: everybody pretty much knew that the bridge being mounted on a vibrating membrane would affect the sound, but a scientist thoroughly analyzed and modeled the whole setup in order to quantitatively figure out exactly how all the parts and vibrations contributed to the sound. Then a dumbass know-nothing journalist wrote an article that misstated what had actually happened ;-)

Re:Banjo = guitar + snare drum

[Kkloe]

yes but have you written it down? have you done the math behind it?
here is the original paper http://arxiv.org/pdf/1406.4907...

Banjo jokes

[tomhath]

Q: How can you tell if the stage at a Bluegrass concert is level?

A: The banjo player drools out of both sides of his mouth.

Banjos...not a fan...

[VAXcat]

One day my old pal David has played a gig with some local musicians, including his roommate, Bob, who was a banjo player. After the set, Bob was going somewhere else with some other people, so he asked Dave to take his banjo home for him. On the way home, David stopped at the convenience store to get a six pack. As he was standing in line, he suddenly realized that he had left the car windows down, and that he was in a bad neighborhood. He rushed out, but, sure as hell, the worst possible thing had happened - exactly what he was afraid of - someone had spotted the open car windows, and thrown two more banjoes in the car.

Sitars

[billstewart]

The sitar has several things going on with it

• -- The body has a chamber made from a big gourd, and a wooden neck with adjustable frets.
• -- There's a layer of strings that you pick, optionally pressing on the string over a fret to change the pitch.
• -- There's another layer of strings underneath that resonate when you play the note they're tuned to on the main strings, which provides some amplification and a lot of sustain; that's one of the things that gives the sitar its characteristic sound.
• -- In addition to fretting a string when you pick it, you can also bend it to the side, changing the pitch dynamically, which is another characteristic sitar sound. Guitar and bass players also use this technique, but sitar strings are long enough that it's easier to do.
• -- Generally there are two or three strings that you'll play the melody notes on, and several more strings that you pick without fretting, letting them drone like a mountain dulcimer; that's another characteristic sitar sound.
• That's most of the technology parts; the rest is about the music itself.

Indian classical music theory is complex, at least as much as European classical music theory or jazz. There's a lot of stuff about "ragas", which are a combination of a scale or scales, melodies, fixed parts and improvised parts, with a lot of rules about which ones are appropriate for which situations.

Not the first one.

[140Mandak262Jamuna]

He is not the first Nobel Laureate to be fascinated by the drums and vibrating membranes. Sir C V Raman, of the Raman Effect fame, was intrigued by the Indian drums, the Tabla and the mridangam. He published why and how they produce harmonics (paywall) back in 1920s. A synopsis.

In some sense it is not a surprise because his main work was on vibrating electromagnetic fields, and the natural modes of vibration of circular membranes is a very good way to practice the mathematics of vibrations.

Interesting spare time

[Anonymous Coward]

Richard Feynman enjoyed playing the bongo drums, picking locks, and whatnot; and also

Then I had another thought: Physics disgusts me a little bit now, but I used to enjoy doing physics. Why did I enjoy it? I used to play with it. I used to do whatever I felt like doing - it didn't have to do with whether it was important for the development of nuclear physics, but whether it was interesting and amusing for me to play with. When I was in high school, I'd see water running out of a faucet growing narrower, and wonder if I could figure out what determines that curve. I found it was rather easy to do. I didn't have to do it; it wasn't important for the future of science; somebody else had already done it. That didn't make any difference. I'd invent things and play with things for my own entertainment.
So I got this new attitude. Now that I am burned out and I'll never accomplish anything, I've got this nice position at the university teaching classes which I rather enjoy, and just like I read the Arabian Nights for pleasure, I'm going to play with physics, whenever I want to, without worrying about any importance whatsoever.

Within a week I was in the cafeteria and some guy, fooling around, throws a plate in the air. As the plate went up in the air I saw it wobble, and I noticed the red medallion of Cornell on the plate going around. It was pretty obvious to me that the medallion went around faster than the wobbling.

I had nothing to do, so I start to figure out the motion of the rotating plate. I discover that when the angle is very slight, the medallion rotates twice as fast as the wobble rate - two to one [Note: Feynman mis-remembers here---the factor of 2 is the other way]. It came out of a complicated equation! Then I thought, ``Is there some way I can see in a more fundamental way, by looking at the forces or the dynamics, why it's two to one?''

I don't remember how I did it, but I ultimately worked out what the motion of the mass particles is, and how all the accelerations balance to make it come out two to one.

I still remember going to Hans Bethe and saying, ``Hey, Hans! I noticed something interesting. Here the plate goes around so, and the reason it's two to one is ...'' and I showed him the accelerations.

He says, ``Feynman, that's pretty interesting, but what's the importance of it? Why are you doing it?''

``Hah!'' I say. ``There's no importance whatsoever. I'm just doing it for the fun of it.'' His reaction didn't discourage me; I had made up my mind I was going to enjoy physics and do whatever I liked.

I went on to work out equations of wobbles. Then I thought about how electron orbits start to move in relativity. Then there's the Dirac Equation in electrodynamics. And then quantum electrodynamics. And before I knew it (it was a very short time) I was ``playing'' - working, really - with the same old problem that I loved so much, that I had stopped working on when I went to Los Alamos: my thesis-type problems; all those old-fashioned, wonderful things.

It was effortless. It was easy to play with these things. It was like uncorking a bottle: Everything flowed out effortlessly. I almost tried to resist it! There was no importance to what I was doing, but ultimately there was. The diagrams and the whole business that I got the Nobel Prize for came from that piddling around with the wobbling plate.

-- Surely you're joking, Mr. Feynman (c) 1985, PP 157-158