At bottom, a strobe and high-speed camera accurately represent the way in which a string is moving. At top, a video taken with an iPhone camera distorts your sense of how the string is moving by capturing instead images of standing waves, caused by the rolling shutter on the device. The video isn’t wrong – it’s just showing you beautiful visualizations of standing waves that make visible how the shutter works on the camera more than they do how the guitar works.

Full disclosure: I love waves. Analog, digital, acoustic, we’re talking vibrations in sound (and other substances, as well — and light). I don’t think you get into this area without having a certain wave addiction. If you love waves, you could easily get lost in exploring videos of vibrating guitar strings and pondering the physics of the string.

This story begins not with how guitar strings actually vibrate, but a curious phenomenon when combining the regular oscillation of the string with the rolling shutter of a CMOS digital camera — namely, the iPhone’s. To accurately capture motion, you need to record an image all at once (or at least come close). Rolling shutter or line scan as exhibited in a CMOS camera sensor like the iPhone is a side effect of the capture being scanned from top to bottom, so the bottom portion of the image is saved later than the top. That causes motion to skew across the image. (Long before digital, people played around with the same effect in analog video and even using film photography – all you need is something moving and a way of capturing the image that moves gradually in a different direction.)

When the regular oscillation of the scanning combines with the oscillation of what you’re filming – as with a vibrating guitar string, or the rotating propeller on an airplane — the two frequencies effectively phase, causing some curious distortion. In the case of the guitar, this means seeing the appearance of standing waves that, while they can occur in nature, don’t occur on any conventional guitar. (You can also think of the basic effect as aliasing, as seen optically when video shutters capture the frequency of rotation of a rotating car wheel in such a way that it appears to move backwards.)

As with many concepts in physics, it’s all easier to see than explain, so I’ll turn it over to some terrific videos. I’ve contrasted two at the top of the story; here are more examples.

Below, a high-speed camera operating at 600 and 1200 frames per second, played back 20x and 40x, respectively, slower than you’d see with your naked eye.

And here’s another example of how that might appear on a camera like the iPhone:

Of course, that means – fodder for our sister site Create Digital Motion – potential for more creative abuse, beyond the mere novelty.

What’s also amusing is the heated discussion this triggered as the iPhone video went viral. Read some angry comments – and some solid science among them — at Reddit:
Guitar string oscillations captured on video [reddit.com]

I’m glad people don’t behave (yet) in person the way they do on the Internet.

In person: “Excuse me; I think your fly is open.”

On the Internet: “That guy’s fly is open. He doesn’t even know his fly is open. That’s bullshit. I mean, what kind of person leaves their pants just *(&$#ing open like that? Look, look, look at his open fly. I’m never talking to him again. I’m not even going to wear pants from now on.”

(Seriously, happily, many of the comments are perfectly polite and well-informed!)

School’s out for just about everyone, but I’m going to remember to file this away for the next time I have to explain sound vibration. Now, back to the beach, or wherever you’re relaxing.

Thanks to Alexander Chen, from whom I lifted this via Google+. (See his own work on CDM here and here. Alex is just the kind of person I want to see this, as he’s been working with the aesthetics of vibrating strings! So, hurrah, Google+…

  • http://music.cornwarning.com chaircrusher

    I remember back in the day the slowed-down guitar (and bass) string wobbling being a fascinating part of XTC's "Senses Working Overtime" video.
    http://www.youtube.com/watch?v=SyX75NpibdU

  • Dan Pat

    There's something decidedly "Human Centipede" about the Plek Technology video. No offense intended to them, but the metallic, grey lab with flashing light and droning detuned guitar just doesn't make for a very attractive product demo. That aside, all of these videos are very interesting visual depictions of waves, accurate or not. Great post!

  • bonjour

    with a double bass : 4041788

  • au revoir

    sorry for failing the link :&nbsp ;http://vimeo.com/4041788

  • http://www.jhhl.net/iPhone jhhl

    You don't need a high speed camera if you use a very long string. see:&nbsp ;http://www.ellenfullman.com/

    Actually, most of Ms. Fullman's sound is not from the side-by-side waving of her strings in air – which is way below human hearing – but from the extremely fast end-to-end vibrations in metal.

  • Daniel Tirado

    this is the solution of a diferential ecuation of a string programed in processing

    x axis is amplitude of the perturbation

    y axis velocity of propagation in the string (tension/density)^2
    http://www.openprocessing.org/visuals/?visualID=3

    sorry the inglish :D

  • Lasse Munk

    I could be cool to see it if you used a "ebow" for your guitar. So you'll have a continous waveform.

  • http://http:/wheatwilliams.com Wheat Williams

    This reminds us that all synthesis and sound generation owes its physics to understanding the mechanics of vibrating strings, and by extension the resonance of all other kinds of acoustic instruments. It all goes back to Pythagoras many centuries ago working out the geometries and maths of nodes, harmonics, and how to tune a scale with pure intervals.

    Some of these videos would be really great for high-school physics classes.

  • http://mikesalovich.com Mike

    Check this out!  I filmed my large baritone 12-string and a tabla with a high shutter speed camera… &nbsp ;http://youtu.be/wdggA-Qc7f4

  • http://mikesalovich.com Mike

    I made cool high shutter speed video of a 12-string using a canon 5d at… http://youtu.be/wdggA-Qc7f4