Musical String Shooter Makes Sound Visible

One reason we really like [Rulof]’s hacks is that he combines the most unlikely things to create something unexpected. This time he makes a fast-moving loop of cotton string undulate in time to music.

To do this he uses cotton string, hard drive parts, two wheels from a toy Ferrari, two DC motors, a plastic straw,  a speaker, and an amplifier.  The loop of string sits in the air by being rapidly rotated in between the two wheels. The hard drive parts, driven by the amplifier, give the string a tap with an amplitude, and at a time determined by the music. The result is music made visible in the air in front of you, or in his living room in this case. Check out how he made it, and see it in action in the video below the break.

And if you want to see more of [Rulof]’s creations, have a look at his leg mounted, underwater, beer bottle propulsion system, his vibrating gaming chair, and another usage of the same hard drive parts but this time to make a microphone that works surprisingly well.

11 thoughts on “Musical String Shooter Makes Sound Visible

  1. The arm acts like an envelope detector since the amplitude just bumps up the string but doesn’t pull it back down. If it had a tube instead so it actually controlled the string actively it would probably look cool

    1. Very good point hap.

      You could also maybe use an UV reactive string and put a blacklight on it to get it even more spiffy.

      Incidentally, I’m impressed by the video’s speedy clarity in instructions, nice cutting and angles.

      1. P.S. I do find it a bit tricky when you just put power straight from an amp on the coils of the arm, some amps would probably damage the thing right quickly, or if you keep the volume very low would not include an audible output on the speaker leaving you just with the string.

          1. The arm works pretty much on the same principle as the speaker. Both are voicecoils, so a coil next to a magnet. Which in turn means that they’re capable of pretty high frequencies. The problem is that you simply won’t be able to see them. Or even hear, if you go waaay high.

          2. I think 50Hz is a bit too low a number, although low frequencies are most noticeable I’d say you can at least go up to 300Hz.
            But I think this was made to be as simple as possible without too much electronics.

          3. Seek time is most commonly specified as an average, but the worst-case seek time (full-stroke) is typically about double the average (or a bit less). We’ll take an average seek time as 10ms, just to put some numbers on it; this means we’ll take full-stroke seek time as 20ms, which is conservative for modern drives.

            We can then picture the head of a drive commanded to read sectors alternately from the inner and outer tracks, as following a sine wave with 40ms period (20ms to seek out, 20ms back in); of course using a half-sine motion profile results in infinite jerk at start and stop, so I expect actual drives use a slower profile, meaning a true sine wave could go some faster, but it’s close enough. This gives us 25Hz, absolute minimum, for full-stroke movement; I wouldn’t be surprised if you could get 50Hz on a pure sinusoid (and if not, there’s always fancy server drives!). But the maximum frequency really depends strongly on amplitude — best-case (track-to-track) seek is typically around 1ms, leading to 500Hz for small amplitudes.

            Depending on the amplitude, then, your 50Hz figure and [Whoknows]’s suggestion of 300 Hz are equally valid; to me, the question is really which looks better on the string, bigger amplitudes that only respond to bass, or smaller amplitudes that cover a bit more frequency? Only one way to find out, of course, but it probably depends on the sort of music you’re playing, and I’m guessing large amplitude wins nine times out of ten.

            (The other option, of course, is to cheat! Use what amounts to a single-band vocoder — an envelope detector (on whatever frequency band you like) modulating the amplitude of a carrier frequency, and drive the voice coil with that; the carrier frequency needs to be just fast enough to keep the head visibly blurred. Now it visibly responds to the higher frequencies, without actually needing to handle those frequencies.)

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