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CSI:NY - Blather, Rinse, Repeat
September 27th, 2006
11:05 pm


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I've been watching the CSIs now for a while - lured in first to Miami when TiVo told me that Emily Procter could be found these days being the ballistics expert in Miami, and then a couple crossovers later, I'm now hooked on all three.

So. Tonight's episode involved using a "Sine Wave Generator Program" which created a waveform that was turned into an MP3, which was subsequently played back with an iPod attached to a battery powered set of speakers. This waveform simultaneously set up sympathetic vibrations in all the glass cases inside the jewelry store. The CSIs were able to track down the distributor of the Sine Wave Generator Program, and track the purchase to a P.O. Box.

There are so many things wrong with this:

  • Sine Wave Generator Program - the criminals involved were college students, it seems likely to me that if they understood enough about sympathetic resonance to conceive of a scheme to shatter glass, they'd be capable of writing a program themselves to generate a sine wave. To prove that it could be done by a hobbyist, I wrote a 30 line program to generate sine wave WAV files. The whole idea of using sound was to eliminate evidence at the scene - why buy a commercial sine wave program (with the associated paper trail) when you can write your own? And, if you're lazy, you can download a free one.
  • audible to dogs, inaudible to humans - Humans can hear in a range from 20 cycles per second (hertz) to around 20,000 hertz. Unless, like me, you've listened to a little too much Phil Collins on the Walkman back in your youth, in which case you can't hear that full range. Dogs can hear about one octave up from us, a range from 40 hertz to 46000 hertz. This means that the glass-breaking sound would have to be in a band between 20,000 and 46,000 hertz.
    There are several problems here. For one thing, somehow they discerned that the theives were using MP3 as their file format, instead of WAV, AIFF, AAC, or whatever else. MP3 is familiar, but wouldn't have been used for a couple of reasons. First, it's a lossy format, which means that you'd get degradation of your pure waveform, which isn't what you want. Secondly, MP3 is designed for human listeners - if you try to MP3 encode a signal outside the range of human hearing, you'll get silence. Not just "inaudible signal" - a total lack of signal. This is actually a feature, MP3 throws away stuff that you don't want to listen to and passes the savings on to you in the form of a smaller file.
  • what's the resonant frequency of a display case, anyway? - here's an exercise you can do at home. Take a wine glass, or a fish bowl, or a plate glass window. Flick it with your fingernail. The better the quality of the crystalline structure involved, the clearer the ringing sound will be. Ideally, you'd get a pure tone, a single frequency. But one thing that I'm pretty sure you'd get would be an audible frequency. The smaller the glass, or the higher the tension on the glass, the higher the frequency. But maybe you just rang a wine glass, and maybe it was around 600 hertz. The display cases pictured were much larger than your wine glass, so their ringing frequency would have been substantially lower than your wine glass. And, as I mentioned above, the range where a dog can hear something and a human can't is up in the higher ranges, so a low note isn't helping us.
    Also, consider that every display case in the store seemed to shatter simultaneously. If you were putting out a pure tone, the shapes of all the panes of glass would have to be the same to get the same ringing frequency. If you weren't putting out a pure tone, if you were using a shotgun approach to cover multiple frequencies at once, you'd have a much harder time getting the glass to resonate with your signal, because you're splitting your power up amongst the different frequencies.
    So, assuming that all the display cases had the same resonant frequency, how do you find the frequency? Mythbusters did a segment where they had a piece of hardware that generated pure sine wave tones, and it took quite a bit of work to find the right frequency, and once they found it, they had to spend several seconds at that frequency for the vibrations to break glass.
  • "Moon River" - yeah, it was cute that their MP3 of an inaudible tone had been titled the same thing as the song from "Breakfast at Tiffany's", but there's no reason you'd need to have a song-length file - three minutes of inaudible tone is certainly overkill.

As an experiment, I wrote this program to prove that generating sine waves is something that amateurs can do with no special training. The output of it is this WAV file, which I proceeded to encode into this MP3 file. WARNING - the audio files have pure tones, and I insist that you not listen to them with headphones. If you want to listen to them on speakers, you might want to turn the volume down before playing them. That said, go ahead and play them - the files are each 16 seconds long, starting at 343.75 cycles per second and stepping up an octave every two seconds. This means that 12 seconds into the playback, the signal is 22,000 cycles per second, a sound that you can't hear, but a dog can. At 14 seconds into playback, the frequency is 44,000 cycles per second, which is getting up to the point where signal reconstruction is problematic (indeed, the WAV file isn't able to reproduce 44,000 cycles per second correctly, and the MP3 doesn't even try).

Did you play the MP3 file? Did glass break? I didn't think so.

(7 comments | Leave a comment)

[User Picture]
Date:September 28th, 2006 12:28 pm (UTC)
I think you were missing one possibility. If the actual music could play (albeit inaudible to us), you could set it so that it played different frequencies for ten seconds each. This would explain the length of the song. Add it to some research, say buying some plates of glass and seeing what hertz they shatter at, and you bring the scenario to the outer edges of possibility.

The shattering wouldn't be as dramatic, but that's television. :)
[User Picture]
Date:September 28th, 2006 02:41 pm (UTC)
I forget exactly how long the "song" was, but it was something like three and a half minutes, which would be 21 frequencies that you could hit during the duration of the song. Assuming we're finessing the fact that high frequencies are even going to shatter the glass, and assuming that they can do the advance work to narrow down the resonant frequency to a narrow window covered by these 21 signals - a three and a half minute song means that they're prepared for the possibility of waiting around for a few minutes before they hit on the right one, which seems very unlikely in a robbery - you'd want to grab the goods much quicker than that and be on your way.
[User Picture]
Date:September 28th, 2006 03:12 pm (UTC)
waiting around for a few minutes

I don't see this as a problem. You simply don't make your move until the glass breaks.

Just looking, just looking....*smash*, grab, run.
[User Picture]
Date:September 28th, 2006 05:20 pm (UTC)
Sure, you could do something like that, but that's not what was pictured - the thieves came into the store in disguise, which immediately drew attention. They didn't have three minutes to wait for their secret weapon to take effect - they had people down on the ground within seconds of entering the store, and the glass was shattered within seconds after they gained control.

Additionally, something that I didn't mention above is that iPods don't reproduce ultrasonic frequencies very well - imagine that. I don't know what sort of speaker system they were using, but it's also possible that the frequency response of their 3" speakers wouldn't have been very effective between 20kHz and 46 kHz.

[User Picture]
Date:September 28th, 2006 05:38 pm (UTC)
You may well be the nerdiest person I know. I hope you're okay with that, because I am.
[User Picture]
Date:September 28th, 2006 06:27 pm (UTC)
Thanks! :)

I prefer not to think of myself as nerdy or geeky, but instead "a treasure trove of useless and sometimes frightening information".

A narrow distinction, perhaps.
Date:October 17th, 2008 06:42 pm (UTC)
I REALLY needed that...

Don't worry. Popular people are popular cause there's so few of them. Once the rest of the nerds & geeks realize that, we win. :P Yes, I am a nerd, and I'm proud that I got a collage offer at the age of 12! (Just got it yesterday. YAY!) I'm the "teachers hate me cause I get in trouble so much, and they still like me (if they say love, I threaten to sue.) cause I sleep through class and still know what they're saying" type nerd.
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