Built on Facts

Laser Safety and the Vuvuzela

As you might expect for a guy who does experimental optical physics, I get to spend a tremendous amount of time in labs with some fairly snazzy lasers. Most of them are fairly specialized pieces of equipment that aren’t really designed simply to dump huge amounts of power in industrial applications. As far as danger goes, they’re not going to come to life and murder you in your sleep. But still, we have open beamlines of infrared lasers with average powers on the order of 1-4 watts. Unfocused they usually won’t do much to exposed skin other than make you uncomfortable (for instance, like a Christmas light pressed against the skin for the 4 watt laser). Focused they’d have no trouble poking a very clean little pinhole in you. Unfortunately your eye is a focusing lens, and direct eye exposure would be instant permanent damage. And the infrared beam is invisible to the naked eye, so that’s an extra challenge.

Of course we have pretty robust safety procedures and I’ve never heard of any injuries in our AMO group. We also have a wide assortment of surprisingly expensive safety glasses that are designed to filter out the wavelength of particular lasers. For lasers with a continuous beam this is not so hard since all the light is emitted at pretty much exactly one frequency. For lasers that generate ultrashort pulses the light spans a wider portion of the spectrum and the glasses have to remove more bandwidth.

This particular laser is a 15 watt Nd:YLF laser we use to pump an ultrafast amplifier. While it’s possible in theory to pop the lid and get a good picture of its innards, it’s not a great idea to shut down and open up expensive equipment for no research-related reason. So here’s a picture of some green light diffusely reflecting from the gap between the pump laser and the amplifier:

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And here’s the same thing with some safety glasses held in front of the camera:

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Though the images are not great, you can see that the green light is totally blocked. You can still see most of the world pretty well because the glasses still let in light with longer wavelengths than green. (For the curious, these block everything from about 540nm to around 160nm. This protects against frequency doubled Nd lasers and excimer lasers, though our lab only uses the former.)

So what’s this have to do with the now-infamous vuvuzela? If you’ve been living under a rock lately, you may have missed watching or at least seeing clips of the World Cup being held in South Africa. Apparently it’s tradition to bring this instrument to the games. It sounds something like an oversized kazoo and in the aggregate makes the stadium sound like the world’s biggest beehive. But in analogy to the laser, it just so happens that the sound produced by the vuvuzela is generated in a relatively narrow frequency range – mostly at 233 Hz, 466 Hz, 932 Hz, and 1864 Hz. Block those frequencies electronically and you remove that buzzing while preserving most of the rest of the noise of the games. Various news organizations have reported that some people are doing just that. Here’s the web site of the guy most of the news articles mention. It’s in German, but if you scroll to the bottom the before/after sound samples are easy to find. It’s not quite perfect. The vuvuzela is not quite a perfectly narrowband instrument and some of those removed frequencies contain significant parts of the spectrum of the human voice which leaves vocal audio sounding a tiny bit tinny. But it’s really a remarkable improvement nonetheless.

Maybe not quite as important to health as a good pair of laser safety glasses, but certainly an application of band-stop filtering that’s likely to be useful to more people.

Comments

  1. #1 Joshua Zelinsky
    June 17, 2010

    As I understand it, part of the problem with the vuvuzelas is that the vast majority of people blowing them don’t actually know how to blow it. A well blown vuvuzela will sound more like a classic horn. But there are lots of people who are blowing them with no knowledge how to old their lips or moderate the air level or anything.

  2. #2 HP
    June 17, 2010

    FWIW, a vuvuzela is nothing like a kazoo. A kazoo is a membranophone, in which a (usually paper) membrane vibrates sympathetically with an external sound source (usually a voice) to amplify and modify the sound. A vuvuzela is a lip-vibrated aerophone (aka trumpet). The lips are used to create a vibrating column of air within the horn, which resonates at frequencies corresponding to the normal modes of the instrument.

    Ordinarily I wouldn’t nitpick, but the fact that the resonant frequencies of the instrument are predictable based on its physical dimensions is what makes this technique possible. It would be impossible to do narrow band-pass filtering on a kazoo because it has (nearly) no resonant frequencies of its own.

  3. #3 6EQUJ5
    June 17, 2010

    The instrument’s output is correlated noise, which is more disruptive to voice communication than random noise at the same volume because it tends to confuse signal processing.

  4. #4 Matt Springer
    June 17, 2010

    Good catch, HP. In general instruments with complicated geometries will usually have a more complicated mode structure. It would (probably) be easier to block out a flute playing a G sharp than a trumpet.

  5. #5 Eric Lund
    June 17, 2010

    One of the experiments in the group where I got my Ph.D. was to look at naturally occurring noise in a frequency band which overlaps the AM radio spectrum. (The electron cyclotron frequency in the ionosphere typically falls within this range, and at times the plasma frequency does too.) Doing the experiment involves putting a receiver somewhere out in the boonies (to cut down on the amount of broadcast signal), but not too far (the receiver needs electric power to run). One of the initial sites was about 20 miles from Fairbanks, which has a powerful AM radio station known at the time for its religious content. As part of the installation they installed such a filter (radio engineers call it a “notch filter”) in order to knock 40 dB off the “Voice of God.”

    As for brass and woodwind musical instruments, they work by having a tube whose effective length can be changed by slide (trombone), valves (all other brass instruments), or keys (woodwinds), along with some means of selecting a particular harmonic (through keys and/or blowing technique). The overtones you hear are always harmonics (otherwise the instrument would be musically useless); you can force the higher normal modes to be harmonics by making the bore of the instrument either cylindrical (flute, clarinet) or conical (oboe, saxophone). Flutes have particularly weak harmonics, but the frequencies a trumpet puts out are equally predictable, so it is not that much harder to filter out a trumpet than a flute if both are playing pitches of similar duration.

    Karaoke machines work on a different principle: usually the lead singer is centered in the stereo mix, so by inverting one of the channels you can cancel out his/her voice.

  6. #6 CCPhysicist
    June 23, 2010

    What does the warning label say on your laser? I’m curious because I’ve only seen the ones on the 1 mW ones we use in class.

  7. #7 zorger
    June 28, 2010

    Mat & HP: Mat you should not give in to such nit picking. HP he said “It sounds something like an oversized kazoo” and did not mention any other similarity between the two as to the method of signal generation.

  8. #8 David Broome
    June 28, 2010

    Have you seen the shockingly affordable blue laser for sale at wickedlasers.com? It’s their new Arctic Blue line. At under $200, it is in the price range of just about anybody, and about as dangerous as a handgun when it comes to blinding yourself or bystanders. I don’t imagine it will be long before we start seeing a flood of blinded, drunken college students popping up in courtrooms.

  9. #9 bomoore
    June 28, 2010

    The fact that human beings who love soccer love the vuvu thing, proves that soccer is the game of morons.

  10. #10 Müzik Dinle
    June 29, 2010

    Vuvuzela bizimde kafamızı sikiyor Türkiye de de izliyoruz maçı napalım

  11. Şuan dada Japonya-Paraguay maçı oynanıyor.Bakalım bu maç sonucu ne olacak.Vuvuzelada sikti beynimizi