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Law, culture, and Catholicism...up in smoke!

Wednesday, October 03, 2007

Interesting and Inexpensive Device Caused Havoc in Multimedia Room

Some time ago, I was kind of surprised that a science fair had something disturbing all of the audio and video equipment in the room. Video had long sequences of snowy interference and audio would buzz making the voice recording unintelligible. At others times the audio became snowy and incomprehensible. Turns out two devices caused all the havoc.

One was a Jacob's ladder (or, a rail gun for electrical arcs). Apparently, such a device creates wideband interference that screws with nearly anything electronic within a given range. It didn't do much more than twenty feet away, but people getting near it couldn't take pictures, the audio on their video was messed up (because the arcing overloaded the microphone in the camera) and all kinds of problems happened with computers nearby. That thing was fun to look at, but definitely messed up stuff nearby.

But the Jacobs Ladder wasn't the cause of why the PA system wasn't working for periods of time. Turns out one of the displays at the fair demonstrated that adults can't hear above 18khz or so. The display had created small tone generators that emitted 16 khz, 17khz, 18khz, 19, 20, 21, etc so that by pressing successive buttons a person could tell at what frequency their own hearing dropped off.

Quite a clever display, the device was very cheap to make, using a 9V battery, a 555 timer chip, a couple little resistors, and a piezo speaker. All in all about $5 for each frequency. All it would take is one tone, such as the 20khz tone to fill the room with so much inaudible high frequency sound that the microphones would be overloaded and not register other sounds. The troubleshooting problem was that nobody except dogs and microphones could hear it.

Eventually, it was discovered that when someone would press a button on the display, the PA system would fuzz and be unable to work. Effectively, the entire room became a cone of silence to the recording devices. Quite an unintended effect for such a clever dog whistle made from such inexpensive parts.

Nobody could hear the tone in the room, so it was quite the mystery.

TRY IT FOR YOURSELF: There's a computer program that can generate high frequency tones up to 20 khz. Set up a microphone for yourself and see if you can overload it with your own computer speakers. Or, try one of your own sound programs and make a tone at or above 19khz to see if you can hear it and if it has the same effect on your own electronics and audio equipment.

There are free recorded tones online. download the wav file, open it in your media player of choice, and set the player to automatically loop the file. Then go get your video camera and see if the microphone picks up the test tone instead of your voice. It's interesting -- you can't hear it, but the microphone can.

20 kz test tone is here. This tone is detectable but not annoying: 19 khz. I can barely make out this tone: 18 khz. I can hear this tone, the 17khz tone.

Apparently, the high frequency tones generate a lot more power and that's how they totally occupy a microphone that is sensitive to the frequencies.

Try all the frequencies to test your own hearing, and to test you microphone's! What fun! Careful not to bug your dog, or anyone else in the room still sensitive to high frequencies, though.