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Electronic Design

What's All This Resonance Stuff, Anyhow?

I was walking down a hallway on the top floor of NSC's building D, when something I heard made me suspicious. It sounded like a series of tiny clicks rattling around the hallway. I stopped and slapped my thigh to make a sharp sound. I heard TICK-TICK, tick-tick, tick-tick. I was surprised, because there was no obvious reason why this hallway should do this. It even had a carpeted floor. I tried the slap in several other hallways—and in tile bathrooms—and got almost none of this effect. What's going on?

I decided to tape the effect, and if I could see the timing of the clicks on my scope, that might be a clue. After all, the hallway goes 270 feet from one end of the building to the other. I'm standing near the middle when I get the best reflections. But there are other frequencies and time relationships in there to study. I'll let you know what I figure out.

I was lying on my bed a while ago, humming, when I heard a strange humming below me. I tuned my hum up and down—there it was at 336 Hz. The mattress was resonating. The funny thing is that this is not just a simple resonance or simple forced damped harmonic motion. When I stop my burst of humming, the amplitude then grows for another half-second, and then it slacks, and then it grows even bigger. Let's see if I can tape this. I may need a limiter amplifier so the recorder isn't fooled by its AVC loops and the initial hum doesn't drown out the rest. See the oscilloscope photo for the results of my taping experiment.

Nope, sorry. Even with the preamp, I couldn't get a clean recording. So the scope photo shown is just a (poorly) simulated waveform.

I've been on the road for quite a few days this fall, and every time I come to a new hotel, I hum at the mattress. I hear a little resonance, but not nearly so much. I haven't got any good calibration for all those frequencies. I'm not going to say that my mattress at home is all worn out or is of lousy construction. I'm just saying that if I had to invent a circuit or structure to make the resonance act like that, I would be puzzled how to do it. I only hear one frequency, not two.

When I go hiking up in the Marin Headlands, or Fort Funston, there are several large concrete tunnels left over from World War II. They're perhaps 20 feet high and 200 feet long. My sons early on figured they could hum and set up some great resonances. I don't know what the Q is, but—several seconds. A Q of hundreds... Of course, they called this "Tunnel Hum." It must drive the other tourists and hikers crazy. We can't resist it!

Many barbershop quartets like to rehearse and record in a concrete stairwell, at least for some songs. They really seem to like that resonance. With only four people in the stairwell, they don't damp out the resonance as they would if there were 40 people. But stairwells seem to provide better results even than a tile bathroom.

I go to church at Grace Cathedral in San Francisco. This is, structurally, a great beautiful concrete barn, and it has a seven-second resonance. This means the organist has to wait after a loud chord and wait a couple seconds more than usual before playing the next chord. When they get the church more than half full, this cuts down to perhaps four seconds. Brass players specifically have to wait for the bright tones to die out. It makes a difference how you play!

What resonant frequency and Q do you get out of your mattress? I gotta slide over to my wife's side and see if its frequency is any different. That might explain why the energy comes up gradually. But the Q has to be well over 100.

Comments invited! [email protected] —or:
Mail Stop D2597A, National Semiconductor
P.O. Box 58090, Santa Clara, CA 95052-8090

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