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I got inspired to ask by Chris' neck block thread - how exactly does one measure the "damping" property of a slab of wood? Impulse response?

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The member formerly known as erikbojerik....

What Chris said, sort of......

In general there are two methods of measuring damping: by bandwidth and by decrement. They give equivalent information, but one or the other might be easier in given circumstances.

The Simpson's are using a decrement method. Rather than measure the time it takes for the wave to die out, one usually counts the number of wave periods required for the vibration to reach some percentage of the initial amplitude. For this you'd get Homer to use a jackhammer (bury a doughnut in the street), and tune the frequency of it until you get the maximum jiggle. Once the vibration has built up to a nice steady state you'd measure the amplitude (perhaps at his belly button) and then switch the jackhammer off. Measure the time it takes for the amplitude to decrease by 3dB (to 70.7% of the original amplitude). Knowing the jackhammer frequency (you did note that down with your Peterson tuner, right?) you can figure out how many periods of vibration went by and thus get the decrement. I must say that I have not used this system myself (especially not in the experiment as stated) so I'm not a comfortable with it. I can't say how Homer would feel, either.

The bandwidth method is somewhat similar. In this case, you would find the jackhammer frequency that gave the maximum Belly Button Bounce, and note that down as Fp. Then you would find the frequencies above and below that the yielded 70.7% of that maximum amplitude with the same input energy; call those Fh and Fl. The damping is given by Fp/(Fh-Fl), a number called the 'Q value' or 'Quality factor'.

If you have software that can do a Fast Fourier Transform you can get the same information from an impulse test. In this case, you'd stick an acellerometer into Homer's belly button, and get Bart to hit him in the gut. The FFT should have a nice peak at the appropriate frequency, and you can find the peak and the 3dB down points graphically. You do need to have a high enough sample rate to resolve the peak well: it should look like a 'bell curve' in the spectrum plot. Given the low frequency and relatively high damping one would expect from Homer's gut, that might be a problem; it's often an issue when impulse testing wood samples, too.

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Bill Hodge


One does not simply, own enough guitars!

you mean to tell me the simpsons ARE educational!

The Simpsons are absolutely educational! William Irwin edited a book called "The Philosophy of the Simpsons."

My favorite episode is when although tempted Homer does not cheat on Marge.

Also thanks for the excellent and concise definiton of damping. More than anything I think we want to avoid a Q much higher than 1 at any frequency the guitar will be likely to make - that's what makes a "wolf note".

Steve

Steve Brown wrote:
"More than anything I think we want to avoid a Q much higher than 1 at any frequency the guitar will be likely to make - that's what makes a "wolf note"."

Actually, most resonances on a guitar have Q values in the 30-50 range, iirc. There's more to wolf notes than that.

Geeze...after all these years sitting at the feet of the master and trying to absorb all of his knowledge, I'm honored to know that at least I share one thing in common with Alan...we both watch "The Simpsons"!!!

Thanks Alan...the analogy was right on and I actually understand Q values much better now! You may have just found the right common denominator.

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JJ
Napa, CA
http://www.DonohueGuitars.com

Man o' Man. I have enjoyed this even tho I don't understand it. Alan, it is good to know that you have a good sence of humor. I have tears in my eyes from laughing. Thanks.

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RJS

Man - I've been watching The Simpson's all week, and still haven't managed to catch the damping episode...

Thanks Alan!

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The member formerly known as erikbojerik....