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Modal Testing Speaker Enclosures
Modal Testing Speaker Enclosures
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Old 15th August 2019, 02:31 AM   #11
George Schmermund is offline George Schmermund  United States
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Join Date: Mar 2011
Modal analysis is usually considered to be a frequency domain method for determining the mode shapes of structures. Time domain modal analysis methods can also be used to evaluate response data from known excitation sources. The data from known inputs can often produce better analysis accuracy.

The time domain method can have advantages over the frequency domain method when applied in the audio world. This is because the time domain doesn't require the full assembly of a loudspeaker enclosure with the drivers installed and driven to produce the modal stimulus. The averaged value from a few simple taps of an instrumented hammer, implemented with the proper hammer tip, can produce all of the necessary frequencies needed to construct a surface plot of a vibrating enclosure panel.

By using the roving hammer method of excitation along with a fixed position accelerometer, simple spreadsheet software can be used for the detection and reduction of unwanted extraneous resonances from an enclosure.
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Old 25th August 2019, 12:27 AM   #12
George Schmermund is offline George Schmermund  United States
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Join Date: Mar 2011
There was a question earlier about the S/N level when the modal hammer and accelerometer were plugged straight into an average DSO with 1 Meg ohm inputs. One photo show the noise floor of the accelerometer when connected straight into ch1 of the hp DSA. The device was left just hanging in the air on its cable. As can be seen the noise floor of the device is below -120 dBVrms out to at least 12.8 kHz.

The screen shots of the LeCroy scope were also taken with the outputs of the modal hammer and the accelerometer connected straight into ch1 and ch2. The signals were produced using the accelerometer attached with microcrystaline wax to the center of a 1 sq. ft. 3/4" MDF in a quasi free-free arrangement with the panel laying on top of a piece of egg-crate foam. The top trace is the output of the hammer's tap and lower trace is the accelerometer's response.

This is a reasonable demonstration that the modal approach to doing entry level speaker enclosure testing can be done without having to over invest in some of the more expensive hardware and instrumentation typically used for these measurements. More hardware and software can be added as the testing advances according to ones needs and desires.

The photo of the flask sitting on the hotplate shows the method I used to put a hot black oxide coating on the accelerometer body. Most of the literature on this method claims that the process of doing the "hot" coating at home should never be attempted unless you're a thrill seeker with a death-wish. I'll allow that there's some merit to that advise.

One of the photos shows the finished accelerometer mounted on the MDF. I'll have to say that I think the coating came out pretty good as far as I can tell using my one remaining eye.
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Old 27th August 2019, 05:16 AM   #13
George Schmermund is offline George Schmermund  United States
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Join Date: Mar 2011
After actually surviving the hot black oxide coating experiment with the accelerometer bodies, I decides to take it up a notch and try coating one of the modal hammer heads. The jeweler's mallets that I'm using to make the instrumented hammers with comes with a nickel plating for the metal finish. I thought it would be interesting to see how it would look with the black finish. This would match the accelerometers and would lend it the gravitas of the Bruel and Kjaer 8207 instrument. I originally thought that I might have to offer some level of apology for the wooden handle on my hammer until I saw the B&K offering. They get $2380.00 for theirs. Mine will be bargain priced if it ever gets to market.

The photos show the evening's results using the new coating "processor" that I pilfered from a kitchen cabinet. It works great and has the capacity to do larger stuff that I have plans for further down the line.
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Old 2nd September 2019, 08:45 PM   #14
George Schmermund is offline George Schmermund  United States
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Join Date: Mar 2011
I thought that it might be interesting to extend the concept of time domain modal analysis using a simple Excel plot of a speaker panel's fundamental resonance. The plot shows what a set of 24 stimulus/response points might look like using the roving hammer technique. Of course this is only a first approximation model of what the actual test outcome would be, but it may still be instructive for anyone following this thread. There are no metrics applied to this plot other than a scale with no specific dimension.


Well, it looks as though I'll have to turn this plot file into an image file so that it can be uploaded. I'll work on that.

Click the image to open in full size.

Last edited by George Schmermund; 2nd September 2019 at 08:53 PM.
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