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Enclosure vibration measurements
Enclosure vibration measurements
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Old 14th January 2019, 10:16 PM   #31
George Schmermund is offline George Schmermund  United States
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I thought that it might be interesting to see what applying a simple bandpass filter would look like on a hammer tap using the same set up as described in post #28. These taps were done on the undamped panel. There was no averaging nor any attempt to reproduce the previous tests.


These 2 DSO images only show different taps with and w/o the filter. Channel 1 is unfiltered so that the force of the taps can be judged to be ~ equal. There was no attempt to be quantitative here, just a filter demo to show how a DSO measuring in the time domain can be used in these tests.
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Old 19th January 2019, 02:56 AM   #32
George Schmermund is offline George Schmermund  United States
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This is a comparison of the 5th generation brass housing accelerometer along side of the previous 12L14 steel housing unit. The mass has dropped from 23 grams to 11 grams. The output signal levels are about the same, but the bandwidth on the new one is better.


The modal hammer is still being refined and everything still needs to be calibrated. When the designs have all pretty much stabilized I'll be able to offer up some real numbers about the capabilities and limitations of these 2 instruments.


Please excuse the focus. This camera seem to have a mind of it's own when it gets near my bourbon.
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Old 20th January 2019, 11:38 PM   #33
George Schmermund is offline George Schmermund  United States
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Now that I'm beginning to look at the modal hammer and accelerometer transducers more seriously it's time to start doing some measurements using charge amplifiers. These 2 DSO images are of the hammer tapping the same MDF panels as before.

The first image is of the hammer's signal going straight into the DSO's channel 2 input. As can be seen (and demonstrated in an earlier post) the half sine wave has been corrupted by the 1 megohm input resistance. The signal's amplitude is also attenuated. The signal has been averaged 4 times.

The second image is of the hammer having been passed through a very high impedance charge amplifier which allows the full signal from the hammer's force transducer to be collected and displayed undistorted by charge leakage. The charge amplifier has a low output impedance and can be adjusted to produce a calibrated force output signal to the DSO. This signal has also been averaged 4 times.
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Old 21st January 2019, 05:03 AM   #34
1audio is offline 1audio  United States
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Enclosure vibration measurements
Is it possible to miniaturze this enough to use on small objects like headphones? Please share some guidelines on building a system.
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Old 23rd January 2019, 12:40 AM   #35
George Schmermund is offline George Schmermund  United States
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I'm pretty sure that most things can be reduced in size by simply making them smaller. I have no plans at the moment to head in that direction. It's an open field, though.
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Old 17th February 2019, 02:35 AM   #36
George Schmermund is offline George Schmermund  United States
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The modal hammer and accelerometer project has been moving forward slowly as I gather more instruments that apply directly to vibration measurements and calibration. The vintage Bruel and Kjaer stuff is really nice to work with and very well thought out for the application. My instrument bench for this project has been slowly growing over the years by patient collecting of B&K sleepers on eBay.


The present calibrating accelerometer is a modified general purpose Endevco device that I've attached a 12L14 steel cap onto with epoxy. This allows my magnetic base accelerometers to piggyback the bottom unit for simultaneous signal comparison.


The mini calibration shaker project is still in the works. It's prototype is designed to produce a 1g acceleration at 120Hz.
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Old 20th February 2019, 02:54 AM   #37
George Schmermund is offline George Schmermund  United States
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As a potentially interesting aside to the hammer/accelerometer project I'll offer up a speaker panel test that can be used as a thought experiment relating to the application of modal analysis.

Since a panel is a simple type of structure you only need to choose how many measurements to make and where to make them. The rule here is that there is no rule about exactly where and how many measurements to use. Good judgement comes with time and some thought. Too many measurements take up unnecessary time and add complexity. Too few can lead to spacial aliasing that leads to inaccurate results.

Wave lengths can be empirically evaluated by changing the the distance between the accelerometer and the hammer tap. When the signal from the fixed accelerometer point has gone through a 360 phase shift from the original tap position the frequency can be determined by simply measuring this distance. Measurement points made at 1/4 or less of this wavelength should give good results.

This is a very simplified concept of how modal analysis can work, but speaker panels aren't really complex structures to evaluate using this type of measurement.
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Old 20th February 2019, 05:56 AM   #38
1audio is offline 1audio  United States
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Enclosure vibration measurements
You have quite an extensive collection of B&K vibration equipment including the big shaker. I'm hoping I can accomplish what I need with the Endevco stuff I have and maybe some eBay accelerometers.
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Old 20th February 2019, 06:04 AM   #39
1audio is offline 1audio  United States
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Enclosure vibration measurements
I have a GR 1557 calibrator somewhere I my stuff. I'll dig it out. Its a pretty clever design.

http://www.ietlabs.com/pdf/Manuals/G...Calibrator.pdf
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Old 21st February 2019, 03:43 AM   #40
George Schmermund is offline George Schmermund  United States
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Thanks for the link to the GR calibrator. I'm a big fan of their instruments and have collected many pieces over the years. Their audio measuring and calibration instruments were second to none in their day as were all of their instruments. The build quality was as good as anything available then and it's still a pleasure to open up the cased and have a look inside.

Then came WW2 and HP pushed GR out of the way for the most part. Post war B&K came along to push HP pretty much to the side in the areas of sound and vibration instrumentation, though HP held their ground in some of those areas that the military wanted to support. Still, B&K is the best of equals when I have my choice for some types of measurements.

So much for reminiscing. The GR 1557 is a nice design and elegant in execution. I'm using a somewhat similar design, but with a compressor amplifier on the pickup coil. There's nothing novel about that. My choice of 120 Hz as the drive frequency is because its twice the line frequency and can be easily had with an unfiltered full wave bridge rectfier. The frequency is stable and constant enough for entry level accelerometer calibration.
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