Panel vibration measurement (accelerometer)

[tiroth]

What is the appropriate measurement range (in G) for measuring the vibration of enclosure panels?

I am thinking it would be very easy to use the output of an analog accelerometer as the "mic" input to Speaker Workshop. Since these accelerometers are not very expensive, I think it would be a great capability to have.

[SY]

First, I note that I just use an uncalibrated piece of PVDF- its low mass means that it won't perturb the measurement much and it has outstanding frequency response.

Second, if you have a feel for the max displacement and frequency, the g force is pretty easy to calculate. I can run through that if you don't know it already.

[tiroth]

Unfortunately, I really have no idea how much the average panel moves in absolute terms, although I have a wealth of data from magazine reviews on the frequencies at which they tend to resonate. If you could just give me some idea, it would be most appreciated. I just want to know if we are talking 0-10G, 0-2G, or something much smaller.

[SY]

If I had to guess (and this is a guess), it would be larger, if anything. Acceleration goes as the square of the frequency.

[tiroth]

I guess, I was thinking that 100G sensors are used on subs that have 1" or so of movement at 100Hz. I guess we need almost the same amount of dynamic range if the wall moves 1 mil at 3kHz. A big "if" thouhg since I have no idea.

I did an experiment last night where I touched the measurement mic to the panel and did a MLS pulse. I found a peak in both the FR and the waterfall that seemed to correspond to the longitudinal resonance of the panel. It was interesting, but I have to assume this is a poor technique. There is no way to remove acoustic contamination, although the nearfield special case certainly applies. Perhaps a reasonable technique if done outdoors? Everything after the initial pulse should be resonance...

[moamps]

Originally posted by tiroth
I am thinking it would be very easy to use the output of an analog accelerometer as the "mic" input to Speaker Workshop. Since these accelerometers are not very expensive, I think it would be a great capability to have. [/B]

Hi,

I'm now working on a couple of experiments with ACH 01 accelerometer. It is also very usable in panel vibration measurements.
You can find some application data at http://www.libinst.com/accel.htm

Regards,
Milan

[catapult]

Someone over on the Mad board was using an accelerometer from Digikey with good results.

http://dkc3.digikey.com/PDF/T051/1460.pdf

I think, but I'm not sure, it was the first one listed, ACH-01-03. The post has dropped out of the archives. Anyway, that looks like a good one with 2 Hz to 20 kHz response and 10mv/G sensitivity which should let you measure down to a small fraction of a G with most mic preamps. The max dynamic range is +/-150 G.

Edit: ah, I see that's the same one moamps is using.

[tiroth]

Great suggestion! Thanks.

It looks easily adaptable to phantom-powered mic input. Even better.

[RHosch]

I suggest an accelerometer somewhere in the 50G range. The goal should obviously be to use the accelerometer to document changes as you try different damping/stiffening techniques, so if you start "out of range" or clipping you know you've improved when you move into linear sensor territory.

I also suggest starting with a simple tap test on the panel and a fourier transform analysis of the resulting impulse response. No need to input any sort of white noise signal with MLS techniques... a true impulse (or close enough for our goals) is easy enough in this case to generate by hand. Just a sharp blow with a hard object, measure, transform, and there is your resonant frequency.

That analysis will help identify appropriate stiffening and damping measures, and will tell you when you've moved the Fn and also when you've reduced its peak amplitude.

If you want to get more serious, I suggest displaying data in a shock spectra format (velocity vs. frequency) wiith G-level isolines running at angles. This type of data presentation is extremely helpful in directing efforts at problem areas.

[cocolino]

originally posted by tiroth
Unfortunately, I really have no idea how much the average panel moves in absolute terms, although I have a wealth of data from magazine reviews on the frequencies at which they tend to resonate. If you could just give me some idea, it would be most appreciated. I just want to know if we are talking 0-10G, 0-2G, or something much smaller.

Below attached a very nice conversion chart of frequency/ acceleration/ velocity/ displacement

scanned from the Brüel & Kjaer book "Piezoelectric Accelerometers and Vibration Preamplifiers"

a note to the graph:
acceleration is not nominated in g but in m/s^2
1g = 9,81 m/s^2

example:
a panel that displace 0,01mm at 1000Hz (I think this would be a more or less realistic value) is accelerated by (about) 400 m/s^2 = 41g