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Accelerometers to measure panel vibrations?
Accelerometers to measure panel vibrations?
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Old 21st January 2021, 02:36 AM   #471
Remlab is offline Remlab  United States
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Accelerometers to measure panel vibrations?
And another to use as a reference. This time, once again, It's Atkinson's CSD panel measurement of the same speaker..
Click the image to open in full size.
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Old 21st January 2021, 02:55 AM   #472
Pano is offline Pano  United States
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Accelerometers to measure panel vibrations?
OK, from plot 2B in post 469, how do we know what is the panel noise and what is left over from the drivers that the null didn't kill?
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Old 21st January 2021, 03:35 AM   #473
Remlab is offline Remlab  United States
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Accelerometers to measure panel vibrations?
The first question I have to ask, just to make sure, you do understand what's going on in graph "1", correct?
(I just corrected this. I had "A" instead of "1". Sorry.)
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Old 21st January 2021, 12:52 PM   #474
Remlab is offline Remlab  United States
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Accelerometers to measure panel vibrations?
Quote:
Originally Posted by Pano View Post
OK, from plot 2B in post 469, how do we know what is the panel noise and what is left over from the drivers that the null didn't kill?
As an example, 1db of differentiation between the speakers at a given frequency in graph "1" will add 1db to the "null method" panel measurement in graph "2" at that same frequency, which is so small compared to the output of the panel, that it's pretty much irrelevant.
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Old 21st January 2021, 01:23 PM   #475
john k... is offline john k...  United States
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That is not correct. For example if one speaker radiates 100dB and the other 101 db but out of phase, the summed signal would be 81.7dB. Let's call it 82dB. Now if the measure the side panel using the null method and got 85dB, assuming the sound from the speaker was in phase with the panel, the panel would be radiating at 74.3 dB.
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Old 21st January 2021, 01:49 PM   #476
Remlab is offline Remlab  United States
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Accelerometers to measure panel vibrations?
John
I trust your judgement on things like this as much as anyone, but just to make sure.. You are fully aware of how the null method is performed, correct? You've made no mention of it until now. If you do understand how it's performed(And I have to assume you do), then based on what you said, I stand corrected.
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Old 21st January 2021, 02:32 PM   #477
Remlab is offline Remlab  United States
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Accelerometers to measure panel vibrations?
Quote:
Originally Posted by john k... View Post
That is not correct. For example if one speaker radiates 100dB and the other 101 db but out of phase, the summed signal would be 81.7dB. Let's call it 82dB. Now if the measure the side panel using the null method and got 85dB, assuming the sound from the speaker was in phase with the panel, the panel would be radiating at 74.3 dB.
Just so I can get this straight.. Let's take 750hz, where not only the loudspeakers match "perfectly", but the panels have the highest amplitude resonance. what is your take on that? The relative comparison will help me understand what your explaining in the above statement.
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Old 21st January 2021, 02:42 PM   #478
Pano is offline Pano  United States
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Accelerometers to measure panel vibrations?
Hey Remlab. What I see in your latest measurements is this:
Illustration 1, post 469 shows measurements of the two drivers on axis at 18". They seem a rather good match and with ideal coupling they should almost cancel out each other.

Illustration 2, post 469 shows A: One speaker on axis at 50". B: Two speakers facing each other in opposite polarity, measured at 50" from the side on the box, the other speaker with blankets over it.

Is that correct?

What I don't know from illustration 2 is the SPl at 50" of the nulled drivers. Is it 10dB lower than a single driver, 20dB lower? We don't know. So what is being measured in plot 2B? Is it the box panels, the residual of the nulled drivers, a combination of both?
The info isn't obvious, if it's there.

It's not going to be easy to measure and separate driver from box, but you might be able to.
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Old 21st January 2021, 03:02 PM   #479
Remlab is offline Remlab  United States
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Accelerometers to measure panel vibrations?
Pano
Yes, that's exactly correct.
How about this idea. I could do the "null", but with several thick blankets over each side panel, and the mic placed nearfield and farfield, on axis at where the baffles meet. It wont be perfect(The lower the frequency, the more transparent the blankets become), but it would get us a little closer to eking out more information about what's going on in the lower frequencies.
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Old 21st January 2021, 03:30 PM   #480
hifijim is offline hifijim  United States
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This may be useful. It is a chart which shows the depth of the null we would expect, based on the difference measured between two signals.

Example of how to use the chart: If two signals differ by 0.25 dB, then if one of the signals is 180 degrees out of phase with the other, the resulting null will be -30.9 dB below the baseline signal. The baseline is the higher of the two.

Example: If the two signals are 3 dB apart, then when one is reversed, the null will be -10.7 dB below the higher signal.
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