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Old 27th April 2005, 08:58 PM   #1
Otter is offline Otter  Belgium
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Question Adire motion controlled sub problem

I've build a low budget motion controlled subwoofer in the past, now I want to build a high quality motion controlled subwoofer using an Adire DPL12 driver. I already mounted my accelerometer under the dustcap, on the mass loading disc. I verified that the accelerometer was outputting a decent signal, and glued a new dustcap over it (which I got from a warranty issue). I'm ready to go, ain't I?

For reasons I won't explain here, I need to know the frequency response of the driver in a sealed box with a small air volume. Now here comes the problem: the shape of the frequency curve is not as it is supposed to be. Please view the measurement results on the following link:

Frequency response

The boost below 100Hz is normal and is caused by the high Qtc of the small box. But where does 600Hz peak come from? This is a 20dB peak, so it should be quite audible, but it's not. This peak is also visible when the driver is working in free air. (I verified this using a sweep generator and an oscilloscope).

So either there is something resonating on the driver (which should be audible), or there is something wrong with my expensive calibrated accelerometer (which is not likely).

Any suggestions on how I can figure out what the problem is exactly? I'm stuck here...

Thanks in advance,

Wouter Minjauw
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Old 27th April 2005, 09:04 PM   #2
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Is that the resonance of the mass loading disc?
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Old 27th April 2005, 09:08 PM   #3
Otter is offline Otter  Belgium
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It could be the mass loading disc resonance, but this disc is made of what seems to be plastic and paper. This disc is one sixth of an inch thick and is very stiff. It will surely resonate at a certain frequency, but 20dB is a lot I think. Could be though, I don't know :-(
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Old 27th April 2005, 10:14 PM   #4
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It's probably the resonance of the accelerometer's mount. If it was a driver resonance, you would hear it; it's most likely the accelerometer mount causing the problem.

Can you limit the bandwidth to below 200 Hz, using a steep filter? That would take care of the problem. Alternatively, go with a different mounting scheme.

Dan Wiggins
Adire Audio
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Old 28th April 2005, 05:38 PM   #5
Otter is offline Otter  Belgium
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A 200Hz filter can take care of the peak, but the fase response will be so bad that I won't be able to use the signal for a feedback loop.

I can't change the mounting of the accelerometer. The accelerometer is a small device encapsulate in a tiny plastic package. This package is glued to the woofer directly.
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