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Old 5th December 2004, 06:45 PM   #1
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Angry realtime excursion calculation

I'm working on a circuit that will attenuate the signal if the sub is reaching its max excursion. Can I use the output from the amp and translate that into excursion (for example: if the output is 65v, then the sub is at max excursion), or do I have to use some kind of a pressure sensor inside the box (sealed) and calculate it from that? Is there any other way to do it?
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Old 5th December 2004, 06:50 PM   #2
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Excursion is frequency dependant, and will have different values with similar voltages at different frequencys. The feedback devices that I'm familiar with use a small accelerometer attached to the cone of the speaker.
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Old 5th December 2004, 07:21 PM   #3
Svante is offline Svante  Sweden
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What type of box is it (closed, bass-reflex...)?
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Old 5th December 2004, 07:33 PM   #4
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Default Re: realtime excursion calculation

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Originally posted by Svante
What type of box is it (closed, bass-reflex...)?
Quote:
Originally posted by xplod1236
...inside the box (sealed)...
closed/sealed...whatever you want to call it
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Old 5th December 2004, 08:22 PM   #5
Svante is offline Svante  Sweden
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OK, so you can estimate the cone amplitude by building a 2nd order lowpass filter with the same Q and f0 as the box (fc and Qtc). If you feed that filter with the music signal, the output from the filter will be directly proportional to the cone amplitude in the low-frequency region. A bass-reflex box is far more complicated.
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Old 6th December 2004, 04:29 AM   #6
Konrad is offline Konrad  Norway
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If max excursion itself is a meashure, two photedetectors can bee used. Bouth of/dark at inner position, and bouth on/light at outer. Simple logic and dampening of signal can bee triggered.
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Old 6th December 2004, 05:23 AM   #7
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Quote:
Originally posted by Svante
OK, so you can estimate the cone amplitude by building a 2nd order lowpass filter with the same Q and f0 as the box (fc and Qtc). If you feed that filter with the music signal, the output from the filter will be directly proportional to the cone amplitude in the low-frequency region. A bass-reflex box is far more complicated.

More complicated? The excursion of a driver in a vented box can be modeled by parking a notch filter after the 2nd order lowpass mentioned above. The fc would be the box resonant frequency, naturally, and I'm guessing Q would be the box alignment Q. Not quite trivial, but not horrendous either.
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Old 6th December 2004, 09:15 PM   #8
Svante is offline Svante  Sweden
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Quote:
Originally posted by DSP_Geek



More complicated? The excursion of a driver in a vented box can be modeled by parking a notch filter after the 2nd order lowpass mentioned above. The fc would be the box resonant frequency, naturally, and I'm guessing Q would be the box alignment Q. Not quite trivial, but not horrendous either.
Ok, so let's take that path too. You are right in that it is doable. The second order filter should now have a Q and F0 corresponding to the driver in an infinite baffle (ie not in the box). The notch filter should have a F0 corresponding to the fh of the box. The depth of the notch is not terribly important, since it corresponds to a low cone amplitude, and we don't need the protection circuitry to be activated here anyway.

I fail to see right now if there is a third parameter, namely the width of the notch, or if that is just a result of the depth of the notch.

As you say, "not quite trivial, but not horrendous either".
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Old 7th December 2004, 08:39 PM   #9
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Default Re: realtime excursion calculation

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Originally posted by xplod1236
I'm working on a circuit that will attenuate the signal if the sub is reaching its max excursion. Can I use the output from the amp and translate that into excursion (for example: if the output is 65v, then the sub is at max excursion), or do I have to use some kind of a pressure sensor inside the box (sealed) and calculate it from that? Is there any other way to do it?
At the same output excursion in a sealed box quadruples (increases 12dB) for each halving of frequency.

Below the pass-band you get to a 12dB/octave roll-off so excursion remains constant for a given input voltage. I don't understand what happens when you get close to DC although with a sub-sonic filter on the input you won't have to worry about it.

Within the sub-woofer's pass-band excursion for a given input voltage is increasing 12dB for each octave lower you drop.

As you go beyond the pass-band output decreases (12dB/octave?), you have an even bigger (24dB/octave?) decrease in excursion, and can assume that with a music signal you're going to be limited by Pmax.


Feed the input signal to a 12dB/octave low pass filter where the low frequency pole matches the sub-woofer's. Additional filters will be needed to adjust for any boost in the plate amp. You probably need to use all-pass filters to get a delay between that and the amp input.

Run the output of this into a peak detector.

Feed that output to the negative input of a differential amplifier with the positive input tied to a higher voltage reference equal to what your VCA expects to provide unity gain.

Feed that op-amp output to a VCA on the sub-amp input.

Attempts to exceed the sub-woofer's mechanical limits will lower the volume.
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Old 9th December 2004, 10:34 PM   #10
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Would it be possible to do this with a microcontroller? I would use a low pass filter with the same q and f0 as the box to estimate the excursion, and I would run that into a peak detector, connected to the micro. The micro would sense that the signal is too high, and attenuate it with a pga2310. That's the simplified version. For the real version, I would need to set up 2 or 3 peak detectors each activating at a different amplitude to figure out how much I need to attenuate the signal. I would also need to add a delay before the amp so that the attenuation can take place before the signal gets to the amp.

Would a microcontroller (pic16f84a @ 20MHz) and a pga2310 be fast enough for this?
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