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MFB for ACI SV12 Drivers using Piratelogic Electronics
MFB for ACI SV12 Drivers using Piratelogic Electronics
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Old 15th May 2019, 06:11 PM   #191
rscamp is offline rscamp  Canada
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Quote:
Originally Posted by bolserst View Post
Back to back measurements with and without the MFB path closed as chriscam suggest would be helpful to determine the open-loop transfer function.
I'm having a busy week, but I did a quick grab for the left (no damping material) channel. I included an image below but the REW file was too large to post here. PM me an email address and I can provide a shareable link to the REW file.
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File Type: jpg Left Channel With and Without Feedback 19-05-15.jpg (69.1 KB, 81 views)
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Old 17th May 2019, 03:15 AM   #192
bolserst is offline bolserst  United States
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Thanks for the REW file.
Here are the steps to calculate the OLTF which is needed to determine phase margin.

Step 1: Label the two measurements appropriately and then click on the [All SPL] tab.
Step 2: Click the "Controls" icon to bring up the dialog box to perform arithmatic
Step 3: Load the "MFB=OFF" measurement into slot A, the "MFB=ON" into slot B, select the arithmetic operation "A-B" and then click the [Generate] button
Step 4: Load the "A minus B" result you just generated into slot A, change the arithmetic operation to "A/B" and then click the [Generate] button
Step 5: Switch to the [SPL & Phase] tab, select the Final result generated in Step 4 and adjust the scales appropriately to be able to comfortably view and identify the phase at zero dB crossing with the cursors.

Looks like you could increase loop gain about 5dB and still have an acceptable 30deg phase margin. I will take a look this weekend to see if there is any minor changes in filter components that would improve this phase margin further.
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File Type: png OLTF_Calc_01.png (121.7 KB, 70 views)
File Type: png OLTF_Calc_02.png (145.0 KB, 68 views)
File Type: png OLTF_Calc_04.png (171.5 KB, 66 views)
File Type: png OLTF_Calc_05.png (181.0 KB, 67 views)
File Type: png OLTF_Calc_06.png (161.5 KB, 13 views)
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Old 17th May 2019, 09:21 AM   #193
bentoronto is offline bentoronto  Canada
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MFB for ACI SV12 Drivers using Piratelogic Electronics
Quote:
Originally Posted by rscamp View Post
I don't think an MFB speaker itself is going to act as an absorber... the better MFB control of the driver should make it a better absorber.
I woke up with this very very obvious decision strategy insight: try it.

Put the MFB speaker in one corner and another speaker in another corner. For greatest test sensitivity, put the mic near the centre of the room.

BTW, we've all seen links to YouTube demo's where you're supposed to hear how great a new tweeter is by listening to the demo on your laptop's speaker. But, not the same idiocy recording an MFB demo for YouTube because the benefits should be hearable A/B. (Maybe not the test suggested in this post which requires comparing two freq curves).

B.
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Last edited by bentoronto; 17th May 2019 at 09:27 AM.
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Old 17th May 2019, 09:54 AM   #194
esl 63 is offline esl 63
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I guess bolserst could explain this better than most of us. Regarding MFB as a noise canceler..
The correction signal is not just the same in the two scenarios.
Scratching my head... is it 180deg phase shift between the two?
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Old 17th May 2019, 04:30 PM   #195
rscamp is offline rscamp  Canada
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Quote:
Originally Posted by bentoronto View Post
I woke up with this very very obvious decision strategy insight: try it.

Put the MFB speaker in one corner and another speaker in another corner. For greatest test sensitivity, put the mic near the centre of the room.

BTW, we've all seen links to YouTube demo's where you're supposed to hear how great a new tweeter is by listening to the demo on your laptop's speaker. But, not the same idiocy recording an MFB demo for YouTube because the benefits should be hearable A/B. (Maybe not the test suggested in this post which requires comparing two freq curves).

B.
One thing to consider with this I suppose is that well below the Schroeder frequency most of what we hear is modal. With the speakers in opposing corners, the microphone in the centre would be at the null for the first plan view modals - the lowest frequencies that are well supported by the room. In a way, these are the most interesting frequencies to compare between MFB and passive subwoofers as the MFB benefits would be greatest. If the room is near square in plan view, almost nothing would be heard by the microphone in this position at the first modal frequency.
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Old 17th May 2019, 05:59 PM   #196
bentoronto is offline bentoronto  Canada
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MFB for ACI SV12 Drivers using Piratelogic Electronics
Quote:
Originally Posted by rscamp View Post
One thing to consider with this...
The test(s) are now in good hands.

I'd say to start, you want the test of principle to have every advantage just to see if it works even a little.

Seems a straight-forward FR test, no conceptual choices to speak of. Should take minimum time to do the runs if you have one MFB sub (esp in a corner) and any other sub that plays low (esp in a corner).

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Old 17th May 2019, 07:35 PM   #197
bolserst is offline bolserst  United States
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Quote:
Originally Posted by bentoronto View Post
… you'd think cone motion (in a dedicated absorber or in an MFB sub) would be adequate for absorption purposes with enough loop gain, even if a cone is a poor mic/sensor.
If sensor = mic, then you need a specific level of loop gain(more is not better) to get local cancellation of the pressure.
This is how the Bag End E-trap works, and why its adjustment knobs must be fiddled with while watching REW type sweeps in real time.

If sensor = accelerometer, remember increasing loop gain increases the effective moving mass, easily over 2kg in most MFB systems. The air pressure “error” is just not going to have much effect on the woofer acceleration, hence why I had said minimal interaction. As rscamp mentioned, any interaction there is would be in the direction to enforce the pressure not cancel it.

You might find it interesting that woofers can make reasonably good passive absorbers with appropriate electrical loads added to their terminals. The effective bandwidth is limited though. This was actually what several of the AVVA developers started out researching before switching to an active approach. (example paper attached)
Section 2 of the Rivet Thesis has thorough treatment of this type of absorber.
https://infoscience.epfl.ch/record/2...PFL_TH7166.pdf

Quote:
… a puzzle to me how you'd demonstrate that independently of general MFB action - other than changes to the far-field acoustic trace?
In laboratory type setups, tubes are used with the absorber device at one end and source at the other.
Then you can easily measure and quantify how well the end reflections are absorbed across the bandwidth of interest.

As you suggested, measuring or listening to other sources in a room while an absorber device is added or removed from the room works as well, although the 3 dimensional aspect of modal behavior makes the results less clear unless you have an array of microphones. The research/developers of the AVAA include many room measurements in their papers and a few audio clips(real kick drum and recording of string bass played thru subwoofer) in their presentations. I extracted some charts and sound clips from one of their larger presentations. Pretty amazing what just a few actively controlled 6” drivers can do to damp a 200m^3 unfurnished room. They look insignificant sitting in the corner…only 0.1% wall coverage according to the presentation. Note that besides frequency response/waterfall plots, they also consider RT60 type decay measurements to be a good indication of how much audible improvement will be perceived.

Last edited by bolserst; 17th May 2019 at 07:50 PM.
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Old 18th May 2019, 11:21 AM   #198
esl 63 is offline esl 63
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Microphone based active sound absorber want to neutralize the soundpressure, so when the pressure rises the cone will move inwards trying to keep the pressure at 0.

MFB works the opposite 180deg, when the pressure wave hits the cone MFB will push back and keep the cone stiff.

So they work in opposite I would say.
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