solderdude
Grand Contributor
It's somewhat similar to room correction (in FR only) up to 1kHz or so (Philips MFB didn't work as high) but on top of that you reduce distortion in the lows (like seen in NC headphones)
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Motional Feedback (MFB)?
- Thread starter DanielT
- Start date
DS23MAN
Active Member
Has nothing to do with room correction.
Some reading:
Some reading:
OP
DanielT
Major Contributor
- Thread Starter
- #23
Thanks to everyone who engaged in this thread for interesting posts. 
No matter how interesting this MFB technology is, I wonder. Modern subwoofer elements, which in themselves have low distortion. Large subwoofer elements with low distortion and substantial ability to pump air. It combined with powerful Class D amplifiers, which are many subwoofer amplifiers W for little money. Then measure with a microphone and set the EQ. With this, does not MFB then become unnecessary? Or what do you think?
No matter how interesting this MFB technology is, I wonder. Modern subwoofer elements, which in themselves have low distortion. Large subwoofer elements with low distortion and substantial ability to pump air. It combined with powerful Class D amplifiers, which are many subwoofer amplifiers W for little money. Then measure with a microphone and set the EQ. With this, does not MFB then become unnecessary? Or what do you think?
The main question is probably audibility. Those large subs have relatively low distortion, but in absolute terms it still not very low. However sub frequency distortion is not very audible. But fact is that MFB gives you much better control of cone movement, so will objectively beat the same system with only EQ. Would be fun to do a comparison!
I’d also like to see differences in waterfall plots. Seems to me things like decay should also improve. Obviously decay products that emerge from the enclosure will not be improved.
I think MFB is provably unnecessary, because many (most) low distortion and high output subwoofers on the market do not have servo mechanisms in them, and yet they still have state of the art performance. A servo mechanism wouldn't prevent me from buying a subwoofer if it had excellent measured performance, and a servo isn't a prerequisite for choosing a sub either. One of my subs is a servo Velodyne, and while it is has good performance for a sealed sub, non-servo sealed subs are available in the same size range that have as good or better measured performance.
I think the idea that it’s only useful for subwoofer is just wrong. There is far more use in smaller drivers with small enclosures.
OP
DanielT
Major Contributor
- Thread Starter
- #27
But Velodyne is a well recognized good brand? I suspect the subwoofer designers have their specifications, goals they want to achieve. Then they start from those that are available to achieve the goals. MFB may or may not be part of the solution.
Of course also related to the extra cost of adding MFB
Well, that is debatable, specifically for a subwoofer. You can use a significantly cheaper driver if you use MFB. You don’t need a massive motor. Just lots of excursion and a stiff cone. Add a bit of cheap class-D power and you can have a state of art performing subwoofer.
OP
DanielT
Major Contributor
- Thread Starter
- #29
Regarding Subwoofer. The laws of physics are not so easy to get around. Good subwoffer = large.
Large subwoofer element / drivers. Ability to pump a lot of air, large x-max, a lot of power. . That is, large.
Low distortion of course, even if we are not so sensitive to it in the subwoofer area compared to higher up in frequency.
DS23MAN
Active Member
One misconception about distortion in the low end is that it should be not audible. But it is, not in the low register but the harmonics clutter the low-mid range. Most of the recording pro's who use the Grimm LS1 in their recording studio traded in the old non mfb subs for the DMF sub. And not only the Pro's....
DS23MAN
Active Member
3 way with DMF with a loopgain more than 30db:
![_DSC2664[1].JPG](https://www.audiosciencereview.com/forum/index.php?attachments/_dsc2664-1-jpg.164365/ "_DSC2664[1].JPG")
solderdude
Grand Contributor
No it doesn't.
My remark was that MFB flattens the FR (below say 1kHz) and extends it which is also what one does with DSP EQ/room correction but MFB also lowers distortion in the lower frequencies.
DS23MAN
Active Member
If you play a single note at 50Hz it is very hard to hear a difference between 1 or 10% distortion several papers state. The main problem is that we never listen to a single note. The distortion of a 50Hz note with 10% distortion in real music will mask the content from 100Hz and up.
But Velodyne is a well recognized good brand? I suspect the subwoofer designers have their specifications, goals they want to achieve. Then they start from those that are available to achieve the goals. MFB may or may not be part of the solution.
Of course also related to the extra cost of adding MFB
Velodyne has been sold off to a German company, Audio Reference, which was Velodyne Acoustics German distributor. Their latest product line, called Deep Blue, is not a servo design. They still sell the old Digital Plus line of servo subwoofers, but they are >10 year-old technology, and IMO not competitive with the latest offerings from other companies. In the US, finding parts or service for the DD Plus series is difficult. I ignore the fact that the brand still exists.
KSTR
Major Contributor
No matter if with MFB or not, at some point you're always exceeding excursion limits at signal peaks and hence you need a limiting scheme. This happens already in the driver when BL drops and suspension stiffness increases at some point.
With MFB, you also need some sort of limiting to prevent forcing the driver to do things it simply can't. The best scheme is to limit excursion in the input signal to the power stage to begin with. For this, one needs to transform the signal into the true excursion equivalent, then soft-clip it, convert back to input signal domain. All this only in the relevant LF region, so a split-band approach. Not exactly easy to implement (DSP helps some, though).
By this, only the LF content is occasionally excursion-clipped and the promised driver distortion compensation by MFB is not degraded at lower momentary excursions. Of course the sensor must be linear to start with and have a higher absolute excursion limit than the driver itself.
As simple and elegant as MFB is in principle, in an actual implementation the devil is in the details.
As for using cheapish drivers and fix them with MFB, I'd say this can be rewarding but it's not a given. Cone breakup and suspension woes are your worst enemy for MFB. But you can get away with lesser motor designs (except that you need to put a good sensor somewhere).
Nevertheless MFB, like any other form of feedback, is best used on already quite linear drivers. Let's say a Purifi woofer, with MFB you could tickle the last bit of LF precision out of that already excellent drivers at low and medium levels. At (overly) high levels, it will (and must, as explained) break into lots of distortion quite abruptly once you max out the momentary distortion. Thermal limits need handling here as well, btw...
With MFB, you also need some sort of limiting to prevent forcing the driver to do things it simply can't. The best scheme is to limit excursion in the input signal to the power stage to begin with. For this, one needs to transform the signal into the true excursion equivalent, then soft-clip it, convert back to input signal domain. All this only in the relevant LF region, so a split-band approach. Not exactly easy to implement (DSP helps some, though).
By this, only the LF content is occasionally excursion-clipped and the promised driver distortion compensation by MFB is not degraded at lower momentary excursions. Of course the sensor must be linear to start with and have a higher absolute excursion limit than the driver itself.
As simple and elegant as MFB is in principle, in an actual implementation the devil is in the details.
As for using cheapish drivers and fix them with MFB, I'd say this can be rewarding but it's not a given. Cone breakup and suspension woes are your worst enemy for MFB. But you can get away with lesser motor designs (except that you need to put a good sensor somewhere).
Nevertheless MFB, like any other form of feedback, is best used on already quite linear drivers. Let's say a Purifi woofer, with MFB you could tickle the last bit of LF precision out of that already excellent drivers at low and medium levels. At (overly) high levels, it will (and must, as explained) break into lots of distortion quite abruptly once you max out the momentary distortion. Thermal limits need handling here as well, btw...
DS23MAN
Active Member
A feedback loop will keep the excursion in control until the amp starts clipping. We use the clip output of the Hypex amps to control a limiter in the dsp control loop. Problem solved.
OP
DanielT
Major Contributor
- Thread Starter
- #38
Yamaha uses a servo system on their YST Subwoofers.
I bought an old one this spring. Have it under the bed. Not a giant subwoofer but suitable for my bedroom.
Edit:
Or wait a minute, Yamaha writes negative outpot impedance amplifier. Maybe it's not MFB?
Hm, negative outpot impedance amplifier... wasn't that the technology Audio Pro used and called ACE-Bass? I think it was the engineer / designer Karl-Erik Ståhls at Audio Pro who received a patent for that technology. If I remember correctly.
Edit 2
Apparently.
Inventor Karl-Erik Ståhl
The engineer Karl-Erik Ståhl is behind many of Audio Pro’s patented ingenious innovations, most notably ACE-Bass, which allows a deep bass from small subwoofers.
Attached image,Karl-Erik Ståhl
www.audiopro.com
I bought an old one this spring. Have it under the bed. Not a giant subwoofer but suitable for my bedroom.
Edit:
Or wait a minute, Yamaha writes negative outpot impedance amplifier. Maybe it's not MFB?
Hm, negative outpot impedance amplifier... wasn't that the technology Audio Pro used and called ACE-Bass? I think it was the engineer / designer Karl-Erik Ståhls at Audio Pro who received a patent for that technology. If I remember correctly.
Edit 2
Apparently.
Inventor Karl-Erik Ståhl
The engineer Karl-Erik Ståhl is behind many of Audio Pro’s patented ingenious innovations, most notably ACE-Bass, which allows a deep bass from small subwoofers.
Attached image,Karl-Erik Ståhl
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KSTR
Major Contributor
As long as the amp power fits for the driver used and starts to clip before it fries the driver this is a completely fine solution, I whole-heartedly do agree. A well balanced compromise.
solderdude
Grand Contributor
When a pure 50Hz sine has inaudible harmonics (10%, mainly 2nd and 3rd ?) at 100Hz and up how can those (inaudible ?) harmonics then mask harmonics of the instrument itself that can sometimes even exceed the amplitude of the fundamental ?
Or are you talking about equal loudness contour effects combined ?
Maybe resonances are a bigger issue in muddying the sound ?
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