Bolserst:
I see your point as to why rising rate suspensions can be useful in some situations.
Bolserst,
40 years ago, after crushing my left hand in an industrial accident (waiting to happen..), tearing off the two small fingers and half my left thumb, the resulting $12,000 workers compensation "fortune" allowed me to buy many "State of The Art" electronics and loudspeakers.
One of the first purchases was a 12" Gauss foam surround woofer, IIRC it had an Fs below 30Hz, had a dual spider, and could handle 400 Watts "RMS", when the usual "Pro" speaker of the era was around 100.
Being a Pink Floyd fan, as well as other LF delving bands of the era like ELP, Yes, Edgar winter Group, and various short intervals from orchestral and pipe organ records, owning a woofer that could reproduce their low notes with flat response to 30 Hz, a full octave below the usual "Hi FI" bottom end.
The "floppy" heavy, low Fs, 12" Gauss delivered exactly what was expected, "flat" response to a very low frequency (for the time) but at a huge sensitivity reduction, the usual PA drivers were 10 dB louder, using EQ (loudness control) they killed the 12" Gauss in pretty much every respect other than "flat response".
Art
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I suppose this is for a standard DVC woofer having the two coils very similar.
Does a specialised coil as used in the Ryhtmik drivers deliver a cleaner signal ?
Apropos the valuable data bolserst provided in post #209, what is amazing is how close VC feedback is to bridge feedback which, in turn, is likely to be very close to simple series resistor sensing.... and maybe accelerometer sensing.
Somehow the benefits of collecting and using MF with a sub is getting lost in a forest of largely immaterial shortcomings. So lets keep things in perspective.
In the temperature error chart in post #209, we see some deviation between the two methods, esp at extra-low freq. But like virtually all the hand-wringing about temperatures, it just isn't a problem except for unrealistic lab experiments.
Coil inductance issues? No problem in the sub band. And what if it is still too imperfect for you taste, what are the consequences?
Which leads finally to this point: once you have the feedback signal, you can manage to address many of the sensor shortcomings easily. And mostly pretty satisfactorily with ordinary analog EQ. As far as the substantial shortcomings of the ancient Rice-Kellogg driver, the MF data is as meaningful and suitable as anyone could want, even if not tracking the driver's systematic/bias perfectly.
Want acceleration data and all you got is velocity: one capacitor, or vice versa.
Inductance rising: one capacitor and a resistor.
But correcting for using a bass reflex box.... I don't know how,
Ben
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Easy:
Karl Erik Stahl
Synthesis of Loudspeaker Mechanical Parameters by Electrical Means: A new Method for Controlling Low-Frequency Loudspeaker Behaviour
A method for extending bass response and lowering distortion in louspeaker systems, which differs from both equalized and servo or feedback systems, is described. It can be seen as an extension of the technique to increase loudspeaker damping by giving the driving amplifier a negative output resistance. However, this method also controls the moving mass and compliance. Very good results, such as flat frequency response down to 20 Hz and excellent distortion data, have been achieved with moderately sized bass-reflex enclosure
AES E-Library Synthesis of Loudspeaker Mechanical Parameters by Electrical Means: A new Method for Controlling Low-Frequency Loudspeaker Behaviour
Yes, but apparently not that easy to understand, as he's been told this several times now, most recently being about 17 hours ago.
This was covered multiple times in the other MFB thread.
You are correct that MFB in the traditional sense (forcing cone motion to match input signal) is not well suited for vented boxes. But, this is not what Rythmik and Ace-Bass are doing. They are using feedback from the woofer to manipulate the T/S parameters to achieve more extended and better damped response for a given woofer and box size. The feedback networks need to be optimized for the desired woofer/box combination. It is not a one-size fits most situation. Perhaps it would be better for you to think of it as an adaptive EQ that is self-correcting for VC temp and T/S parameter drift that just happens to use feedback from the woofer. - http://www.diyaudio.com/forums/subwoofers/296342-diy-servo-sub-kit-6.html#post4835459
Unfortunately there will be no end to this - either he is being willfully negligent by refusing to accept the proof that's been provided several times or he simply can't understand it.
Refusal to accept the data is not immaterial. You've made several errors already that have been pointed out.
If this is all so easy why haven't you done it yet? You've been talking about this for EIGHT YEARS now.
On this you refuse to accept data as well. YOUR antique ancient drivers have some substantial shortcomings, but if you move into products made in this century (or even the last half of the last century) you would find that these products have improved substantially.
Just like a lot of other things. If you judged all cars based on the Model T you might say they have some pretty substantial shortcomings. But if you look at modern state of the art products it's impossible to deny that a lot of those shortcomings have been addressed.
I could show you Klippel reports of a modern driver that are picture perfect out to about 13 mm at Klippel verified acceptable levels of distortion that are on the borderline of being audible but I doubt you would look at it or accept the data so there's not much point.
No, no...
It's easy to understand that the characteristics of the box have to be corrected to provide a (almost?) linear overall characteristic.
On the other hand a part of the control of the MF loop is compromized by that correction... since the MF loop does not control the acoustic effect of the BR port.
But I guess the end result could well be better than without any MF system...
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Ben,
Although you presently don't know how to correct the phase response of nearly any driver/box combination (not just bass reflex) using FIR filters, it may be possible for you to learn how, and apply them to your system.
Since you already own the hardware, learning how will only cost you some time.
Cheers,
Art
Come now, few of us have affordable access to AES reports, even 1978.
Most of the academic world is miffed at publishers who are gouging the public esp for research (maybe not this paper) already paid for by public funds. Shame on AES.
Glad to read posts from esgigt who seems to have a grown-up grasp of the issues, as contrasted with some others. And to also mention the in-depth posts of forr. And bolserst, of course. And others too.
Ben
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Ignoring weltersys patronizing remarks, I think I have a better understanding of the bass reflex question.
Josh Ricci and almost every authority doubt the applicability of cone-motion MF to bass reflex boxes while bolserst, Sonce, and some obviously self-interested manufacturers do not.
The problem is simple: if MF provides a signal that regularizes* cone motion, then it would counter-act the cone motions of BR boxes which are meant to sum with port emanations to yield a regular total output. The cones in BR boxes are not designed to have motions proportional to the input signal. Therefore MF would correct that wrong.
But after MF is applied to an existing BR system, do you now have a smart or electronic driver in the wrong box, as I think, bolserst would say? Does that e-driver behave according to the driver model long in use and sufficiently close to that model to use it in a BR suspension? After all, in a feedback system, the feedback is "the model" and determines the behaviour of the system.
Ben
Ben
*I use "regular" or "preferred" instead of flat here
It's not patronizing to say you don't know something if you specifically say you don't know it and keep demonstrating over and over that you don't know it.
Your exact quote was this - "But correcting for using a bass reflex box.... I don't know how,"
And of course this post reiterates the fact that you don't know how it works despite being told the same thing 3 times now.
Bolserst posted this yesterday, I just posted it again today and here it is for the third time. It would be in your best interest to read it, it answers all your questions.
This was covered multiple times in the other MFB thread.
You are correct that MFB in the traditional sense (forcing cone motion to match input signal) is not well suited for vented boxes. But, this is not what Rythmik and Ace-Bass are doing. They are using feedback from the woofer to manipulate the T/S parameters to achieve more extended and better damped response for a given woofer and box size. The feedback networks need to be optimized for the desired woofer/box combination. It is not a one-size fits most situation. Perhaps it would be better for you to think of it as an adaptive EQ that is self-correcting for VC temp and T/S parameter drift that just happens to use feedback from the woofer. - DIY servo sub KIT?
Read that simple paragraph and understand it before you build up the audacity again to say that others don't have a grown up grasp of the issues.
Your exact quote was this - "But correcting for using a bass reflex box.... I don't know how,"
And of course this post reiterates the fact that you don't know how it works despite being told the same thing 3 times now.
Bolserst posted this yesterday, I just posted it again today and here it is for the third time. It would be in your best interest to read it, it answers all your questions.
This was covered multiple times in the other MFB thread.
You are correct that MFB in the traditional sense (forcing cone motion to match input signal) is not well suited for vented boxes. But, this is not what Rythmik and Ace-Bass are doing. They are using feedback from the woofer to manipulate the T/S parameters to achieve more extended and better damped response for a given woofer and box size. The feedback networks need to be optimized for the desired woofer/box combination. It is not a one-size fits most situation. Perhaps it would be better for you to think of it as an adaptive EQ that is self-correcting for VC temp and T/S parameter drift that just happens to use feedback from the woofer. - DIY servo sub KIT?
Read that simple paragraph and understand it before you build up the audacity again to say that others don't have a grown up grasp of the issues.
The problem is simple: you don't know how Stahl's method is working. So, you can't say it will not work.
Pay 33 dollars and read that damn AES paper! You will discover a whole new universe.
You may doubt, but Stahl's technology was implemented long time ago in a successful commercial product Audio Pro B2-50 subwoofer. It was reviewed in Hi-Fi Choice magazine (got "Recommended") and also won gold medal at 1979 CES show.
Some measurements here:
http://user.faktiskt.se/RogerGustavsson/Audio Pro/Audio Pro B2-50 sid 4.jpg
Not bad for a subwoofer with two 6.5 "woofers".
Fairly new model is Audio Pro ACE-bass 2 Subwoofer:
http://www.audiopro.com/sites/audiopro.com/files/expired_product_folders/en_ace-bass2.pdf
However, I agree with just a guy - modern big woofers in a proper box are good enough even without MF.
You can get a downloadable PDF of the entire AES Volume 2 Anthology(which includes the Stahl paper) for a mere $40.
Table of contents attached…lotsa interesting papers.
https://customer258769455.portal.me...tegoryID=dfb59c4a-0066-c63d-c975-0b3b93bf27d5
The attached Stahl patent has nearly as good descriptions and practical examples as the AES paper.
Look at the circuits and read the descriptions. You will see that they are not deriving or using any signal that is proportional to cone velocity or cone acceleration. So, you don’t need to worry about the feedback controlling the cone motion in an inappropriate manner in a ported enclosure. Rather, current feedback(negative and positive) is used to modify the output impedance(complex impedance not just resistance) of the amplifier. This complex impedance combines with the mechanical impedance defined by the woofer physical properties to make the (woofer + amplifier) system act like a normal amplifier driving a woofer with different T/S parameters(mass, compliance, & damping). It is a clever idea.
Again, it is not MFB in the sense that you are thinking of where the motion of the cone is forced to match an input signal. I think this is the distinction you are getting hung up on. Feedback from the woofer can be used for purposes other than to directly control its motion. Any distortion reduction that occurs with the Stahl technique is not from MFB controlling the cone motion, but from MFB linearizing the large-signal T/S parameters.
If the Stahl technique is of interest to you, there have been several simple analog circuits posted where you can independently tweak or dial in the desired mass, compliance and damping using this technique. A fascinating illustration of the validity of the combined electro-mechanical model of a loudspeaker and enclosure.
Just noticed I missed your response from a few days ago….
Are you being serious? or joking. I can’t tell sometimes.
(ie how can you have erroneous velocity but non-erroneous acceleration)
They are both zero.
Hi Y'all,
I'm pretty sure bentoronto is aware of ACE Bass and Stahl.
Also, there are a bunch of very good threads on that subject here at diyAudio. In the Loudspeaker forum do an advanced search for, e.g.: 'Keyword(s): ACE Bass' and 'User Name: bjorno'. There are discussions, builds, circuits, Eagle files, and so on...
And thanks for the additional links in Post #313/314.
Regards,
I'm pretty sure bentoronto is aware of ACE Bass and Stahl.
Also, there are a bunch of very good threads on that subject here at diyAudio. In the Loudspeaker forum do an advanced search for, e.g.: 'Keyword(s): ACE Bass' and 'User Name: bjorno'. There are discussions, builds, circuits, Eagle files, and so on...
And thanks for the additional links in Post #313/314.
Regards,
Since the Rythmik sensor coil is longer than the driving coil, it should provide a lower distortion MFB signal than just using a normal DVC. However since it is wound on the same former as the drive coil, it is still subject to the same problems of mutual coupling with the drive coil that a DVC is. Based on a Rythmik post on another forum, I would say the special coil used doesn’t provide any increase in useable signal range above 100Hz than what I showed in Post#209.
“…I guess you meant mutual inductance. Earlier approach from other people had tried to compensate for such effect. However, we just leave it in the system (by first minimizing the amount of mutual inductance using shorting rings etc which is good for sound quality anyway) and use that as the natural roll-off at the upper end extension of F/R. So far that that upper extension has been around 100hz for 12"…Those tried to solve it with compensation would only look good on paper and then later becomes a source of long term stability problem”
Pros and Cons of Servo Subwwofers
Wreckingball just purchased a Rythmik kit.
Perhaps we can talk him into measuring the output of the sensor coil before completing assembly of the system.
http://www.diyaudio.com/forums/subwoofers/296342-diy-servo-sub-kit-5.html#post4831819
Is this the B&K coil sensor you are talking about? Very unique flat coil in a separate long magnetic gap.
Have you ever seen measurements of its linearity?
http://www.diyaudio.com/forums/subw...trolled-woofer-controller-20.html#post3557261
Thanks for Stahl's old patent.
1. Um, seems to be using the plain simple series resistor for feedback. Works as well as anything needs to and that's why the majority* of MF systems in the universe uses it.
2. Is he using bass reflex box for his FR and distortion tests? If so, you can see how MF is pushing the driver into a little boost right at the resonant point where a BR box routinely brakes the driver. Sure you can hook MF up with your BR box at home but it will make the FR go funny (while reducing "group delay" and distortion).
3. As per the form of discourse bolserst offers, when you look at the driver in terms of T/S analogies, MF makes them change. But now we can see from Stahl's data, that they move around, perhaps in odd ways. Well, that's feedback, folks. It also suggests that the usual way of looking at drivers as God-given static T/S parameters isn't meaningful here.
Ben
*a rough Fermi-type estimate
Just read the Stahl document (1978), for which I thank bolserst.
In short my understanding, Stahl uses current-sensing to "correct" the bass response, and passive correction to extend bass. And yes, these kind of architectures are effective to some degree in improving bass-response and distortion. You're also correct stating it's not MFB..
But I think stating "You will discover a whole new universe." is at least a bit misleading. In my opinion the Stahl method does not go the way MFB goes.
Just noticed I missed your response from a few days ago….
Are you being serious? or joking. I can’t tell sometimes.
(ie how can you have erroneous velocity but non-erroneous acceleration)
They are both zero.
You are wrong on both counts: (1) I only joke when I think nobody will miss the joke* and (2) what I said works fine.
I was quoting your thought experiment with a voice could epoxied in place. My sentence means:
1. there is no velocity in this case and so YES there is corrective velocity feedback (which is just as it correctly should be in this odd case with the cone stuck); because the cone is stuck, all motion (really none) is erroneous vis a vis the amp signal
2. there is no acceleration but in this case, there is NO acceleration feedback (again, which is just as it should be for the glued cone). There is no acceleration, which is correct in a manner of speaking.
Do you agree that I have your experiment characterized correctly even if the words are warped a bit?
As I've said before, it really doesn't matter if you talk in term of acceleration, velocity, or wandering-about T/S parameters, all can be addressed by circuitry based on a simple series resistor sensor.
Ben
*almost nobody
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The voltage across the series resistor that you refer to as current sensing, varies according to back-EMF (which reflects velocity in a magnetic field, I think) and (almost) nothing else, absenting the amp signal of course.
That is motional feedback.
(Oooops, you've discovered the secret sin of velocity MF: needs to be reconfigured into correct acceleration. Takes just capacitor to integrate (or is it differentiate?). The velocity feedback corrects a Rice-Kellogg drivers too well by successfully making it a constant velocity device. Otherwise, the bass droops. Not a power issue, takes no more power than any other boost routine to get flat output.)
I hope those lurking in this thread will consider this and my previous post as general orientation to MF and get going with their experiments.
B.
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