Re: Benefit of Feedbacked Speakers

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Cay-Uwe
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Re: Benefit of Feedbacked Speakers

Bericht door Cay-Uwe »

Unfortunately I do not find a section in this forum for general discussion and that is why I placed this thread here. If for any reason it is not welcomed, the administrators may place it in another section or even delete it.

In the mean time Hans was so nice to open a new section for "General Discussions". Thanks !!!

I would like to introduce myself, although some of you may know me already from my MFB586 thread at MFBfreaks.com.

My name is Cay-Uwe, I live in Germany and since over 30 years I have dealed with loudspeaker design, a passion initially as DIY and since a few years by running a little loudspeaker manufacturing company, called Sonus Natura. Since last year I have started evaluating feedbacked speakers and already run some test.

In this thread I would like to give a brief explanation about how feedbacked speakers work and the benefit one gets of them, even knowing that many may be aware about it.

Feedback is something we experience every day, sometimes we even do not notice it. I think a good example is while one is driving on a road having a speed limit of 100 km/h. When you enter the road you notice that you are to slow, because you look at the tachometer and as an result of it you accelerate. While doing so you continue watching the speedometer and once you notice that you are too fast you stop accelerating and the car slows down. Again if you see that the car is getting too slow you accelerate again to maintain the 100 km/h and so on.

http://www.cay-uwe.de/FB4.jpg

Actually what you are doing is applying feedback or regulating the speed according to your speedo.

Looking a bit more in detail to it, you are applying what is called a negative feedback, meaning that once you see you are to fast you slow down, once you recognize you are to slow you accelerate. You apply the opposite of what you are detecting, thus negative feedback.

Nothing else is what happens in any type of feedback and like in our example the important thing to know about is what one would like to regulate or feedback.

Coming to speakers there are many things one could think about to be regulated and that can be practically anything causing distortions in a speaker, like resonances, reflections or deformations occurring on the diaphragm, but one of the worst ones is the excursion of the voice coil. Ideally the voice coil of a dynamic speaker should have a linear movement with piston alike characteristics, which means while moving the diaphragm should not suffer on bending, deformations, etc. which is very well approached by hard diaphragm materials like ceramic, aluminum or magnesium. While that can be addressed, a problem still exists and that are irregularities a speaker motor has. The cause of that is its electromagnetic construction having to deal i.e. with magnetic hysteresis, mass of the motor assembly, etc. That factors cause the movement not to be linear, it has also non linear components and that leads to a simplified movement as shown on the following picture.

http://www.cay-uwe.de/FB1.jpg

The red line represents the ideal excursion and the blue one a simplified real excursion. How that affects the reproduction of signals is shown in the following picture.

http://www.cay-uwe.de/FB3.jpg

As one can see applying a signal to the linear excursion function results in an ideal reproduction, but reality is shown in the blue shaded signal, which shows that due to the non linear excursion characteristics of the speaker, the reproduction suffers on distortions.

To correct that in principle one has to do something similar to what we learned with our speed example, we have to check how the signal looks like ( sensor ), compare it with the input signal and regulate it by feedback. Applied to the real reproduction signal it means that if the reproduction gets less one has to increase the output signal or the other way around ( slower then 100 km/h = accelerate, faster the 100km/h = slow down ) and that is done with a simplified circuit shown in following picture.

http://www.cay-uwe.de/FB5.jpg

An essential part of a feedbacked system is the sensor that is used to query the actual status of the magnitude to be regulated, being this the excursion the speaker motor is performing.

There always have been a lot of discussions what the right sensor is to pick up the movement of a speaker and in general there are three types that have been used most:

1.Microphone on the diaphragm ( measures SPL = proportional to acceleration )
2.Inductive or capacitive sensors ( measure speed of the diaphragm )
3.or accelerometers ( measures acceleration = proportional to SPL )

It sounds logical that a microphone is the ideal sensor since theoretically it gives also information about other effects that may come up. Anyhow since it also picks up surrounding sounds that are not related to the original signal that will affect the accuracy of the sensored signal. Since a microphone delivers acceleration information, one will need to determine from the sensor signal the position of the coil for feedback purposes.

Inductive sensors are fairly common and some examples where they are used are speakers of Backes & Müller, a company with a long tradition using them, but also newer companies like Schanks Audio ( a good friend of mine ), AGM or Silbersand owned by F. Müller who was co-founder of Backes & Müller. F. Müller is technically far in the front when coming to this type of sensors since he even mounts them on tweeters. The advantage of this type of sensor is the relatively simple circuit needed for the feedback and the good SN ratio. On the other hand to build an inductive sensor is not easy and needs a lot of know-how making it difficult to implement and it also suffers in the maximal excursion it can handle since the overall construction of the speaker will define the limits. Also twin coil speakers can be seen as having inductive sensors since the second coil delivers in a specific frequency range similar capabilities as inductive sensors.
Capacitive sensors have been used by Backes & Müller in the past for their midrange speakers and tweeters. Inductive and capacitive sensors deliver velocity information about the diaphragm and proper calculation is required to get the excursion position for feedback purposes.

Another way of getting excursion position information is by using accelerometers which is done i.e. in Philips MFB speakers. The most popular technology used are piezo elements that are mounted either close to the voice coil or on the diaphragm to pick up the movement as acceleration and by proper conversion one will get position information for creating a feedback. A big advantage of these type of sensors is that they do not suffer in excursion limits as inductive sensors, since they travel with the voice coil or diaphragm. Because of that they pick up any movement up to its limits. They are also quiet simple to implement. Anyhow piezo elements and as well newer MEMS ( Micro electro mechanical systems ) suffer on bad SN ratio especially at low levels and special care must be taken to make the signal usage valuable.

When implementing any of the described methods one will need to ensure that the movement of the diaphragm is as good as possible and close to the ideal piston movement. The reason for that is that all described sensors will only deliver information about the position or excursion of the voice coil. Any other failure a speaker may do should be optimized to avoid further distortions since the sensors cannot pick up them properly. As a rule of thumb one can say that paper cones will work ideally up to about 300 – 700Hz, while some stiffer materials as magnesium or aluminum cones show effects of resonances at much higher levels, often above 2kHz.

Personally I have had already good experiences with twin coil woofers and depending on their quality I measured distortion improvements of around 30 – 50% for better woofers in the bass reproduction and even more the 80% for lower quality speakers. Here is one example of distortion improvements for one and the same woofer having the same frequency response, once without feedback and then with feedback.

http://www.cay-uwe.de/TwinCoilEFSKlirrmitohneFB.jpg

The upper measurement shows the distortion without feedback and the lower with feedback. Especially below 60Hz one will see that for the harmonic distortion K2 it is up to 30% less, but more important the non harmonic distortions K3, which relate to non linear excursion, it is lowered by up to 50%.

That is the benefit one gets from a feedbacked speaker, it suffers less in distortion due to excursion irregularities. That will result in a „cleaner“ sound and the possibility to drive the speaker to higher levels as without feedback. This should not be mixed up with hearing loud, it is more a matter of covering higher dynamic ranges with lower distortions.

Right now I own a pair of Philips MFB 586, which I have optimized in the bass and crossover area and in the next days I will run some similar measurements as I did with the twin coil woofer to see what improvement one can expect from such a feedback system.

A detailed explanation of what I have been doing can be read in MFBfreaks.com:

http://www.mfbfreaks.com/forum/viewtopic.php?f=2&t=7346

I will keep you updated.
Laatst gewijzigd door Cay-Uwe op do 09 jul, 2015 15:38 pm, 6 keer totaal gewijzigd.
Happy listening, Cay-Uwe
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Cay-Uwe
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Re: Benefit of Feedbacked Speakers

Bericht door Cay-Uwe »

Hans,

thank you very much for creating a new section and you also may use Dutch manes for it. I do not want to be the cause that suddenly section have English and Dutch names.

General Discussions = Algemeen Discussies ( trying a bit on Dutch :mrgreen: )
Happy listening, Cay-Uwe
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Re: Benefit of Feedbacked Speakers

Bericht door Sandrowski »

Personally i don't mind this section being in english. This forum might draw some international attention and by using English, it will be more accessible to non-Dutch speakers.

Your explanation of MFB in it's various forms is one of the best i've seen so far on the internet. I might try to play around with inductive feedback some time in the future. Good results seem possible with this technique and i have a nice dual-coil woofer lying around (Visaton GF200) as the victim of experimentations. Some day...

Greetings,
Sander
Groet,
Sander
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Re: Benefit of Feedbacked Speakers

Bericht door Cay-Uwe »

Sander,

thanks very much for your compliments which a very much welcomed ;)

I have to say that I am a very pragmatical, sometimes also a bit lazy, guy and anything that can make my live easier I will take advantage of. That is why I spend a longer time experimenting with dual coil woofers. The results are very good and simple to achieve and most important when designing feedback loops it is absolutely stable.

The only draw back is that the second coil after a specific frequency works as a transformer and does not pick up valuable information for feedback purposes, but for others. Usually this frequency is something around 2x fc of the build in speaker in a closed enclosure. In other words, if your fc = 80Hz expect this to work fine up to about 160Hz ( rule of thumb ;) ).

Actually this is an interesting work covering the basics: http://www-personal.umich.edu/~hpeng/IMechE2000.pdf
Happy listening, Cay-Uwe
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Re: Benefit of Feedbacked Speakers

Bericht door Karsten »

Hi Cay-Uwe;

Very clear story! ;)
The systems by Backes & Müller I’ve read about in Stereoplay. I’ve been a long-time reader of that magazine because they take measurements, listen and try to find correlations between the two; That's very interresting for technical guy like me. Lots of English magazines talk about “flow”, “tempo” and other vague terms; Not my cup of tea. :mrgreen:

Not too long ago there was a thread on MFBfreaks about an MFB-system from a Begian firm that (I think) were using a pressure sensor inside the box in the early seventies. That’s probably the microphone-method you were talking about. :?
Addition: Found the topic: http://www.mfbfreaks.com/forum/viewtopi ... 129#p60783

The dual-coil method is used quiet a lot in modern-age (sealed acoustic) subwoofers like the ones by TMA. Apparently a feedback system in my car-subwoofer (Infinity BassLink T) is also based on the dual-voicecoil-technique. I don’t quiet understand how that can function in combination with the two passive membranes that are used. At their resonance frequency the excursion of the active woofer is reduced strongly. If the feedback system corrects for that, it will result in a large peak in the acoustic output. The only way (that I can think of) to correct for that is by means of some kind of forward correction. :?
What are your thoughts on that?

Greetings;


Karsten
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Re: Benefit of Feedbacked Speakers

Bericht door Cay-Uwe »

Karsten,

there is a company in Germany that builds subwoofers ( A. C. T. ) and holds as far as I know a patent for feedback of vented boxes. They use a microphon on the diaphragm of the speaker.

Because you like Stereoplay, here is a review of one of their subwoofers ( sealed speaker ): http://www.benedictus.de/produkte/al4x4sp.pdf

I have a woofer with an inductive sensor and one of the reasons to have it was also to test on vented speakers, but as usual time flies and up to now I was not able to test anything ;)

It was my intention already a few months ago to check how that may work, because basically it is just a matter of the feedback loop design. Anyhow that is easy said ...
Happy listening, Cay-Uwe
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Re: Benefit of Feedbacked Speakers

Bericht door Sandrowski »

Cay-Uwe schreef:here is a review of one of their subwoofers
That is a large subwoofer... :shock:

I'm suprised that it works actually, the microphone seems to be in strange position to pick up the sound from the woofer. Do they use it as a sound pickup or is it the motion of the entire capsule that generates the microphone signal.

The SA-S1 from good old Sony also uses MFB in a bass-reflex enclosure. I've misplaced the service manual for this, but from what i can remember they used a dual coil woofer feeding a remarkably simple feedback loop.
Groet,
Sander
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Re: Benefit of Feedbacked Speakers

Bericht door Karsten »

Cay-Uwe schreef:Because you like Stereoplay, here is a review of one of their subwoofers ( sealed speaker ): http://www.benedictus.de/produkte/al4x4sp.pdf
Thanks! :D
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Re: Benefit of Feedbacked Speakers

Bericht door Cay-Uwe »

@ Karsten,

I have had a look to the Korn-Macway Thread you have linked. What is used there is actually a positive feedback by putting a shunt in series to the speaker. That will typically increase the SPL at areas where the impedance is high, i.e. resonance frequency of the build in bass driver. An advantage of this method is that it compensates thermal effects of the coil. Here an there it also may reduce the effect of resonances as long as they are visible also on the impedance of the driver.

Some German manufactures I know like to use it for the tweeter. Choosing the right tweeter one can get an simple way to linearize the frequency response.

To be honest I do not think that the Infinity subwoofer has any motional feedback build in. At least I do not find it described in the specs.

@ Sander,

A.C.T uses microphones for feedback and they are mounted 90° to the excursion direction to avoid motion interference.

Would be interesting to see what Sony has done in their SA-S1 ;)
Happy listening, Cay-Uwe
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Re: Benefit of Feedbacked Speakers

Bericht door motoindo »

Wie op 5 mei toevallig in de buurt is van Keulen : Cay zijn Sonus Natura Audire staat dan klaar voor een demo.
Chris Camphuisen
mfb does to the ear what a pretty face does to the eye
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