I've posted a few times how hard it is to implement commercially, in particular because speakers are complicated in phase and unstable in various ways and generally hard to mass produce MFB.
ironically, the Philips "silk purse from sow's ear" speakers were OK commercially but hardly of interest to this forum crowd. They applied modest feedback to cheap drivers and got pretty good sound, as I understand the results.
But - and this is important - MFB is just peachy for the DIY crowd to implement. No trouble at all keeping your own MFB system working fine.
B.
In-floor subwoofers - 8x8" drivers each, highest WAF factor, lowest THD
I built in-floor sealed subwoofers consisting of 8 x 8" drivers each (Dayton Audio DCS 205-4). Those were the only 8" drivers which I found to fit in the subfloor cavity with a box built around them. Each sub has the cone surface area of 2 x 15" subs and 1 x 15" sub displacement. The drivers were not the reference type but I had to compromise since they were the only ones which would fit. The whole build was designed around the subfloor cavity restriction.
The purpose of the subs was to use them below 40Hz. The box size gives each driver Qtc=0.9, Vb=5 Ltr, , Fb=82Hz .
When the house was still renovated and did not have the drywall installed yet I tested them both with 120Hz x-over at full displacement and they played really loud - I would say around 120dB at 120Hz. The garage floor concrete slab was shaking.
When still a teenager living in my parents home, in what was probably close to an 750 sq. ft. bungalow, I managed to talk my parents into cutting a 3' high x 4' foot wide hole in the living room wall to accommodate a pair of stacked horn loaded 15" Electrovoice drivers. It was located to back into the lower half of my sisters closet with just enough clearance to avoid the closet door from striking the back of the drivers. In retrospect my parents were highly accommodating... as notwithstanding of my sisters considerable objection. In any event, it had problems with standing waves, though with enough power to consider the integrity of the ceiling remaining as such. Mom and dad gave me the impression they were proud.
I later built a pair of 6 x 8" all Philips MFB drivers in two "conventional" looking boxes (below with grille cloth removed). It is considered that a good MFB system (in the absence of a signal being applied) ought to break a hammer if the hammer is applied to the cone.
I haven't tried this though. In any event my preference was to a pair of Philips 12" MFB drivers each in a slightly smaller enclosure. Most striking was hearing what can be described as thundering vibrato of simultaneously occurring lower register organ notes that the multiple 8" drivers couldn't do nearly as well. The MFB subs contain an input parametric equalizer to "mostly" remove the dominant room resonance along. This feeds a multistage low phase shift loop filter. The feedback was sufficient to drive the resonance down to about 10Hz with considerable peaking that warranted some consideration. In reflection of what you are doing I redesigned the back wall of my basement listening room during a reno. It has two integrated floor to ceiling equipment/junk collectors integrated on either side of a central space that was projected somewhat into the living space. Each is about 29" outside, has 3" laminated side walls and 1 1/2" back wall. Each has a pair of rectangular 15" deep spaces at the bottom with 13" high openings to accommodate records or a 12" sub or two. Each space has a volume of about 2.5 cu. ft, hence 5 cu. ft per side (same as yours by the sound of it). The central area between these two verticals has a flat surface built out to allow for a cavity behind the central area as a kind of large volume open baffle air space that could be used to increase the space. Because the space is hidden it has both WAF and MAF (My Approval Factor). There isn't any schedule for completion though.
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Attachments
Following whatever maximal room correction I could achieve acoustically, for my four 12" Rythmik sealed subs (cabinets by https://www.salksound.com/model.php?model=Rythmik 12 Subwoofer), I thought of using four channels in this DAC https://www.oktoresearch.com/dac8pro.htm , with DIRAC Live software to select/design filters to further smooth bass response. My midwoofers play down to 70Hz.
I'm still much the novice at room correction, with no hands on experience. Btw, I've read at audiophilestyle and audioscience that it's possible to use a multichannel DAC's inputs to somehow function as an (multichannel) ADC? Any truth in this? If yes, exactly how it that implemented?
OTOH, this Motu has ADCs. https://motu.com/products/avb/8a
And how would you rate its DAC chips vs. other chips, and by what performance criteria ?
Since the other comments are a bit sarcastic, I'll bite and try to help. Servo subs measure the output of the subwoofer or they measure some characteristic that we can derive the sound pressure from (usually cone acceleration is used). This happens in real-time and thus delay is important, otherwise we can't correct for errors. This is something completely different as room correction as servo subs operates in closed-loop. Room correction is just glorified compensation for room issues.
DAC stands for digital to analog converter. It can not be used as ADC (analog to digital converter). In fact, they are quite opposite devices.
The performance criteria ds23man stated are latency specs that are very important in closed-loop operating systems (such as servo subs). For these systems it is important as it significantly limits the performance we can achieve.
That being said, since room correction doesn't operate in closed-loop it's not relevant. Judging whether the proposed MOTU device is good depends on all your requirements. For simple home hifi it is more than sufficient.
My input was not sarcastic. Its just a fact. Ask sound engineers who sometimes meets good servo sub equipt fronts. You can't make it sound full and the audience says "Where's the bass?" -They just don't have enough distortion. Also look up "brownian noise" that most incompetent people don't even consider.
Cheers!
Cheers!
With PA applications MFB may not be the best idea, as the PA man is used to drive any speaker into limitation. "If the limiter is not on, what is the use of it?" he will ask.
What I have experienced, if you drive a MFB bass far out of the chassis linear limit, power consumption suddenly gets excessive. The correction soon reaches a point where it can not do it's job any more.
In a PA situation this is a very good way to destruct something.
As a frequent life listener, I'm quite often surprised by the sound engineers incompetancy. Probaply you need so many years near stages to qualify for this job, that most of this rare species, finaly at the mixer, don't have a functioning hearing any more. Bass is in most cases just some mashed up movement of air with no real connection to any instrument. Yes, you are right: You can't generate THAT sound with MFB.
What I have experienced, if you drive a MFB bass far out of the chassis linear limit, power consumption suddenly gets excessive. The correction soon reaches a point where it can not do it's job any more.
In a PA situation this is a very good way to destruct something.
As a frequent life listener, I'm quite often surprised by the sound engineers incompetancy. Probaply you need so many years near stages to qualify for this job, that most of this rare species, finaly at the mixer, don't have a functioning hearing any more. Bass is in most cases just some mashed up movement of air with no real connection to any instrument. Yes, you are right: You can't generate THAT sound with MFB.
I've only heard servo subs in a domestic setting. They had more than enough bass output. I know it's not acceptable language on diyaudio but all the usual 'fast clean' words describe them. They integrate smoothly with the main speakers and make it difficult to locate where the subs are.
Shame there seems to be no feasible diy solution for them.
What live sound / PA servo subs are your sound engineers listening to ?
Rob.
Im not an engineer. Ive just talked to quite a lot of them. Im sure that with the right material servos will sound awesome.
Its just very hard when the keys are direct clean outputs from clean sampling of vintage keys without the distortion of a guitar- or Lesley-cabinet.
Guitars are often fully or partly modelled sound without the sound of output tubes and speaker drivers struggling to keep up.
Bass is often direct output, so might as well be synth bass.
I dont know the brands they use, but our new local music venue has supposedly state of the art equipment and despite acoustically designed room it doesnt sound right.
Despite the challenges of servo subs, I will admit that deaf spl-addicted engineers are a bigger problem there too.
Its just very hard when the keys are direct clean outputs from clean sampling of vintage keys without the distortion of a guitar- or Lesley-cabinet.
Guitars are often fully or partly modelled sound without the sound of output tubes and speaker drivers struggling to keep up.
Bass is often direct output, so might as well be synth bass.
I dont know the brands they use, but our new local music venue has supposedly state of the art equipment and despite acoustically designed room it doesnt sound right.
Despite the challenges of servo subs, I will admit that deaf spl-addicted engineers are a bigger problem there too.
Because I lack the design knowledge and woodworking skills to build passive sealed subs or could find any -and because I failed to find any professional or user review expressing any kind of dissatisfaction with Rythmik sealed servo subs, I ordered the Jim Salk versions. Otherwise, I likely would have pursued use of mini-DSP or other proven solutions for integrating the subs with my main speakers, and for minimizing room modes using multiple non-servo subs.
While "all in one" solutions are usually frowned upon by enthusiasts, and by the DIY audio community in particular, Brian Ding (despite his sometimes less than perfectly manicured English) doesn't miss a trick with justifying his specific seroved subwoofer design. Some among other informative links.
https://www.rythmikaudio.com/technology.html https://www.rythmikaudio.com/servosurvey.html https://www.rythmikaudio.com/loop.html
https://www.rythmikaudio.com/memory.html https://www.rythmikaudio.com/servo_dsp.html
As human ear sensitivity to noise and distortion decrease with decreasing frequency, the multichannel DAC for my subs need not be quite as advanced as the two channel DAC I'll choose for my main speakers. Thus, the 8 channel Motu, which happily includes both 8 DAC and 8 ADC channels (Of course, I know the difference between a DAC and an ADC) should be an excellent choice for using both mulitple subs and software (e.g. REW, Audiolense, Camilla DSP, et al) for room correction.