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Old 29th May 2007, 01:55 AM   #901
ScottG is offline ScottG  United States
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Originally posted by Lynn Olson

I was thinking of a sealed box, actually, since the feedback servo takes care of most of the distortion. The primary benefit of vented is substantially reduced excursion (and distortion) close to the vent frequency. With servo, that benefit mostly disappears, leaving increased SPL as the remaining asset - but with the drawbacks of vent coloration and a 4th-order highpass characteristic.

In a servo setup, what are the sealed-box DISadvantages? Curious to know.


It is interesting that Rythmik is addressing the same concerns that I'm looking at, just in a different frequency band. It does make you think about custom drivers for the Bass array in the OB, although that's a project for another day.

Ah - but the servo does NOT take care of most of the distortion - at least not around resonance. Think of it more as:

1. A very good motor design with triple shorting rings that lower distortion above resonance. The Dayton HiFi sub drivers from PE are largely comparable here.

2. Unlike the Dayton drivers - transient capability for an underhung, long gap high excursion design with a lossy surround, is radically improved *except* near resonance (..and even there its improved somewhat).

Basically the servo is increasing driver "control" both in the mass controlled and compliant controlled region. Its more effective in the mass controlled region (no surprise there), but while being less controlled at the lower end in the compliance region - a servo is perhaps better here than most designs. At resonance though, (and near resonance - which is "q" dependent), its still not very good.


Any given design shifts driver resonance up depending on driver param's and cabinet size.

The first question then is:

Where is the resonance in relation to the operating passband?

If possible we do NOT want this in the range of fundamentals. IF thats not possible we would rather have:

1. a narrow "q" design (usually a low qts value), and/or
2. a mechanical limiter.

Because we are operating the sub at higher freq.s - pushing the resonance up beyond our crossover isn't really an option (..and even if it was compliance related distortion is still a problem even with a servo - and then there is the excursion vs. spl problem and a distinct potential to "bottom-out" the driver).

None of the drivers are particularly narrow "q" designs - the lowest being the DS12CV with a qts of .327. So we move to a mechanical limiter.

Now the servo IS a mechanical limiter.. to an extent. I liken its capability to that of a "fine adjustment" control. What we need then is a "coarse adjustment" control - one that is specifically designed to keep the driver "in the gap" at and near resonance (..which is hard to do electrically). Of course a ported design provides this "coarse adjustment" control.

A ported design mechanical dampens and limits a driver's operation ( least where the port air-load is mass controlled - i.e. above port resonance). It is however crude and "lossy" when compared to a servo (hence the "coarse adjustment" designation).

Now to get "maximum" value from the port design it needs to be "tuned" at *least* an octave away from the in-box driver resonance, and preferably further than that. (..again, its basically a extending mass controlled character to lower freq.s.) If its to close to the driver's resonance then you are potentially compounding the error by adding poor driver control to poor port control.

Of course at, (and particularly just below *port* resonance), the port is horrible. Linear and non-linear distortion are very poor in this region. The best thing you can do here is move it out of your fundamental range, (and even as far out into the harmonic range as is feasible). (Understanding of course that this puts additional excursion & thermal strain on the driver.. the lower the port tunning freq. is.) Another reason for a lower port tunning freq. is group delay, push it low enough in freq., (half octave + below any fundamental), and it won't be an audible problem even if at resonance its hovering a bit above 50 mil. sec.s. More importantly, combined phase has a nice gradual rotation in such a design (in the region above resonance).

A few more notes on Ported designs:

1. There is an obvious constraint on port tunning and port length - especially when factoring in a larger port "area". Unfortunately..

2. The port diameter should be large to keep the "slug of air" in the port and limit its excursion. (..otherwise you'll suffer from excursion limiting that will limit output at higher spl's.) Don't think that multiple ports vs. one large port are comparable, they aren't. While multiple ports have greater surface friction and reduce air speed and ultimately port turbulence, they do not decrease air excursion to the same extent (..and of the two decreasing air excursion is more important for most sub designs).

3. A good port design is preferable to a PR.. Linear decay behavior above resonance is usually a LOT better - in fact it may be better than the driver anywhere near this freq.., at least for the first few mil. sec.s (..which seem to have the largest effect on perceived clarity/transparency).
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Old 29th May 2007, 06:01 AM   #902
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Need to chew on this for awhile.

Feedback systems are nothing more than input/output comparators. The Rhythmik is a feedback system that compares the input voltage of the amplifier with some sort of analog of driver acceleration (which directly corresponds to sound level). Since their sensor is not an accelerometer, there must be a conversion from sensor output (velocity?) to acceleration - I'm guessing there's a differentiator in there, as well as FR shaping to the desired contour.

The feedback summing node itself has no intelligence, and automatically compares input voltage to a phase and frequency-shaped feedback signal. Rhythmik doesn't disclose the feedback ratio itself, but it can't be too much (10~20 dB?), since the system is limited by stability criteria - this would be most troublesome at the top end of driver response.

I can see how the system operates in a closed box, since there's a 1:1 relation between driver acceleration and total sound output. Put in the shaping network to simulate the accelerometer and the desired frequency response, and boom, feedback will accomodate us. Not perfectly, but if there's 20 dB of excess gain (probably not that much) we'll see a 10:1 improvement in distortion.

Things are straightforward with a closed box, since the sensor is measuring real sound output, and correcting for distortion, level, and FR shaping, just like it would in an amplifier. Just like an amplifier, the degree of correction is limited by system-stability considerations - and just like an amplifier, full-power oscillation would probably destroy the speaker driver, so the design must be conservative.

Where it gets kinky is a servo-feedback vented system. At the vent frequency, most - 80% or more - of the sound is coming from the vent, not the driver. The Rhythmik system has no way of directly sensing the vent output - there's no mention of a vent microphone, which would be the only method that would work.

So it has to fake it, with a network in the feedback loop that simulates the vent, probably by peaking the feedback signal where the driver sensor output starts to drop thanks to the box frequency restricting driver motion. So the feedback network needs to be critically tuned to the Q and Fb of the vent/box system, otherwise system stability will be degraded due to excessive phase shift.

It bothers me that the servo-feedback vented system is not actually directly sensing the total output level, unlike the closed-box system. The proportion of system output that is coming from the vent is simulated, not measured directly.

This indirect approach seems to defeat much of the benefit of feedback. The whole point of feedback is accurate comparison of input and total system output, yet there's no way to sense the 80% or more of sound output coming from the vent (at the box frequency). It can't detect vent turbulence, it can't detect vent rectification effects, and the exact ratio of driver/vent output is only a mathematical approximation, not a direct measurement at the sensor. As we know, small-signal behaviour is not the same as large-signal behaviour, with the driver Qms, Qes, and compliance shifting fairly dramatically with high-level drive - the notorious in-and-out driver "breathing" we see with pulsed high-level drive being only one example.

The whole servo-feedback vented-box system just strikes me as kind of squirrely, while the action of the closed-box system seems simple and direct, since the sensor is directly sensing the total output waveform. With a vented system, it's only guessing what's coming out of the vent.

P.S. I am complete agreement about passive radiators, which seems the worst of all worlds - a vent with suspension nonlinearities thrown in for free. The finite compliance - the source of the nonlinearities - also adds a zero in the response around 1 Hz, which really gums up the transient response in a system that's already a 4th-order highpass filter.
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Old 29th May 2007, 09:47 AM   #903
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Originally posted by Lynn Olson

Within this range I'll be using separate amplification and equalizing as necessary, measuring on a flat boundary surface (the rear wall of the room, which I used before to confirm room energy spectra). There will be three sets of amplifiers: my Class A1/A2 Karna triode amplifiers, powering the WR and HF drivers; a Class AB or D amplifier for the Midbass and Bass drivers; and another Class AB or D amplifier for the stereo subwoofers. I'll use a Rane or DBX parametric equalizer for the MB, B, and SW amplifiers, and use passive crossover EQ (if necessary) for the mids and highs.
I hesitated quite a bit before posting this (it can be misinterpreted in so many ways and I don't know what it will achieve), but here it comes.

I fail to grasp what exactly are you aiming this project at in terms of "targeted audience". By its current looks -- large OB (ala "airplane rudder") with four large(ish) drivers + CD/ribbon _and_ subwoofer requiring tri-amping this project seems -- in my ignorant oppinion -- a bit steep even for the (semi)ambitious DIY-ers.

That you are not aiming having this an "easy", ready-to-build pseudo-kit / speaker plans -- that's fine and dandy. If OTOH this is closer to the other end of the grey scale of build difficulity/acceptance/applicability i.e. only intended as an exercise/Proof of Concept in "all-out dipole speaker" that will be attempted by a handful of ppl (at most) then I don't understand the point of having an "open discussion" in a thread of nearing 1000 posts...

Again, pls don't take this the wrong way, it's sincere sign of confusion. It's a very interesting and instructive discussion to follow, but the end goal (as per above) not very clear (to me at least)...

Best regards,

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Old 29th May 2007, 01:08 PM   #904
MBK is offline MBK  Singapore
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I didn't mean to criticize or pontificate, just to share the results of many a trial and error I made, especially error . I am still uncovering the many inane oversights I was and still am making, especially where measurements are concerned.

Your configuration has some elements I have also thought of or am still planning. I ended up not going the double mid route because I thought it was just too hairy to measure and optimize, however, I still plan on adding two sealed subs below my mains, as you are planning with the double 15", because I have just enough space for two 10" which I have already lying around. And as I said I found that the midbass driver shouldn't do lower bass duty, so all in all quite similar thinking here.
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Old 29th May 2007, 04:53 PM   #905
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Originally posted by Lynn Olson
It bothers me that the servo-feedback vented system is not actually directly sensing the total output level, unlike the closed-box system. The proportion of system output that is coming from the vent is simulated, not measured directly.
Nice thread. The directservo configuration uses a combination of sensing feedback and current feedback. The purpose of the current feedback is to prevent local high Q peaks that can lead to instability at extreme operating conditions. It also provides a means for graceful degradation (the accelerometer based design does not have this, which is covered by our patents). In the vented box, it turns out the same concept can be used to flatten the output around the tuning frequency. For simulation, one can take a regular driver, keep all other parameters the same except with the Re value reduced to 1/3 to 1/4 of its original value, update the Qes and Qts values, and plot the FR. You can see it produces a basically flat frequnecy response with some shelf-up characteristic at mid bass. Our servo network can flat that out too. If the result still have slight peak at the tuning frequency, we uses a 2nd order HP to make it a flat 6th order FR.

Most people have a very narrow interpretation of the servo. In my view, servo is based on the principle of remote sensing. As long as one can insert a 10ohms resistor into the speaker wire and still have the same FR (max output will change though), that is servo. It does not need to have sensing feedback from all radiators. The advantage of this remote sensing configuration is that it increases the cone movement control, together with all other benefits I put on the web site. Reducing the Re to its 1/3 value is like using a magnet 3 times as big.

I once filed a patent application for a servo based on PR with sensing feedback from both active and passive radiators. It is mathematically sound and perfect. But I cannot get it to sound right, which to me is far more important. I think I figured out why it will never sound right. Later in the process the patent examiner incorrectly cited a Japanese patent, I didn't bother to pursue further.

Personally I like "sealed" servo over "vented" servo. I can voice the latter with a couple of different components to sound close to sealed. I am constantly puzzled by this, BTW.


Rythmik Audio
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Old 29th May 2007, 06:53 PM   #906
Variac is offline Variac  Costa Rica
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Nice to get response from Brian- the "horses mouth" on the subs..

I am certainly going to let Lynn reply to your question, but your same thoughts occured to me also. Here are my conclusions:

-It is amazing the amount of good knowledge about open baffles, and drivers and more that has been uncovered here due to our members- including Lynn of course! Although I have been a member about forever I have been incredibly impressed with the quality of comments in this thread, and how they are being brought together in one place.

-This probably is an all-out, proof of concept design, but think of how manufacturers start out with the all-out model, then there are "trickle down" benefits to less ambitious speakers. There are lots of things that he can determine that will help in future designs- possibly by him in the future:

If he determines which drivers are the most appropriate and how many are needed then that can be used for other similar projects. I certainly would benefit from knowing that in his opinion whether it is best to use horn drivers or ribbon drivers? Tone Tubbies, Hemp Acoustics, 18 Sound? which are the Holy Grail? Maybe he will find out..

Some parts of the design might work better than he expects. Maybe the 12" wide range can actually go higher than he thinks, and a smaller and cheaper ribbon or a 1" CD will work fine and maybe be more cohererant. We just don't know yet.

The shape of the baffle and anti-diffraction methods. Wings? curved edge? holes? Enable?

A project could be developed for those that don't need as much maximum volume, possibly with half the drivers. A LOT of this design is extreme to get the huge dynamic range Lynn wants. Possibly this isn't required for those with smaller rooms..

Another example: If Tone Tubbies end up being chosen , then maybe the ferrite driver could be substituted for the Alnico at about half the price. As above, the tweeter might not need to be as expensive, and maybe one 15" can be used in smaller rooms. I see a whole family of speakers coming out of this research., But would be thrilled to have just two designs, a high end and a moderately priced one.

As far as complexity goes, Lynn prefers reasonably simple crossovers, and is choosing drivers partially based on this, so while the speakers will have lots of drivers, I 'm not sure that they will be that complicated to build. It is perhaps apparent complexity that is actually simpler. Again, not arguing with you, you just got me to thinking more, and I am just mentioning the possible benefits that I can think of.

One of the appeals of the Ariel was that the drivers were quite reasonable in price, and a DIYer could get some very nice speakers by a lot of work rather than spending a lot of money. These speakers seem to be almost the opposite- Easier to build but more expensive- possibly $4000 for the drivers/horns.

I do think that this thread has already helped Lynn make them a lot better than they would have been without it though...
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Old 29th May 2007, 08:12 PM   #907
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Originally posted by ScottG
Note: as a result I would choose the 2 12's over the single 15.
All other things being similar i'd always choose 2 drivers over 1 so that one could take advantage of push-push cancellation and the inherent increase in downward dynamic range.

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Old 29th May 2007, 08:13 PM   #908
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First of all, many thanks for your input. Really did help see things in perspective. And I fully agree: As a "brewing pot" for discussing issues dipole speaker design the content here is invaluable.

But then again, there's a whole dedicated forum for this exact purpose, with quite a bit of expertise in this very area...

The problem seems that I kept thinking about this in terms of a single design -- which, again, seems pretty "niched" for several reasons (price, applicability, equipmnt/knowledge reqrd to get this tuned up, etc, etc). OTOH as _platform_ out of which a family of designs could trickle down is a different beast. But yet again, wouldn't each such "family member" be better served by addressing sepparately its own issues i.e. as a specifically tuned speaker, with its own limitations/compromises ?

Anyway, thanks again for the input. Enough OT babbling from me already

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Old 29th May 2007, 09:06 PM   #909
AJinFLA is offline AJinFLA  United States
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Originally posted by rythmikaudio

Personally I like "sealed" servo over "vented" servo.
Rythmik Audio
Me too .
Hope things are going well for you in TX.


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Old 29th May 2007, 09:50 PM   #910
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Default Point of this thread

Florian et al...

This isn't me being preachy or arrogant but sometimes I stick my oar in so here goes.

Surely the point of this thread is to discuss theory and design ideas and for Lynn to get the speakers HE wants, anyone who wants to follow this is welcome but I see this as a side issue.

Personally I'd never build speakers like shinobiwan as i don't like boxes, but damn they're pretty, the construction and finish are faultless, and the discussions always interesting.

just my take.


(Now Planning something big, maybe with the 15" 18 sound drivers and 1m+ DIY ribbons I always promised myself)
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