[pjchappy]

In a thread in the GR Research forum, there was a discussion about PRs w/ an active.

This sums it up: "As for two actives with one passive, that's a bad idea. There always needs to be twice as much volume displacement from the passive radiators as there is from the active driver."  Later someone said 2 actives w/ only 1 passive (of equivalent size) would be like have a port too small.

Big B, I am technically clueless on audio. . .I was just looking for your input, since the Larger only has 1 passive 15" w/ a 15" and a 12" active.  How does your design differ as to defeat what is said above?  Or is what is said above false? etc. . . .Does slot-loading have anything to do w/ it?

Thanks!

p

[maxwalrath]

I'm also clueless about this stuff, but I think passive radiators can make up some of the difference by having a larger excursion (if that's the right word)...I think on the RM1's I had, the 10" passive would travel a lot further than the 8" woofers would.

Not that I'm at all sure about this.

[covermye]

Big B addressed this quite a while back.  It may have even been on the HD boards before Audio Circle's time.

Basically, he said he has no explanation for why the speakers sound best as allocated ("under-PR'd" compared to most modern-day PR sizing mathematics), but that they definitely sound better as they are.  He had tried some designs following "conventional PR wisdom", and the results were not very impressive, according to him.  

If anybody remembers this discussion, please provide a link.  It was interesting.

[jgubman]

This it?

http://www.harmonicdiscord.com/forums/viewtopic.php?t=6687

and the thread that started it all:
http://www.hometheaterforum.com/htforum/showthread.php?s=&postid=826914&t=4520

[John Casler]

This it?

http://www.harmonicdiscord.com/forums/viewtopic.php?t=6687

and the thread that started it all:
http://www.hometheaterforum.com/htforum/showthread.php?s=&postid=826914&t=4520

Hey Jon,

I almost forgot about those silly postings on HTF.

I guess some would rather take a formula, a slide rule and a calculator to tell them what they're hearing, and others would rather just place a woofer in their room, pull up a chair, and listen.

I have been told for years that with a "physics formula" we can prove that a "bumblebee" cannot fly.  Problem is, some one forgot to tell the Bee

Well in this case, someone forgot to tell Big BEE!! that he can't woof either.  

[pjchappy]

So, I guess there is no technical answer. . .what technically SHOULD be right, didn't SOUND right, is THAT right?  

p

[JohnR]

I think the thread you are looking for is here:

http://www.audiocircle.com/index.php?topic=8300

[pjchappy]

Thanks!

p

[Rory B.]

Perhaps the reason for our not being able to get a straight answer on this issue has something to do with the effects of the unique slot-loading of the passive radiator. That would lead me to believe that the real reason that Mr. Cheney is not being forthcoming with details about this is that it is a novel idea which has not yet been patented, and he doesn't want to see everybody and his brother's little Hi-Fi Wood Shop coming out with a speaker system that uses slot-loaded PR bass loading. I have a feeling that this actually behaves like mass-loading of a transmission line. After all, I would find it hard to believe that this is all designed by trial-and-error.

So my suggestion is for Mr. Cheney to go ahead and get the thing patented. After all, he's been using it for quite a while (since the early 70's, I seem to remember reading). Then we could find out some more about what it actually does.

As for concerns about things like the RM40 and Larger Subwoofer being underported by the PR used, I think that perhaps the passive radiator operates nearer to the limits of the suspension when the speaker is at full honk but it does not run out of excursion. I also hypothesize that a much stiffer PR suspension is used, which introduces a whole set of weird variables to the mix, and also that the slot-loading adds apparent mass to the PR which allows the PR to run with a lot less mass on the diaphragm itself, also making the downfiring passive radiator more viable.

[pjchappy]

It is too late to get it patented.  He has been using this slot-loaded system for a long time now.  

If something is made public or published, one cannot receive a patent on it after 1 year of it being made public or published. . .that is what I remember from my Intellectual Property class in law school.

p

[Rory B.]

The thread about putting glue on the passive radiator (PR) for stiffening of the paper cone makes me think that the PR's interaction with the air in the box is very heavily influenced by the stiffness of the spider, whereas conventional passive radiators are not quite as much influenced by the spider's stiffness. The foam surround also probably contributes some extra stiffness since foam is stiffer than paper. (That means you can't just go out and get a PR from PartsExpress or Stryke - they have to be built to spec.) That leaves me with the question of how the PR system appears to "load" the active driver. Since a stiffer PR requires there to be more air pressure in the box to generate the same type of excursion of the passive, concerns about underporting on VMPS speakers using the down-firing passive radiator aren't justified. I am still left wondering about some things though. Since a stiffer PR can be expected to behave less and less like a port as the suspension becomes stiffer and stiffer, I would guess that the sound, instead of being either like a ported or aperiodic box, is instead somewhere in between. I have to wonder, then, whether the function of the Passive Radiator in VMPS speakers is to control the driver's resonance frequency like a typical bass port or simply to add output as the drivers in the apparent sealed/aperiodic box begin to roll off, lowering the F3 (-3dB) point in the speaker's response. If the PR doesn't need to control the resonant frequency of the drivers because they already see something more like an aperiodic sealed box, that explains why Brian ("Big B") can use multiple types of woofers, likely with very different Thiele-Small parameters, loaded to the same passive radiator, all in a single chamber. This isn't passive radiating as an extension of existing vented box theory, as we see in most designs that sue passive radiators. This is a whole new way of passive-radiator loading!

What does this mean, then, for those who take jabs at the design and bass loading of VMPS speakers because they appear to be underported? They know what they are talking about, but they don't have any idea that what they are talking about isn't the same as what Brian is thinking of in the design of his speakers. This is because until now they have always been exposed to passive radiator theory simply as an extension of bass-reflex port theory. This requires the passive radiator's diaphragm to be most compliant suspension you can get, for the closest approximation of a large, long vent, where the air mass is not limited by any suspension. (NOTE: Compliance of the suspension becomes less of an issue as mass becomes larger and larger because the mass becomes so heavy that the stiffness of the PR's suspension does not affect the diaphragm's motion as much and is simply used to keep the diaphragm centered and moving linearly instead of cantilevering out of control. Perhaps if the stiffness of the PR's suspension remains proportional to the mass of the cone as the mass is increased, then we could expect the stiffness of the suspension to become an issue once again.)  In the case of VMPS implementations of Passive Radiator theory, the passive radiator needs a very stiff suspension, so it no longer behaves the way we would expect if we were trying to use the passive radiator as just a bass port. If I were to experiment with such designs, my first try would be to design a sealed box, and then add a passive radiator with a very stiff suspension that would be tuned to a frequency just below the cutoff frequency of the sealed box. I would need to be able to build my own passive radiators, though. My second try might be to design an aperiodic box but then use a passive radiator of the type VMPS uses (stiff suspension), tuned below the predicted cutoff frequency of the aperiodic box. There wouldn't be any aperiodic vent used.

I still need to do more research on the effects of slot-loading the passive radiator. I have heard things that say that the slot-loading of the passive radiator can have the effect of adding extra apparent mass.

I hope Mr. Cheney will stop in here and see whether I've come close to describing slot-loaded passive radiator theory. After all, he's the only one who knows for sure.

[Rory B.]

Actually, I just read that the passive radiator is designed to be very compliant, not very stiff. That makes me think it's all in the slot-loading.

[Rory B.]

It turns out that, according to the VMPS Europe web site, VMPS already patented the Slot-Loaded Passive Radiator, in 1979. That patent would have expired in 1999 (20 years later). So, does anyone know the patent number?

[Brian Cheney]

Very nice analysis of the function of the PR which actually does all the things mentioned and more.
We have two PR suspensions, one stiff, one compliant.  Moving mass is very low with both but much higher with the stiff suspension.
Since a PR is driven equally over its entire surface by the active woofer's backwave, the diaphragm will move pistonically even if it is not rigid.  However, I recently discovered great advantage to stiffening the paper PR cone with a vitrifying agent (Elmer's carpenter's wood glue).
It is true that our use of the PR is unlike others and underporting is not an issue.
I invented the slot-loaded PR and decided not to patent it, since I would spend my life in court defending against encroachment.  I knew I was on the right track when I saw a Klipsch monitor at the AES in 1984 with a slot-loaded PR.

[Brian Cheney]

Very nice analysis of the function of the PR which actually does all the things mentioned and more.
We have two PR suspensions, one stiff, one compliant.  Moving mass is very low with both but much higher with the stiff suspension.
Since a PR is driven equally over its entire surface by the active woofer's backwave, the diaphragm will move pistonically even if it is not rigid.  However, I recently discovered great advantage to stiffening the paper PR cone with a vitrifying agent (Elmer's carpenter's wood glue).
It is true that our use of the PR is unlike others and underporting is not an issue.
I invented the slot-loaded PR and decided not to patent it, since I would spend my life in court defending against encroachment.  I knew I was on the right track when I saw a Klipsch monitor at the AES in 1984 with a slot-loaded PR.

[Rory B.]

Somehow, though, I still think the slot-loading performs some function other than the filtering of any high frequencies, and the whole floor-boundary thing. I remember hearing that the slot-loading technique mass-loads a woofer somewhat, and thus a tuning that would require a more massive passive radiator can be accomplished with a less massive diaphragm, preventing sagging of the diaphragm. Something tells me that this is not supposed to behave like a pure helmholtz resonator but is only intended to have partial helmholtz-radiator functionality. I am still looking for something that will clue me in as to how the slot loading affects the operation of the passive radiator's diaphragm.

I am also trying to think through how something like this would resolve the different electromechanical parameters of the different woofers used in a single air space into a coherent bass loading scheme. (i.e. graphite woofer and Megawoofer in an RM40 operating in the same air space loaded to the same passive radiator.

[Rory B.]

I think I may have a hypothesis for why a design with very different woofers in the same enclosure (ex. the Larger Sub) with the same passive radiator works. As you already know, the purpose of the Helmholtz resonator in a speaker box is to control the behavior of the woofer's cone near the driver's resonant frequency (Fs). As you go along a driver's impedance curve, you notice that the impedance curve is continuous, and there is a peak at Fs. That tells me that the driver doesn't simply see that the frequency it is trying to reproduce is its own Fs and then start freaking out. Instead, I imagine that the rapid increase of the impedance curve as you get closer and closer to the maximum impedance at the center of the peak is indicative of a gradual loss of control of the driver's suspension over the cone. The Helmholtz radiator is added such that at the enclosure's tuning frequency (affected by the length and diameter of the Helmholtz resonator) the Helmholtz resonator is operating in phase with the woofer's cone and absorbing the acoustic load, damping the cone motion at Fs. Now, my hypothesis is this: The Helmholtz resonator is suitable for a single driver loudspeaker, since it is only effective over a very small bandwidth, which ideally should be centered at the driver's Fs. Now, this slave cannot serve two masters. It will either love one and despise the other, or love the other and despise the one. (Don't you love the Biblical metaphor?) That means that the usable bandwidth of the Helmholtz resonator is so narrow that it can only handle one driver's Fs. If you have two different drivers and one port, one driver will be excursioning wildly while the other is nearly standing still. If the other driver happens to be putting out midrange, the midrange will be hosed by the wild excursions that are causing the mid-woofer's motor to be operating outside its most linear BL range. We could try to solve this one by putting in a second Helmholtz resonator (port) tuned to the other driver's resonant frequency, but then if one woofer is operating at the tuning frequency of the port that is intended for the other woofer, there will be a big peak in the overall system response at the other port's tuning point. Many manufacturers solve this one by isolating the two different woofers each in their own enclosure. With a passive radiator we can control many more parameters of its operation than we can with a tuned port. We can also do things to load the other side of the passive radiator's diaphragm. If we stick a big, round pipe with the same radius or diameter as the passive radiator such that one side of the passive radiator faces the box's interior and the other side faces the pipe, the moving mass of the passive radiator will now appear to be that of the mass loaded on the diaphragm itself plus the mass of the air in thelength of pipe, and the whole system will function as one big passive radiator. That is just one example of what you can do to change the operation of the passive radiator without actually touching it or changing its mass. My hypothesis is that perhaps slot loading of the passive radiator helps it to be effective over a wider range of frequencies than an ordinary mass-loaded PR or a Helmholtz resonator. The result is that if there are two drivers whose resonant frequencies are near each other, both will be damped by the passive radiator. The tuning frequency of the passive radiator should be selected such that it is nearer to the most violent resonance peak. If the two drivers behave differently at their resonance, the passive radiator itself should be tuned nearer to the driver that will have the most violent, uncontrolled diaphragm excursion. If both drivers have the same suspension and moving mass, the tuning frequency of the passive radiator should be directly between the two resonant peaks of the drivers. The bandwidth of the operation of the passive radiator is affected by the dimensions of the slot and the compression chamber between the radiator and the slot opening. That only leaves the question, what happens between the resonant peaks of the two drivers? Will the passive radiator behave badly and cause muddiness of the frequencies in the band between the two resonant peaks, or will it sound very dry and damped? I think that's what you adjust with the mortite putty. (My logic has the potential to fall apart here.) Little variations in the moving mass will affect the way the passive diaphragm allows transients to decay. The result is that the user of the speakers can select between this overdamped or underdamped sound, or hit a sweet spot right in the middle.

Now, that's just my hypothesis of what the slot-loading does. Still, if it's right, it will be pretty cool.

Addendum: Yes, I realize my keyboard has a "tab" key. I just didn't want to derail my train of thought.

[Rory B.]

I read something that kind of freaked me out. In the "Fabulous Free Factory-Authorized Tweak thread, someone noticed that if the cone moved out, the passive radiator would have to move in to compensate, to which Brian (Mr. Cheney?) replied that phase reversals at low frequencies are not as perceptible to the human ear. I certainly hope he's not implying that what comes out of the slot is out of phase with any of the woofers. If what comes out of the slot is out of phase with any of the woofers, there will be very prominent and measurable cancellation because at those frequencies, bass becomes more and more about room pressurization (which is why with bigger rooms you need more woofers but not more tweeters). The idea with any sort of bass-loading is that the cone excursion at the resonant frequency of the drivers is controlled (unless the Qts of the driver is 0.707 or greater which means that the resonant frequency (Fs) of the driver is already damped by the suspension), and that the output of the bass loading device makes up for the reduced output at the enclosure's tuning frequency caused by damping of the driver at that frequency.

(Ooo, I think I get it now...The slot loaded passive radiator as used by VMPS does not control the drivers' own resonant frequency because the drivers have a Qts of 0.707 or thereabouts but instead it reinforces gradually falling-off response at the bottom of the frequency range. Then the adjustable bass damping is accomplished by getting the operating range of the passive radiator to overlap just a little bit with the woofer. What's a mystery to me still is how the woofers interact with the box at frequencies higher than the PR tuning frequency - do they see a sealed or an aperiodic box? I could be completely wrong here.)

[Redbone]

Will the passive radiator behave badly and cause muddiness of the frequencies in the band between the two resonant peaks, or will it sound very dry and damped? I think that's what you adjust with the mortite putty. (My logic has the potential to fall apart here.) Little variations in the moving mass will affect the way the passive diaphragm allows transients to decay. The result is that the user of the speakers can select between this overdamped or underdamped sound, or hit a sweet spot right in the middle.

I agree with this analysis.  I have been experimenting with the 40s and have found some interesting points:

1) The upper mid-woofer, when decoupled from the cabinet (removed) has a much louder, richer and wonderful sound.  It has much better attack and decay characteristics IMHO, as perceived in string plucking and violin sounds.  I really like it that way !

2) The lower mega-woofer, when removed, doesn't sound much different than it does in the cabinet, it is very low volume except at  very low frequencies which are loud.  It only reproduces frequencies below ~60Hz well, it muddies higher frequencies.

3) There is interference between the two drivers when installed in the sealed cabinet, IMHO degrading the sound of the mid-woofer.  Tuning of the PR will both enhance the bass response of the mega-woofer and reduce the degradation of the mid-woofer, but the mid-woofer is still very much damped from its native sound.

I am going to remove the PRs this weekend and perhaps try the Elmer's tweek.  In any case I plan on listening to the speakers for awhile with the PRs removed.

[Rory B.]

That's the big concern with two different types of drivers loaded to the same air space. (It's also the reason why you generally find sealed-back cone midranges instead of open-back except in the crappiest of crappy speakers.) They both have different transient response characteristics, and one will sometimes drive the other as a badly-tuned passive radiator. This is the only part of Brian's design methodology that, I fear, is still stuck back in the 1970s (that is, of course, unless Big B can enlighten us as to how this very outside-the-box bass loading principle actually works in the case of his designs to resolve different types of bass transducers into a coherent bass source). The 10" and 12" woofers in the RM/X are so similar that I don't think it makes too much of a difference in that case. I would expect the QSO 626R, the RM1, the RM2, and the Super Tower III to have the most consistent bass loading in this regard, as well as the other QSO speakers, the Focused Field speakers, the Original Subwoofer, the 215 subwoofer, the Dedicated subwoofer, and the Smaller subwoofer. The similarity between all these speakers is that they have only one type of woofer loaded to the passive radiator or reflex port.

On the other hand, I would expect the RM30, RM40, and RM/X to have some very weird properties to their bass loading, as well as some older SuperTower designs and the Larger Subwoofer. Two 6.5" woofers loaded to two 6.5" passive radiators makes sense, but when you bring the side-firing 10" Megawoofer into the mix, that's when I would expect the weirdnesses to begin, according to conventional speaker design wisdom. Perhaps using a stiffer spider is what makes the passive radiator work over a broader range of frequencies, not the slot loading.

I'm not trying to say in any way that Brian's bass loading designs are crap. Indeed, VMPS bass is famous for its great sound, so Big B is doing something very right, without a doubt. I am just wondering why this technology works as well as it does.

When you listen to the speakers with the passive radiators removed, don't crank it up too much or you could damage something. After all, speakers rely on their bass loading for support (though as I said earlier, the VMPS woofers could be such that they are supported mainly by their suspensions.) The slot may begin to act as a Helmholtz Resonator, and the sound will be boomy and muddy.