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In Reply to: 4435/4350 posted by djk on May 12, 2003 at 16:45:50:
Hi Dennis!Sounds like a great solution. I'm fond of a similar system, a three-way using the JBL 2245, 2123 and 2426 or a 2142, 2012 and 2426. Using the 2206 in place of the 2012 or 2123 would be excellent too. There are a few guys running Professional Series three π's, which are a 2206 and a 2426, and when this is combined with the 2245 sub, it's exactly the same system you described. So I'm with you on this one all the way. I like the simplicity of the large-format two-ways, and they're very popular. But splitting the band out to form a three-way or four-way has advantages too.
On the slot loaded subs you're talking about - You got me thinking about something though. I'm very familiar with the JBL and Eminence product offerings, and I like 'em both. I've built lots of systems that have similar drivers paired on the same baffle, but I'm not familiar with slot loaded or isobaric configurations.
It's easy to see that increased diaphragm decreases excursion requirements in a baffle mounted configuration. I would expect the same for the slot loaded configuration you've described. Perhaps it might emulate a stronger motor by improving force to weight. It certainly reduces distortion by reducing excursion requirements, but this improvement is similar to what would be expected by reducing power by 1/2. It's not like a 10dB change, more like a 3dB, and the distortion difference is a small percentage of that.
How does the slot loading mechanism suppress distortion greater than a pair of baffle mounted drivers? Have you seen evidence of this somewhere?
That gets me thinking of another interesting configuration - Isobaric. Seems like when two similar woofers are connected in an isobaric configuration, the diaphragms are coupled so that they act as one with increased mass. They're usually situated "face-to-face" and driven out of phase, electrically so that their cones move in unison, but I suppose they can be installed any way that has a common chamber between the diaphragms that doesn't become pressurized as the cones move. Seems like that one would make deep bass possible in a smaller cabinet, and power handling might be improved a little bit. But I wouldn't expect distortion performance to be considerably better. Do you know if that's the case or not?
I can see how some of these configurations can reduce distortion by virtue of reducing excursion requirements, but I don't expect they would be able to reduce the distortion caused by the motor's magnetic structure. Every reputable source I can think of indicates that low frequency distortion from voice-coil driven woofers is mostly caused by the magnetic structure and not the diaphragm, suspension or even the cabinet.
I suppose this could be largely due to the fact that the magnetic structure is modified by the voice coil's presence when the mechanical structure is in resonance, so maybe that's key here - If the cabinet can damp the diaphragm sufficiently, perhaps this will reduce distortion caused by the magnetic structure. Then again, I've seen data over and over again that shows distortion rising dramatically down low, regardless of the acoustic load. That would seem to indicate that while you can damp or shift the resonant frequency, this doesn't change the distortion performance of the magnetic structure. You see a lot of this in work discussing Faraday rings and the like.
Where I'm going with all of this is that I'd like to know specifically what kinds of distortion improvements you've found with dual woofers in this slot loading configuration. It "squeezes" the pressure built between the two. Have you seen anything that compares it with two woofers connected the same way but baffle mounted? What about the opposite connection, the isobarics? I've always treated dual (baffle mounted) drivers as having double the surface area and double the impedance if connected in series, or half the impedance if connected in parallel. But I've never given any thought to the other configurations, and I'm curious about their performance.
Follow Ups:
The reductions in 2nd harmonic distortion from PP mounting on a flat baffle are well known, the non-linearities are equal and opposite and thus the 2nd harmonic is canceled. Because of the driver spacing it only happens in the bottom octaves. The cheaper the driver, the bigger the improvement. I have measured as much as a 20dB change.By mounting in a plenum several things change. The spacing becomes much tighter so the cancellation will work at higher frequencies, and a cavity is formed.
The cavity does several things, in no particular order:
The size, especially the depth, acts like a low pass filter. This can reduce 3rd harmonic distortion products near the top end of the passband, the effect is small, but audible. The cavity sizes I use have to be crossed in the 150hz~250hz region. It gives a slope of roughly 12dB/oct that must be taken into consideration in the design of the crossover. There is a small peak about two octaves above this point, ~800hz on many that I have done. By varying the Q of the 12dB/oct lowpass filter a LR4 I can get a LR4 transfer function. This filter also reduces the out-of-band peak to -30dB or better. There is also a big reduction in FMD, on the order of 6dB. The air mass at the exit of the plenum seems to act like a point source with no doppler, the cones are moving back-and-forth sideways rather than towards-and-away from you. At first glance this would seem to eliminate the FMD, in practice the reduction is on the order of 4dB~10dB depending on the frequencies.
I mounted a pair of drivers in another cabinet in a face-to-face, push-push mode. The box became very 'thick' sounding and had a broad peak centered at 160hz, just like the EV MTL4. In room the response of the push-pull was 6dB smoother in the 100hz~200hz octave.
Being compared side-by-side with the Klipschorn and Cornwall was interesting. At first the push-pull sounded 'funny'. Couldn't put my finger on it. Went back to the push-push cabinet. The push-push cabinet had that 'hi-fi' sound, similar to the Cornwall, but a bit 'thicker'. Klipschorn, much cleaner sounding. Back to the push-pull.
The reason the push-pull sounded 'funny' was the total lack of distortion. Much less than the Klipschorn.
I have not built a basshorn since.
I still use horns, but only above 150hz.
There are some drivers that are not suitble for home hi-fi, mainly those with small magnet vents that can make 'chuffing' sounds when up close.
If enough interest, I can discuss other practical points related to construction and use.
Hi Dennis!I agree with others on this thread - I'd like to see more discussion of this. Can you show some more detail and maybe link some references?
I can see how second harmonics would be suppressed if they were made to be out of phase. And so I can see some merit in the idea - Sort of like complementary-symmetry amps. But the thing I'm wondering about is how do you get the anomalous harmonics to be 180 o out of phase while having the fundamental in phase. In an amplifier, this is done in the coupling mechanism, where the two out-of-phase signals are combined to be in-phase. It creates a condition for common-mode rejection, where the artifacts cancel out. But in a loudspeaker, no similar mechanism can be employed that doesn't cancel the fundamental. At least, that's where I'm drawing a blank on this one.
Seems to me that if two similar motors are used, the phase relationship between these components would be the same. Even if you drive the two diaphragms in opposite directions, if you house them in a cabinet that combines their pressure waves for summation then the results are the same. And so then when you connect them so that the pressure wave of each creates summing pressure - as opposed to cancelling pressure - then it would seem that the same relationship would be maintained for any anomalous harmonics that were generated.
I mean, I think that if there are non-linearities that are made by the mechanical suspension - something that, in effect, causes a sort of rectification-like property whereby the cone actually goes further in one direction than it does in the other - I could see how this push-pull arrangement might reduce second harmonics. This sort of non-linearity is, after all, what causes second-order artifacts, so you might be able to cancel the effects of high-excursion mechanical suspension non-linearities with this configuration. But the thing I'm not understanding is that I would expect that sort of distortion to be onset at high levels, and not down low.
Distortion caused by the magnetic structure isn't something I would expect to be improved on by this configuration because it seems to include odd-orders harmonics as well as even-orders, and that's a different issue entirely, different causes and so requiring different solutions. But then again, the voice coil energy is polarized the same as the magnet on one half cycle, and opposite on the other, so I suppose that's a condition that tends to increase second-orders as well. I guess I need to do some reading about these kinds of systems, because I'm completely at a loss to this sort of arrangement. It's all speculation on my part.
So I'd appreciate any links you could share that I could peruse. I agree with you 100% on the big basshorn thing. It would be great to be able to have 'em, but horns for low frequencies must necessarily be large. So for deep bass from speakers that aren't as big as a house requires other technologies. Horns are great down to 40Hz, but below that they become pretty big. You just can't make a basshorn have 30Hz f3 in 20 cubic feet or less.
"Can you show some more detail and maybe link some references?"At the time I was looking at both the EV MTL4 and the KEF 104, both had their strengths and their weaknesses. I tried to get the best of both worlds.
EV no longer has the datsheets for the MTL4 on line, and the KEF 104 has been discontinued for awhile too.
"But the thing I'm wondering about is how do you get the anomalous harmonics to be 180o out of phase while having the fundamental in phase."
The wavelength of 200hz is ~68", my first model had the magnet structure of the inverted driver only 1" away from the front of the other cone. The cones are so close together and the wavelengths so long that it still subtracts.
"Distortion caused by the magnetic structure isn't something I would expect to be improved on by this configuration because it seems to include odd-orders harmonics as well as even-orders"
The 3rd harmonic reduction is a function of the low-pass behavior of the plenum. IE: if the plenum acts like a 12dB low pass at 200hz it will roll off the 3rd harmonic of 100hz by roughly 6dB. Obviously no useable reduction can occur much below 80hz or so.
Two interesting stories:
On a box that had the woofers hooked up wrong, ie: in phase electrically, the woofers would just flap and absolutely NO sound could be heard.
Car stereo installer just finishes carpeting a 4 X 12 box, wants to check it out before the customer shows up. He's running a 30W Pyramid car stereo amp full range, no 6th order EQ, no HF, no crossover. It sounds real wimpy in his 30 X 24 garage, he's worried. I hooked up the electronic crossover I brought over that I added 6th order EQ to, along with his 100W Fisher/Sanyo garage system, and the same 30W Pyramid amp. He's stunned, it really thumps now, and can play loud enough to rot your face off. Goes into a Ford Escort with 1.2KW on it, played so loud standing in the garage that I refused to get in the car.
Hello again Dennis!This configuration looks interesting, and I'm starting to look at it closely. The π cornerhorn can easily be built with dual woofers and it has a plenium of sorts. Since the woofers aren't exposed, it would not be as unattractive as a baffle-mounted setup. Then again, baffle-mounted speakers can be built that have one woofer only run to 100Hz or so, to reduce interference anomalies in the lower midrange. Those are just a couple of my thoughts for implementations of this configuration in addition to the plenium you've described.
You wrote:
> > EV no longer has the datsheets for the MTL4 on line, and the
> > KEF 104 has been discontinued for awhile too.I assume these two manufacturers used this configuration, yes? I wonder why these models were discontinued. It might have been aesthetics - Speakers made this way might be perceived as being "peculiar looking."
A quick search on the internet shows that EAW is using the push/pull woofer configuration , and I also notice that they use some other design techniques that I personally find attractive.
I appreciate your discussions here and I'll check into this some more.
I wrote:
"But the thing I'm wondering about is how do you get the anomalous harmonics to be 180 o out of phase while having the fundamental in phase."
And you replied:
> > The wavelength of 200hz is ~68", my first model had the magnet
> > structure of the inverted driver only 1" away from the front of
> > the other cone. The cones are so close together and the
> > wavelengths so long that it still subtracts.I wasn't worried about distance problems shifting the phase because the frequencies of interest are second harmonics of the woofer's resonant frequency region. You've rightly shown just how long these wavelengths are.
More what I was concerned with was a confirmation that the second harmonics would be generated exactly in phase with the fundamental as opposed to 180 o out of phase. I wouldn't expect any other phase relationship, but it would not be unreasonable to expect either or both variants, and I'm not sure at what proportions.
For example, if the suspension were more resistive in the forward direction than the reverse on one cone and just the opposite on the other, then I would expect second harmonics to be of opposite phase in these two speakers. In that case, it would actually perform better to have the woofers connected in the "normal" configuration, as this would cancel the harmonics due to their opposite phase relationship.
Then again, this would be an example of poor quality control and I'm not sure that's the sort of issue we're dealing with here. I suspect it has more to do with non-linearities near excursion limits, and where the amplitude of the voice coil's magnetic polarity becomes significant.
I wrote:
"Distortion caused by the magnetic structure isn't something I would expect to be improved on by this configuration because it seems to include odd-orders harmonics as well as even-orders."
And you replied:
> > The 3rd harmonic reduction is a function of the low-pass behavior
> > of the plenum. IE: if the plenum acts like a 12dB low pass at
> > 200hz it will roll off the 3rd harmonic of 100hz by roughly 6dB.
> > Obviously no useable reduction can occur much below 80hz or so.Yes, the configuration can do nothing about odd-order harmonics. This is very similar to a complementary-symmetry amplifier configuration, which have benefits in the reduction of even-orders. I understand what you're saying about the acoustic chamber, but in regards to the summing of the two speakers, we're talking about even-order improvements here.
EAW makes a lot of nice stuff.At one time they were the sole OEM source for RCF in the USA. At one time I was on a scheduled release program and getting 250pc price breaks. N280 and N480 compression drivers. The N480 diaphragm fits 1" JBL structures(2410/2420/etc), sounds better, costs less, and is more rugged.
"I wonder why these models were discontinued."
KEF was sold in 1992, floundering even before the death of Raymond Cooke(1996), I can't remember what year Laurie Fincham left. Here is a description of their push-pull 4th order bandpass design:
The Dual Cavity Woofer System
Model 103/4 incorporates two 160mm woofers mounted in double coupled cavity configuration and linked with a force cancelling rod. This metal rod couples and thereby cancels the drive unit’s identical, but opposing, mechanical vibrations, whilst reducing distortion. Linking the woofers also prevents the transfer of energy to the main enclosure, a cause of the delayed resonances which often give rise the ‘boxy’ colouration thus reducing musical clarity. Model 103/4’s entire bass output is radiated by a smoothly contoured duct placed below the Uni-Q unit. This acts as an air diaphragm of very low mass, with an effective diameter similar to that of the midrange unit. Thus directional characteristics match, ensuring exceptionally smooth acoustic integration throughout the entire frequency range.http://www.kef.com/kefimages/image240.gif
(in case the image doesn't load)
The 103/104/105 used dual 6.5/8/10 woofers. All were a 50hz~150hz BP4 and with the KUBE had a Q=.5 20hz cut-off. The 107 had dual 10s, a bigger box, less effficent, and would do 20hz without the KUBE. The 105/107 had the MF/HF in pods atop the woofer cabinet, ala the B&W 801. The version of the 103 that only had one 6.5" mid sounded really good, but wouldn't handle enough power to get good 'n loud. The 104 almost got loud enough, but sounded funny in the MF. The 105/107 were too expensive(in 1988 a pair of 103s were about USD$1800
depending on finish, I think the 107s could get up to 10K$)A friend of mine, Bruce Knight, went to work with dB Sound in the early 80s. Bruce told me about these four 18" driver manifold boxes that had as much low end as their much bigger four 15" horn boxes.
• Manifold Technology •At the heart of these systems are the world renowned Electro-Voice“MT” enclosures. The “Manifold Technology” enclosures were co-developed by EV and db Sound to combine all of the sonic and cost advantages of reducing system size.
http://www.dbsound.com/images/MTStkLgo.jpg
Here's Bruce's small convex arrayed system for Aerosmith:
http://www.dbsound.com/images/Aeropic1.jpg
When Bruce did our hometown with Aerosmith he brought the 80,000 seat stadium rig into our 10,000 seat auditorium. Jackal opened up with their 'Chain Saw Boogie', I've run jackhammers that were quieter and had less impact that their mic'd chainsaw. I couldn't handle it, even with plugs.
The typical stadium system consists of 54, EV MTH4, Mid/Hi enclosures, 54, MTL4, Lo enclosures, and 16, MT2, Hi and Lo enclosures(a total of 248, 18" woofers). 56 Crest 8001 amplifiers are utilized to provide the system with 168,000 watts of power.
I told Bruce it was stupid to run a convex array, and that trap cabinets ought to face in, not out. He said it would be an all day afair to rig the fly-bars, but he would try it when he had an extra day to set up the gig.
"db Sound has taken the development of the MT system one step further with the introduction of the unique Concave Line Array. By arranging the enclosures in 40° opposing angles to one another all lobbing is eliminated horizontally. This results in 180° of side to side coverage without the “dead” or “hot spots” that are normally experienced with other systems.
The system effortlessly produced levels of 126db at the mix site as well as 110db, 450 feet from the stage, with no delay towers. Implementing a combination of proven acoustical principals, the Concave Line Array is virtually unrivaled anywhere in the world.
The system was critically acclaimed by the press as “the best outdoor sound system ever heard”."
http://www.dbsound.com/images/CLACntr.jpg
A a 1,200 seat club gig the MTL2 rig was being loaded out and I was waiting to load in (you can move the MTL2 without a forklift and get them up a fire escape without killing yourself). I suggested we grab some breakfast and let the stage hands do all the hard work, and afterwards he could hear what a 'real' system sounded like....TBC
This is interesting about the 2nd harmonic cancellations. Is the equal and oposite reaction limited to 2nd harmonics, and does it make the system less efficient.With the drivers close together like this. Just how high can this system be used.
What kind of parameters do you use when calculating the rear chamber, and can you actually build the box so it would act like a low pass filter down to 80hz if needed.
thanks,
Bill
"Is the equal and oposite reaction limited to 2nd harmonics, and does it make the system less efficient"Yes, the equal and opposite reaction only reduces 2nd harmonics, it does not change the system efficency.
"With the drivers close together like this. Just how high can this system be used."
The closer the better seems to be the rule as far as HF response goes. A four 12 box I made could be crossed at 400hz, some dual 18 boxes were able to cross at 250hz. The majority that I have built are dual 15 and crossed far enough below where the plenum rolls off to use an off-the-shelf LR4 crossover.
"What kind of parameters do you use when calculating the rear chamber"
Use normal TS parameters and size like a regular box, I use low Qts drivers with a step-down tuning and equalization. Drivers with Qts=.312 give the deepest bass response from the smallest box size. Alternately one can seal the box and use a Linkwitz transform, or leave the back open like the Linkwitz dipole.
"and can you actually build the box so it would act like a low pass filter down to 80hz if needed."
Yes you can, but now you have a bandpass box. That may better suit your needs, and many modeling programs are available for those.
At the time I was looking at both the EV MTL4 and the KEF 104, both had their strengths and their weaknesses. My design had all their strengths and none of their weaknesses.
Thanks for the info, I appreciate it. I like the tower style speakers because the horn is more ear level. Using a design like the PPSL as the lower portion of a cabinet might come off like having a sub built in to each cabinet. That could leave me enough cabinet under a horn for a nice midrange driver.I was going to start working with a built in powered sub, but that begins a whole new story. The efficiency isn't necessarily there and the choice of built in amp will always be questionable.
Dont have any experience with the KEF you mention. I have thought about using two drivers and basically just use one with a linkwitz transform to augment the lower octaves to the other.
Hi Dennis!I've been thinking about this configuration since you brought it up, and it seems to me that it might reduce second harmonics whether they are caused by non-linearities in the suspension or the magnetic structure. Pretty cool idea, really, and I'm surprised it isn't more commonly used on systems that have multiple woofers. I never gave it much thought before now, because none of my stuff uses more than one woofer. But it does seem like a good configuration for dual woofer designs whether they be baffle loaded, plenium loaded or horn loaded.
Please do post more references if you know of any. I'd like to see more quantified data, if there is any that's available. Looks to me like you've found a good thing here.
Would be very interested in tyou comments and ideas with respect to this design. Thanks. Moray James.
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