Push-pull Slot Loading (PPSL) sub drivers? - AVS Forum
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post #1 of 36 Old 12-26-2010, 07:24 PM - Thread Starter
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Would love to have a discussion about the benefits/drawbacks of PPSL sub drivers.

Theoretical advantages:
1) non-linear distortion cancellation
2) Force cancellation

I've seen third hand confirmation that both exist. Would love to see any other data.

In case anyone isn't clear what I mean, here are some links detailing the concept...

First Link with DJK's drawing

Siegfried Linkwitz's example for different purpose, but same concept.
http://www.linkwitzlab.com/woofer.htm
The picture in that link shows the driver's motion direction.

Note: I find the push-pull name slightly misleading, but its DJK's name and I get that he called it such due to one driver being wired out of phase and reversed with the other but I still see the mechanics as push-push.

Both SL and DJK are smart cookies. They chose this topology for very good reasons. I wonder why it isn't used more often here.

** There are a couple of other threads on this forum that mention the concept yet go nowhere with it, IMO, so I thought it was worth further discussion.

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post #2 of 36 Old 12-26-2010, 07:30 PM
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Wow, trying to wrap my head around that concept and how to model it just gave me a whopping headache. The only ones I saw referenced for builds used high efficiency woofers, and had a relatively high low frequency dropoff (30hz or so).
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post #3 of 36 Old 12-26-2010, 07:31 PM - Thread Starter
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Another great thread mentioning the uses....

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post #4 of 36 Old 12-26-2010, 07:41 PM - Thread Starter
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Thread discussing some of the technical merits between Wayne Parnham (Pi Speakers) and djk.
http://www.audioasylum.com/cgi/t.mpl?f=hug&m=47995

I'm still looking for the winISD models I've seen posted.

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post #5 of 36 Old 12-26-2010, 07:47 PM
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I'm trying to postulate what benefits this config provides. Would it not just effectively double your motor strength?

i've been thinking more along the lines of corner loaded clam shells like the pro-audio JBL cabs can be configured.
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post #6 of 36 Old 12-26-2010, 07:53 PM - Thread Starter
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Quote:
Originally Posted by Cerdic View Post

Wow, trying to wrap my head around that concept and how to model it just gave me a whopping headache. The only ones I saw referenced for builds used high efficiency woofers, and had a relatively high low frequency dropoff (30hz or so).

No reason why it can't be used for low efficiency low Fs drivers, it was just demonstrated for different means there. SL used more standard subwoofer drivers in his dipole design.

2nd Order distortion is canceled and force is canceled (meaning the box won't move a lot).

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post #7 of 36 Old 12-26-2010, 08:18 PM
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AJ, I think most people simply don't know about it, or from the posts so far, don't get why yet.

I saw Dennis mention this several years ago and had a lightbulb moment with it, same as I have with a lot of his comments over the years. I learned a lot off him when I was designing and building horns.

Quote:
Originally Posted by Cerdic View Post

Wow, trying to wrap my head around that concept and how to model it just gave me a whopping headache. The only ones I saw referenced for builds used high efficiency woofers, and had a relatively high low frequency dropoff (30hz or so).

It's just the same as any other two driver ported box, so you can use whatever drivers. They are just mounted differently in the manifold for the reasons AJ outlined in the first post.
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post #8 of 36 Old 12-26-2010, 09:53 PM
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Thanks for making the thread AudioJosh.

I am all for learning new techniques but have no idea how to model an enclosure for a given set of subs.

I already have a bunch of subs I have not gotten around to using like these:

ACI SV12 (already pre-loaded in WinISD)

Impedance: 4 ohm
RMS Power: 350 watts (system power 500 watts)
Sensitivity: 90dB
Fs: 17.5Hz
BL: 8.9 Tm
Qms: 8.35
Qes: 0.41
Qts: 0.395
Vas: 250 liters
Cms: 1041.67
Mms: 99 grams
X-max: 12.87mm peak
VC Ø: 2" (51mm)
VC Height: 31.75mm
Air Gap Height: 9.652mm
VC Inductance: 1.45mH
VCR: 2.97 ohm

NHT Sub2 10"

Re: 9.58
Fs: 21.5
Qms: 2.952
Qes: 0.444
Qts: 0.386
Vas: 103.84 L
Cms: 0.000561
Mms: 97.8
Dia: 215
BL: 16.87
Spl: 85.47
Zmax: 73.2
Sd: 0.03631
Xmax: 13.0
Hg: 8.0
Hvc: 34.0

Maybe it would help if you explained how each would work in a PPSL?

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post #9 of 36 Old 12-26-2010, 10:04 PM
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It may cancel out asymmetry differences, but it can't do anything for non-linear effects innate to the motor or suspension.

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post #10 of 36 Old 12-27-2010, 12:16 AM
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this model was employed by m&k in the thx5000 a long while back.

drivers are non-linear in their force factor.

one in, one out, helps to average the force factor.

this results in better second order distortion.

how important is that? minimal.

otherwise, you would see a lot more of them.

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post #11 of 36 Old 12-27-2010, 12:21 AM
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there was a monster sub from electrovoice, iirc, that used this strategy for another example. let me see if i can find a technical paper or description.

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post #12 of 36 Old 12-27-2010, 12:27 AM
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for something related, have a lookup of "manifold subwoofer" on google images.

that will call up many builds related to your question.

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post #13 of 36 Old 12-27-2010, 12:29 AM
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post #14 of 36 Old 12-27-2010, 07:36 AM - Thread Starter
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Quote:
Originally Posted by LTD02 View Post

this model was employed by m&k in the thx5000 a long while back.

drivers are non-linear in their force factor.

one in, one out, helps to average the force factor.

this results in better second order distortion.

how important is that? minimal.

otherwise, you would see a lot more of them.

I think the minimal part is in question. I've seen claims otherwise and I was hoping to have this thread to hopefully explore just how (in)significant the reduction is.

I think the reasons for not doing it more has more to do with complexity thereby creating more cost in production. Its not super easy to construct the manifold, plus the manifold takes up volume making the box bigger.

According to djk the effect is rather significant. He claims to have built over 100 such subs, so I take it he has more data on the subject than any of us.

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post #15 of 36 Old 12-27-2010, 09:25 AM
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Quote:
Originally Posted by AudioJosh View Post

I think the minimal part is in question. I've seen claims otherwise and I was hoping to have this thread to hopefully explore just how (in)significant the reduction is.

Quote:
Originally Posted by AudioJosh View Post

According to djk the effect is rather significant. He claims to have built over 100 such subs, so I take it he has more data on the subject than any of us.

DJK has been spot on with everything he's suggested over the years and he's an excellent engineer, one of the small number who's comments I take at face value as correct.


Quote:
Originally Posted by AudioJosh View Post

I think the reasons for not doing it more has more to do with complexity thereby creating more cost in production. Its not super easy to construct the manifold, plus the manifold takes up volume making the box bigger.

Plus, it can be a mongrel to get drivers in and out. For a touring rig that can be a nuisance if it needs to be done in a hurry. I'd say there's also a bit of the 'reluctance of the unknown' for some users in trying it.

Interesting to note in the AA thread, the Bassbox 6 simulations show that the boxes are tuned 6th order.
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post #16 of 36 Old 12-27-2010, 09:54 AM - Thread Starter
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I also think he has no agenda since he isn't selling anything. He works in pro-audio not hi-fi.

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post #17 of 36 Old 12-27-2010, 06:46 PM
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I think it is an interesting concept. If you cabinet size is not an issue for the builder then I think it is worth trying as an enhancement to any of the traditional build techniques. I'm actually as interested in the slot loading low pass filter as I am the 2nd order distortion cancellation.

When the AE15's become available again I'm thinking about building a low tuned dual PR18-2100, PPSL AE15H sub in about 10 cu ft. I've attached a quick sketch.
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post #18 of 36 Old 01-04-2011, 01:36 PM - Thread Starter
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I've though about just that sort of alignment you posted. I wonder if it makes any sense to also slot load the PRs. Actually, I can't think how that would help any.

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post #19 of 36 Old 01-04-2011, 02:05 PM
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Push/pull and its benefits is totally separate from slot load (not sure what it's supposed to do).

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post #20 of 36 Old 01-04-2011, 02:12 PM - Thread Starter
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True, in think it is partially to protect the drivers in the original PA use case, rather than having one basket sticking out the box. The slot does act as a LPF, for better for worse.

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post #21 of 36 Old 09-18-2013, 09:25 AM
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With apologies for bumping an old thread (PPSL doesn't seem to be discussed much!) is the following a correct summary of the benefits / drawbacks of a PPSL design?

Good:
- Force cancellation due to opposing driver movements
- Even-order harmonics reductions due to 'push-pull' design
- Can be applied to a variety of box designs (sealed, ported, bandpass...)

Potentially Bad:
- difficult / impossible to remove drivers once installed if the slot is smaller than the depth of the drivers
- Odd-order harmonics remain much the same (and are potentially more obvious due to the other harmonics reductions? making them more prominent??)


Does the slot do anything else? From what I have read it can also seemingly act as a LPF?


How does one determine how large to make the slot?

Do you have to work out the displacement (peak XMax to peak XMax? or resting to one-way peak XMax?) then design a slot with an opening that is equal to or larger than this amount of displacement?

Can you have a slot open to both sides of the box?


Apologies for all the questions, I am struggling to find one definitive collection of information on PPSL! smile.gif
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post #22 of 36 Old 09-18-2013, 09:43 AM
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Now i haven't read other than the last post.... ,but there is another disadvantage to mounting a woofer with the motor outside of the cabinet. Some motors exhibit a good deal of noise that normally would be contained inside the box. I guess the audibility depends on the actual mounting and how the woofer is positioned in relation to the listening position, but it is there to varying degree. Some time ago i saw a video of the old M&K sub (mx350 i think?) playing a low sine-wave, so that there was no masking effect and the motor noise was very obvious.

Oh.. I also remember there was a good thread over at diyaudio about PPSL. Maybe you can look it up :-)
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post #23 of 36 Old 09-18-2013, 09:44 AM
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"Difficult / impossible to remove drivers once installed if the slot is smaller than the depth of the drivers ..."

This is not the case if you make a point to build your slot slightly wider than is the depth of the drivers that you use. That is what I do when building PPSL cabs and have never had a problem swapping drivers IN/ OUT. I have built two sets of cabs using the design published here by DJK and have been very happy with them.
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post #24 of 36 Old 09-18-2013, 12:44 PM
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Thanks for the replies! smile.gif


I was thinking that slot cavity would have to be back of driver depth + front of driver height + room for the front-firing driver to reach XMech (worst case scenario) + additional space for magnet cooling if required?

My designs I've been playing with seem to suggest a 14.5" high slot for drivers that are just under 10" deep (basket side) so removal shouldn't be an issue, but I thought I'd check!

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Now i haven't read other than the last post.... ,but there is another disadvantage to mounting a woofer with the motor outside of the cabinet. Some motors exhibit a good deal of noise that normally would be contained inside the box. I guess the audibility depends on the actual mounting and how the woofer is positioned in relation to the listening position, but it is there to varying degree. Some time ago i saw a video of the old M&K sub (mx350 i think?) playing a low sine-wave, so that there was no masking effect and the motor noise was very obvious.

Oh.. I also remember there was a good thread over at diyaudio about PPSL. Maybe you can look it up :-)
I think you mean this thread?

http://www.diyaudio.com/forums/subwoofers/177905-thread-those-interested-ppsl-enclosures.html

I've not had a chance to read through it yet, it's 51 pages... lol


With regards to motor noise, I guess a box could be designed to allow the drivers to be run PP as intended or swapped to push-push (i.e. mounted 'normally') if motor noise is an issue. Hopefully it won't be but I guess we don't know until we try!


I'll have a read of that DIYAudio thread and I'm sure I'll be back with more questions... lol

Thanks again! smile.gif
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post #25 of 36 Old 09-18-2013, 12:52 PM
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"Apologies for all the questions, I am struggling to find one definitive collection of information on PPSL!"

it can be modeled up in hornresp. the slot provides a low pass effect, but it can also present a "peak" just before rolloff, which i have seen measured, but i forget where. just be aware that it is there if pursuing ppsl.

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post #26 of 36 Old 09-18-2013, 01:40 PM
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MemX

Yes i think that was it.

As I understand it, you can reverse the "inverted" woofer if it is too noisy, but then you wont reap the benefit of distortion cancellation. I cant remember the exact explanation but as i recall the cancellation effect is linked to the fact that woofers do not move exactly evenly in either direction. By inverting one woofer the effect of that phenomenon is canceled out. The better way to go is to find, in advance, a suitable woofer that doesn't exhibit objectionable noise. I have a dim recollection that this issue have been brought up in relation to Linkwitz dipoles.
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post #27 of 36 Old 09-18-2013, 07:59 PM
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Quote:
Originally Posted by MemX View Post

With apologies for bumping an old thread (PPSL doesn't seem to be discussed much!) is the following a correct summary of the benefits / drawbacks of a PPSL design?

Good:
- Force cancellation due to opposing driver movements
- Even-order harmonics reductions due to 'push-pull' design
- Can be applied to a variety of box designs (sealed, ported, bandpass...)

Potentially Bad:
- difficult / impossible to remove drivers once installed if the slot is smaller than the depth of the drivers
- Odd-order harmonics remain much the same (and are potentially more obvious due to the other harmonics reductions? making them more prominent??)


Does the slot do anything else? From what I have read it can also seemingly act as a LPF?


How does one determine how large to make the slot?

Do you have to work out the displacement (peak XMax to peak XMax? or resting to one-way peak XMax?) then design a slot with an opening that is equal to or larger than this amount of displacement?

Can you have a slot open to both sides of the box?


Apologies for all the questions, I am struggling to find one definitive collection of information on PPSL! smile.gif

Very interesting! I designed a transmission line enclosure using a cone-to-cone isobaric setup. 2nd shameless plug for my own thread below.

I have an opening on both sides but don't call it a "slot". I did not attempt any simulations as I was not trying to make a traditional horn.

Great results IMHO. My buddy was over last weekend and called it movie theater bass. I guess that's good. biggrin.gif

http://www.avsforum.com/t/1438061/transmission-line-isobaric-subwoofer-for-home#post_23737631


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post #28 of 36 Old 09-20-2013, 02:17 PM
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That looks great! smile.gif I'm guessing it would have been a lot larger without the isobaric layout? I've not even started to read up on Transmission Line speakers, my head is already too full of bits of info that don't always make sense... lol

I'm not quite sure where 'slot loading' turns into 'just a massive hole where the driver is located' biggrin.gif but I think I will just have to design to the smallest plenum/slot/vent I can as I am so limited on space! Hopefully it will sound ok...


Quote:
Originally Posted by splotten View Post

MemX

Yes i think that was it.

As I understand it, you can reverse the "inverted" woofer if it is too noisy, but then you wont reap the benefit of distortion cancellation. I cant remember the exact explanation but as i recall the cancellation effect is linked to the fact that woofers do not move exactly evenly in either direction. By inverting one woofer the effect of that phenomenon is canceled out. The better way to go is to find, in advance, a suitable woofer that doesn't exhibit objectionable noise. I have a dim recollection that this issue have been brought up in relation to Linkwitz dipoles.
Cool, I think I finally have my head round how the cancellation works smile.gif I'm hoping that the SIs are pretty quiet spider-out, if I design the box so they are vertical and not horizontal (to reduce / remove cone sag issues) then I won't be able to swap the outside one into the box.... lol


Quote:
Originally Posted by LTD02 View Post

"Apologies for all the questions, I am struggling to find one definitive collection of information on PPSL!"

it can be modeled up in hornresp. the slot provides a low pass effect, but it can also present a "peak" just before rolloff, which i have seen measured, but i forget where. just be aware that it is there if pursuing ppsl.
Ahhhh, I seeee... smile.gif

I've not downloaded hornresponse and I'm not sure I have time to learn it before I have to get the box builder guy started lol It looks complicated!!

I'm guessing that Audyssey XT32 will be able to tame any major peak, if there is one? In a worst case scenario I do have a BFD but I've not got it set up yet because I haven't got REW-capability lol
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post #29 of 36 Old 09-20-2013, 02:33 PM
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here is an example of response in a pseudo slot ported design:

http://www.jblpro.com/BackOffice/ProductAttachments/ASB6128V.pdf

it can be seen that there is a significant rise in the frequency response at around 200hz. that is the characteristic behavior that i am talking about. it gets more peaked the smaller the "slot"

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post #30 of 36 Old 09-20-2013, 02:54 PM
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^^ Not even remotely a manifold design: more a non flat baffle.
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