A Thread for those interested in PPSL enclosures - Page 4 - diyAudio
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Old 7th May 2011, 02:11 AM   #31
hawkson is offline hawkson  United States
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Yes, I am aware
I asked him about the benefits of a Compound Sealed PPSL vs his slot loaded manifold PPSL.
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Old 7th May 2011, 02:44 AM   #32
Djim is offline Djim  Netherlands
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Quote:
Originally Posted by tb46 View Post
Hi Djim

I made a very quick overlay sketch of the dual 15" by Les Hudson in Post #10. It looks like the plenum has an opening of about 16.5"x7.5", or a little less than one typical 15" driver's Sd.

Regards,
Oliver, I think I need to explanation myself a little more. Sd (cone surface) only matters for calculating Vd (Volume displacement). But that’s just half of the story. The minimum opening also has a relation to the total volume of the Plenum.

In other words;

1.) The bigger the Plenum Volume the bigger the opening needs to be.
In case the plenum volume is too big in relation to the opening it can start to act as an extra chamber or resonator in front of the driver.

2.) The more Vd (more excursion) the bigger the opening needs to be.
My worry is that if the opening is to small you will measure excellent results from 1w/1m but as soon you power it to max it will compress at these points where Xmax is the highest (like if there is extra pwr compression).
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Old 7th May 2011, 03:37 AM   #33
tb46 is offline tb46  United States
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Hi,

Just pondering: I think PPSL stands for: "push-pull slot loaded". So maybe djk's should have be called PPPSL: "plenum push-pull slot loaded" :-). I think originally the slot loading referred to the reflex port (?), naturally if you use the sealed version you loose the slot, but you gain "sealed loading" :-). The size/volume of the plenum should be kept to a minimum, size reduction in the deep/back corners of the plenum is a possibility. A sealed PPSL could be modeled as a simple sealed enclosure (preferred), or as a 4th order bandpass; whereas a vented PPSL should probably be modeled as a bass reflex (preferred), or alternatively as a 6th order bandpass. The use of a plenum in conjunction with the push-pull mounting is a really neat-and original-way of reducing driver induced distortion without the efficiency loss that results from isobaric mounting, so even less expensive drivers should give good results. The drivers have to be wired out-of-phase. And, finally, another overlay sketch, this time from djk's 15" PPSL; if the opening is about 24"x7.75", we are looking at an area of 1200cm^2, or about 1.4 times the Sd of an average 15". This gives us an initial range, with Les Hudson's plenum opening being about .5x Sd.

Regards,
Attached Files
File Type: pdf PPSL_djk_dual_15_inch.pdf (14.1 KB, 511 views)
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Last edited by tb46; 7th May 2011 at 03:55 AM.
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Old 7th May 2011, 03:52 AM   #34
tb46 is offline tb46  United States
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Hi Djim,

"...My worry is that if the opening is to small..." You just worry too much :-), just have a Heineken, and relax a little :-). But seriously, I don't know of any way to model this with Hornresp or AkAbak, so it's back to building and measuring.

Regards,
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Old 7th May 2011, 04:53 AM   #35
djk is offline djk
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I don't worry to much about trying to model it, I designed it BC (before computers) with a four function calculator and 6th order formulas from Keele.

The first dual 15 PPSL had an exit of 16.5"x5" (3.3:1) and I was concerned about the intense pressure it had. Subsequently I changed it to 22.5"x7" (1.7:1) and feel a lot more comfortable with the pressure. A Klipschorn runs a 13"x3" (3.4:1) in the throat.

The main thrust to minimizing plenum volume is to extend the midrange and minimize cavity resonances. The depth is the main issue, and yes, blocking off the back corners helps.

I have measured an organ pipe resonance out of one of the systems (where I had made a very deep plenum for no good reason), up around 800hz, but that is so far beyond the intended crossover point that I didn't care about it.
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Old 7th May 2011, 05:28 AM   #36
tb46 is offline tb46  United States
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Hi djk,

Thanks for the additional information. I ended up with a pair of Lab-15 drivers, and have not made up my mind what to do with them, should I ever find the time to build anything again. If I do I would like to try them in a tapped horn (single driver), and in your PPSL (dual drivers). A while back I found two 24" sonotubes on the side of the road, I may use them for initial PPSL testing.

Regards,
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Old 7th May 2011, 06:43 AM   #37
Djim is offline Djim  Netherlands
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Lab's huh.... Who needs to worry ?! (says a relaxed man while he enjoys one of his Belgium beers)

Cheers!
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Old 7th May 2011, 03:45 PM   #38
hawkson is offline hawkson  United States
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Quote:
Originally Posted by tb46 View Post
Hi,

Just pondering: I think PPSL stands for: "push-pull slot loaded". So maybe djk's should have be called PPPSL: "plenum push-pull slot loaded" :-). I think originally the slot loading referred to the reflex port (?), naturally if you use the sealed version you loose the slot, but you gain "sealed loading" :-). The size/volume of the plenum should be kept to a minimum, size reduction in the deep/back corners of the plenum is a possibility. A sealed PPSL could be modeled as a simple sealed enclosure (preferred), or as a 4th order bandpass; whereas a vented PPSL should probably be modeled as a bass reflex (preferred), or alternatively as a 6th order bandpass. The use of a plenum in conjunction with the push-pull mounting is a really neat-and original-way of reducing driver induced distortion without the efficiency loss that results from isobaric mounting, so even less expensive drivers should give good results. The drivers have to be wired out-of-phase. And, finally, another overlay sketch, this time from djk's 15" PPSL; if the opening is about 24"x7.75", we are looking at an area of 1200cm^2, or about 1.4 times the Sd of an average 15". This gives us an initial range, with Les Hudson's plenum opening being about .5x Sd.

Regards,

Where would a Plenum mounted PPSL offer benefits over a dual sealed Push Pull design? The 2 drivers are operating in the Push/Pull arrangement, they share the same space. So whats the difference?

What exactly does the plenum offer?
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Old 7th May 2011, 04:31 PM   #39
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Quote:
Originally Posted by hawkson View Post
Where would a Plenum mounted PPSL offer benefits over a dual sealed Push Pull design? The 2 drivers are operating in the Push/Pull arrangement, they share the same space. So whats the difference?

What exactly does the plenum offer?
The closer the two drivers are to each other, the higher in frequency you will get the benefits of cancelling distortion (excuse the poor grammar).

You need to be realistic however. The distortion components at the higher freqs are being attenuated by the plenum (the cavity itself acts as a LP filter).
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Old 7th May 2011, 05:15 PM   #40
hawkson is offline hawkson  United States
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So, assuming a crossover at 80 hz or 120hz, is the plenum adding any benefit?
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