Looking to build 2x18" enclosure - PPSL? - diyAudio
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Old 17th September 2013, 08:14 PM   #1
MemX is offline MemX  United Kingdom
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Question Looking to build 2x18" enclosure - PPSL? - *Now with build pics*

Hey all!

I am relatively new to the Audio world but have managed to set up a reasonable selection of AV toys at home to enjoy, which I am happy with so far

I currently have an SVS subwoofer that does not dig as deep as I would like it to (ported with 16Hz tune), whereas I would like to get into single-digit Hz playback, if at all possible.

I have therefore picked up some of the Stereo Integrity 18" HT drivers: Stereo Integrity | HT Subwoofers
and am currently attempting to design a box for them

I have been wanting to go Push-Pull (or Compound Push Push, if that's the correct terminology for one driver having the spider/magnet outside the box? ) because I have been reading up on its reduced harmonics (thank you to those who have taken the time to post up graphs illustraing the benefits on here!)

Can those who have a PPSL design comment on whether the Odd Order harmonics become much more noticeable when the Even Order harmonics are reduced/removed? And if that negatively impacts the listening experience?

With regards to my design, I started by putting one driver on the top and one on the bottom of my box, the top one facing up and the bottom one facing up but firing into the box, with supporting columns and a base plate at each end so I could stand them up safely and allow the drivers to actually move lol

However, due to space constraints, I decided to move the drivers into the middle of the box and have them firing into the same space, removing the need for the extra height of the base plates and associated columns.

I therefore appear to have inadvertantly started to design a Push Pull Slot Loaded box, if I'm correct??

My current design looks a bit like this:
Click the image to open in full size.
(taken from this thread: 10" PPSL Vented Enclosure Design Help)

but I would like to build a sealed version instead.

Please may I ask those on here with far more knowledge than me (!) to confirm the effects of slot loading?

From what I've read on here (and hoping I've recalled correctly...) the slot can act as a filter above and below certain frequencies? i.e. if the slot is too small then it will 'strangle' the output through compression.

Does the slot therefore need to be large enough to allow free movement of both cones from XMax to XMax?

i.e. if total XMax to XMax displacement of the drivers is, for example, 25 cubic inches, the slot would need to be 5 inches by 5 inches, giving 25 square inches of slot? (ignoring the dimensions of the drivers etc)

Or does it just need to be designed for Resting Position to XMax?

Also, what happens if you open the slot to both sides of the box and not just one? Would there be phase issues with the wave that moved away from the back of the box and bounced off the wall behind?

If the box is designed correctly and a suitable size slot added, are there really no benefits/downsides to slot loading, other than mounting the drivers in a 'safe place', out of harms way?

Or am I being really simple and not taking everything into account?!?

I'm sorry for all the questions, I've been reading and reading but there is a lot to get through (my head hurts!! lol) and I am trying to get the box design finished ASAP!

Many, many thanks in advance for any assistance you are able to give, it will be much appreciated!!

Last edited by MemX; 12th May 2014 at 11:36 AM.
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Old 19th September 2013, 02:48 AM   #2
djk is offline djk
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I would suggest doing a sealed PPSL with a Linkwitz transform.

I think the ports would be hard to make work properly with those drivers.
Candidates for the Darwin Award should not read this author.
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Old 20th September 2013, 09:12 AM   #3
MemX is offline MemX  United Kingdom
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Thank you for your reply, djk, it is very much appreciated!

I would like to build a sealed version

I just need to get my head round plenum and 'port' sizes, and their effects on output and bandwidth...

Current workings seem to suggest a minimum possible plenum size of 19.5" wide by 13 or 14" tall (to account for driver depth + XMech) by c.20" deep (width of the box) assuming I mount them horizontally. Assuming the 'port' is the same 19.5"x14", that seems large!
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Old 20th September 2013, 12:21 PM   #4
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PPSL sound awesome.
I've made 2 (12's and 18's).

It costs about the same.
They are the only way I'd build subs.
Why not add a little complexity to the build for better sound ?

I know you said 18's, but the dayton 15" rss390Hf has a sealed F3 of 30hz in 5.5ft3 (each), 14mm Xmax, all for $173. That's about the lowest sealed F3 that I've seen lately in a medium sized box, regardless of driver size.

Oh yea, I'd have the drivers firing side to side, not up and down.

Last edited by norman bates; 20th September 2013 at 12:23 PM.
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Old 20th September 2013, 10:10 PM   #5
MemX is offline MemX  United Kingdom
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The daytons do sound interesting I have already got the SIs in transit, though, so I am pretty much committed to them!

I am getting the boxes built because I live in an apartment and have no outdoor space available for DIY Well, that and the fact I am useless... so you are right, one might as well design in a little extra complexity for some added benefits!

I may struggle to get the drivers fitted side by side but I will gve it a go...

I really don't think I could do a LT, but Audyssey XT32 will hopefully provide a decent EQ and boost!
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Old 20th September 2013, 10:38 PM   #6
tinitus is offline tinitus  Europe
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Originally Posted by djk View Post
I would suggest doing a sealed PPSL with a Linkwitz transform.

I think the ports would be hard to make work properly with those drivers.
Fs: 17 Hz
Qes: 0.41
Qms: 5.8
Qts: 0.38
Le: 3.2 mH
Sd: 117841 mm^2
Vas: 411.9 l

with those specs a PPSL would be HUGE

I would suggest two ordinary closed boxes
and I really doubt you will need any EQ at all
sometimes we know very little, and sometimes we know too much
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Old 20th September 2013, 11:39 PM   #7
MemX is offline MemX  United Kingdom
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For info, my thoughts are around 15 cubic feet for four SI18 D2 drivers, fed off a Cerwin Vega CV5000, two drivers per box, with each pair of drivers getting 2500wRMS

I think they might come out nearer 13 or 14 cubic feet due to bracing and size constraints, but I have modelled them in a sealed setup in WinISD, as per the attachments, and I would appear to be within XMax all the way down to c.8Hz, with XMech of c.35mm being reached around 2Hz.

If I can get them into a PPSL setup then, as they will be using a lot less than their maximum possible output once EQd using Audssey XT32, I am hopeful that they will sound very clean

As above, the slot will have to be 19.5" wide by about 14.5" tall for driver safety, with the plenum being 19-20" deep, as any smaller and I risk driver contact with each other!

I'm not sure whether to have a slot on just one side of the box or both I have a feeling that it won't really be 'slot loaded' with that sort of port size, even just one side! but if that means it's just effective a sealed push-pull box without any slot-loading, I'm not too worried as I'm sure it will still sound great!
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Old 20th September 2013, 11:44 PM   #8
MemX is offline MemX  United Kingdom
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Oops, perhaps if I attach the WinISD graphs for the sealed PPSL at 15 cubic feet with 5000w like I said I would...
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Old 20th September 2013, 11:46 PM   #9
scholl is offline scholl  United States
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Can you do a PPSL in infinite baffle? Despite the .38 QTS they would work and save space.

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Old 21st September 2013, 12:18 AM   #10
MemX is offline MemX  United Kingdom
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IB? You mean wall off the alcove where they will sit, build a plenum into the wall and then mount the drivers on/in it?

Hmm, that's an interesting idea...

I don't think the alcove is big enough unfortunately I think it works out about 50 cubic feet, which is not that large if each driver has to have 10x Vas? (IIRC??)

There is the other issue that the missus would kill me... haha
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