Looking to build 2x18" enclosure - PPSL?

[MemX]

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:

(taken from this thread: http://www.diyaudio.com/forums/subwoofers/239503-10-ppsl-vented-enclosure-design-help.html)

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!!

 

[djk]

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.

 

[MemX]

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!

 

[norman bates]

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.

 

[MemX]

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!

 

[tinitus]

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

 

[MemX]

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!

 

[MemX]

Oops, perhaps if I attach the WinISD graphs for the sealed PPSL at 15 cubic feet with 5000w like I said I would...

 

[scholl]

Can you do a PPSL in infinite baffle? Despite the .38 QTS they would work and save space.

Scott

 

[MemX]

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

 

[tinitus]

For info, my thoughts are around 15 cubic feet for four SI18 D2 drivers ...

mine says about 7 cubic feet/200liter closed per woofer

but if you are happy with the higher Qtc ...
will it produce more sloppy bass ?
is it audible ?

 

[MemX]

As a newbie to all this, juggling all the constraints and options is not easy! so I apologise if I say anything dumb.

Am I right in thinking that the 7cuft model is for the rated 600w RMS of the SI18s?

I was under the impression that the higher Q number would produce a tighter sound than a lower Q number (e.g. 0.707) at the expense of adding more power?

If that is correct (which it probably isn't??!) my plans for the CV5000 running each channel at 2Ohms / 2500w RMS to two of the SI18 D2s, giving them ~1250w RMS each, seems to overcome the smaller box size?

One another forum several members are running multiple SI18s off CV5000s and also the (in)famous Sanway amp, giving them ~1100w each, and so far I don't believe there has been (m)any problems, so I am hopeful that all will be fine... ??


I fear I am in slightly over my head here!

 

[tinitus]

0.7 Qtc gives you a system without bass overshoot(peak)
higher Qtc also usually rolls off lower bass faster

lower Qtc will show you a higher F3(-3db roll off point)
but watch the -6db roll off point
it will be lower because bass roll off is different(less)

woofers with the lowest Fs around 20hz and medio Qts 0.38-0.40 are the dream woofers for closed/sealed
and good for lower Qtc
to put this in vented is madness
but Im sure some do it just for the dream of straight bass to 20hz
just remember you might have substantial room gain down there
some say up 12db, maybe
and there could be dips and suck outs, or whatever
almost anything seems possible when it comes room ressonances

VAS can be a tricky player and result in too big design to be practical in a living room
and if 0.75 Qtc gives acceptable size, then it might be ok too

but I do not have any real experience with this

 

[MemX]

Thank you for the explanation and kind help

Is it correct to say that the smaller you make a Sealed box, the more it behaves like a Ported box? i.e. the output stays higher further down the scale but then drops off more steeply than a larger box?

But that you can overcome a steeper drop-off caused by a smaller box, by adding more power and an EQ solution to boost the lower end?


I would prefer to build a large Sealed box and require less power/EQ but I have limited space therefore I must build a smaller box and add big power as a solution!


I have finished reading the PPSL thread (http://www.diyaudio.com/forums/subwoofers/177905-thread-those-interested-ppsl-enclosures.html) and I think I understand the concept now:

- try to have both drivers in the same internal air space (but not 100% essential, as long as each chamber is equal in internal size and takes account of the volume taken up by the driver);
- make sure the drivers are as close together as possible to increase the coupling (and hence cancellation effects) across a wide range of frequencies;
- aim for a combined driver Sd to slot dimensions ratio of 2:1 to 3:1;
- have the drivers vertical instead of horizontal to reduce/remove driver sag issues.


I think that's everything...??!

I will get a design finalised and try to upload it here for review

 

[tinitus]

- make sure the drivers are as close together as possible to increase the coupling (and hence cancellation effects) across a wide range of frequencies;

ehh, I don't about that

large Xmax woofers going very low could be different in that respect

 

[googlyone]

A couple of comments:
- Four of these drivers in a vented box to go "really low" needs a truly massive box. A start at 20 cubic ft for a half decent alignment and something more like double that...
No - I don't play games with "room gain" and suchlike.

- You are in an apartment? Is the rest of the building empty? As if you actually did build a sub using four of these it would be capable of a metric shitload of SPL at frequencies that will go straight through your walls.

- You will end up with an IB or sealed box by the time you finish - unless you can fit a box of 20-40 cubic feet, which is more like a small room in itself.

- I note you referring to driving these with some high power levels.
Did you notice that these drivers use a 2.5" voice coil. I dont care what the data says, if you put a 600W sinewave near them for any length of time, you will have some really interesting smells and not a lot of subwoofer anymore.

Have you thought of starting with one or two single driver boxes and working from there. If you really want to go push pull, a box of about 12 cubic feet will work pretty well, but to get to the lower octaves you will need masses of EQ. Again, watch the "real world, hype free" power rating.

 

[MemX]

Thanks for the input, both


I haven't done the calculations for Sdort size ratios yet so I'm not sure how they work out, but perhaps I should try to aim for 2:1 to try to minimise pressure in the plenum at very low Hz but still have a boost, going from what's been said in the PPSL thread about increasing output with close coupling and ~3:1 ratios?


I am definitely planning to make a Sealed box and not a Ported box I want to be able to reach single digits (or try to reach single digits!) without risk of bottoming drivers under a port tuning frequency. Not only that, I only have ~21 cubic feet of room for both dual boxes, including the plenums!

I work the plenums out to be ~3 cubic feet each, which should leave me with about 7 cubic feet internal volume for two drivers in each box. The WinISD graphs I posted on the previous page seem to suggest XMax wouldn't be reached until it got down to about 7Hz or 8Hz with that size box and the amount of power I'm planning to use (the CV5000 is seemingly pretty accurately rated at 5000w RMS) so hopefully I will have protected them that way, and also through signal chain roll-off reducing output at the lowest frequencies.

It is quite a lot of power, though, granted... but I have Audyssey XT32 EQ on my Onkyo 818 AVR, so I will only be using a fraction of the power most of the time because it EQs to something like 105dB peaks rather than the crazy 130dB+ that it seems could be achieved if I really cranked the amp gain up! If I have issues with boominess or peaks then I will look at a MiniDSP or similar.


I hear what you are saying about the volume in an Apartment Fortunately it is constructed with solid concrete blocks and seems to be very soundproofed! I currently have an SVS PB12-Plus/2 (2x12", ~900w RMS) that I run up to about -6 on the AVR when I watch films, and so far I've not had any complaints. I always make sure that I only really turn it up when the downstairs neighbours are out (which they are quite a lot because they are young) and I don't very often listen to music at loud volumes. Any bass that does travel is relatively short-lived and random, from sound effects in films rather than a repetitive bass beat of music, so hopefully that is not too annoying for the neighbours!


I am hopeful that music and movie content will be a lot less stressful than Sinewaves and that the drivers will therefore be ok - some members on another forum are using around 1100w RMS into these drivers with no real reported problems so far, so I will cross my fingers

 

[djk]

If you want the smallest box size, try a Qtc=1.1 with a Linkwitz transform.

 

[badman]

My current design looks a bit like this:

(taken from this thread: http://www.diyaudio.com/forums/subwoofers/239503-10-ppsl-vented-enclosure-design-help.html)

Make sure, as mentioned in that thread, to rotate the manifold 90 degrees.

 

[MemX]

Hey all

Thank you all again for the input and help with this, it is much appreciated!


I've managed to confirm the driver surround to dustcap measurement on the SI HT18 drivers (c. 3.5"), which with around 36mm XMech would mean they could be mounted in a slot with the back of the 'out' magnet in line with the top of the 'in' driver cone surround, to leave 2" clearance between the 'out' magnet and the 'in' cone at XMech (worst case scenario).

However, because I'm getting the box made and then fitting the drivers (I have no space / tools / skills to DIY it myself!) I have no choice but to make the slot the width of the whole driver's height from top to bottom, plus the height of the driver surround on the 'in' driver (so that I can fit the 'in' driver and then fit the 'out' driver without taking the box apart).

If that all makes sense... ?


So, details are as follows:

- My box dimensions should be 36" wide by 20" deep by 33.5" tall.
- The slot would be 12" wide by 19.5" tall, around 3.5 cu ft displacement (including the double layer driver surrounds)
- Box internal volume would be between 7 and 7.5 cu ft (taking into account driver displacement and bracing).
- With driver Sd of 117841 sq mm (182.654 sq in by my calculations?) and a slot of 234 sq in, I work out the Sd:Slot ratio to be 1.56:1.


Does the above look acceptable and like it will produce a good sound? I have Audyssey XT32 for system EQ but I may need to look at boosting the bottom end via other means