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-   -   Help with 12" sub design for donation! (http://www.diyaudio.com/forums/subwoofers/226035-help-12-sub-design-donation.html)

jerryo 19th December 2012 02:45 PM

Help with 12" sub design for donation!
 
Hi,
I'm looking for some help to see what would be the best sub design that I could achieve using 4 x 12" bass drivers with the following known T/S parameters, with or without 4 passive radiators based on the KEF B139

Resistance: 3.4 ohms
Watts: 160
Sensitivity: 95 db/w
FS: 27
VAS: 268 litres
QMS: 2.34
QES: 0.26
QTS: 0.24
Sd: 514 cm/2
EBP: 103.8

I have tried WINisd with the best looking graphs giving a Vb at about 40 litres with a port tuning of about 47 Hz. But without the necessary experience of translating graphs into an actual design I am hesitant to go any further. Also, the passive radiators that I have just acquired add another dimension to the possibilities.
I need to keep the box volume to a minimum so a certain amount of compromise will be inevitable. I'm thinking along the lines of Isobarik loaded active drivers with 2 passives on opposing side faces, but I'm probably wrong about this!
I'm not expecting to achieve a flat response to 20 Hz, as I know that this would not be possible with these drivers.

I would be prepared to make a donation to the forum if a good design could be created, if that is possible with these drivers; or are these drivers not worth the effort?

I could do with some help here.

Cheers and Happy Christmas

pacificblue 19th December 2012 07:01 PM

Specify "best sub design". Do you mean by best
- lowest flat response?
- optimised step response?
- optimised group delay?
- smallest enclosure? With 4 x 12" plus passive radiators probably not.
- high efficiency?

Be aware that some of those criteria are mutually exclusive. Be also aware of the speaker's interaction with the room, so the best design for one room and position therein may not be the best design for another room or for a different position in the same room.

Due to the low crossover frequency on a subwoofer, it is often good to tune the vent or passive radiators slightly lower than in a standard woofer design. That way you get a lower flat response than with a tuning that would be considered perfect in normal circumstances. The slightly mistuned curve adds up with the crossover's roll-off favourably at the cost of efficiency. That means of course it will be difficult to devise a design based on theory only. You will need to resort to the proven method of trial & error, i.e. build, measure, listen, change, measure, listen until you are satisfied.

Four drivers in Isobarik have little to no advantage over using only two drivers in a normal fashion.

If you can afford to put drivers on opposing sides, you should mount active drivers on opposing sides, not actives on one and passives on the other. That helps to cancel out their effects on the enclosure.

With 4 x 12" it should be possible to achieve 20 Hz or even lower with a bit of equalising or a Linkwitz transform.

jerryo 20th December 2012 10:27 AM

Hi pacificblue,
thanks for your reply.
I understand and take note of what you say, but I do not want to get too technical with these drivers as they were cheap "old stock clearance" drivers, and therefore do not warrant
a large amount of time on. I can model a basic vented set up, but as I said, I lack the experience of translating graphs into actual designs. When modelling, a number of graphs/frequency responses are given according to changes in box and vent parameters that look like they would be viable options, but which one is something that I need help with.
Maybe it would be better if I simply asked
"what would you do (anyone) with 4 bass drivers with the T/S parameters given"

I tried a "Ripole" set up, but was not convinced really, especially with the overall efficiency losses. The drivers have an EBP of 103.8 which suggests a vented box, so I thought that a Linkwitz transform would not be an option as this only applies to sealed boxes.

chris661 20th December 2012 10:30 AM

Quote:

Originally Posted by pacificblue (Post 3291724)

Four drivers in Isobarik have little to no advantage over using only two drivers in a normal fashion.

An isobaric pair require half the volume of a single driver.

2x pairs of isobaric pairs (ie, 4 drivers) would require the volume of 1 driver, but have twice the displacement capability, and 4x the power handling.

Seems like a useful advantage to me.

jerryo 20th December 2012 10:49 AM

Hi Chris661,
yeah, I would agree with that, and is why I thought it might be an option for reducing box volume. I'm not going to be lacking in amplifier power!
Cheers

sreten 20th December 2012 11:13 AM

Hi,

Any driver that has realtively high efficiency isn't go to make a great
subwoofer and really should be used a a bass unit in the sort of
speaker it was designed for, here PA / disco types I'd guess.

40L per driver tuned to 40Hz is doable. Isobaric is possible,
and two B139 PR's might suit one driver or two clamshelled.

Hard to say without knowing the drivers excursion capability.

As your after small and isobaric clobbers sensitivity, two isobaric
boxes with two PR's each is probably the way to go, 20L to 30L.

A sub plate amplifier with a bass boost option is probably needed.
Your simply not going to get deep extended bass from these drivers
due to the low Qts requiring boxes much smaller than the driver Vas.

rgds, sreten.

jerryo 20th December 2012 11:49 AM

Thanks sreten,
this is what I had kind of come up with myself, so it is nice to have someone with more knowledge to come to the same conclusion. The next step would be for me to ask about the passive radiator loading; going by what sreten has said, what tuning to go for?

pacificblue 20th December 2012 06:46 PM

Quote:

Originally Posted by chris661 (Post 3292497)
2x pairs of isobaric pairs (ie, 4 drivers) would require the volume of 1 driver, but have twice the displacement capability, and 4x the power handling.

They require more volume than one driver, because there are additional chambers between the drivers of each isobaric pair. For each 12" woofer they would at least be 10 l, probably more. The resulting enclosure will not be much smaller than a non-isobaric speaker with two drivers which will also have twice the displacement capability of a single driver. Advantage isobaric, but not as big as expected.

The isobaric version with four drivers has twice the power handling of a non-isobaric speaker with two drivers, but also only half the efficiency. Advantage non-isobaric due to lower power requirements.

A non-isobaric enclosure is easier to design and build and therefore less expensive. Advantage non-isobaric.

sreten 20th December 2012 07:26 PM

Hi,

For use as a 4x12 bass guitar cabinet its a no brainer,
but please don't try and tell me about isobaric loading.

I know the territory and the consequences, clamshell
needs extra volume to clear one driver magnet, and
it cancels even harmonic distortion, so called doubling.

More to the point the 4 B139 PR's are a lot more likely to
suit isobaric pairs (2 each) than any other arrangement.

Minimising box volume and using the PR's points to isobaric.

rgds, sreten.

I'd expect the PR's tuning to moreorless fall into place,
though if they don't you can always add some mass.

chris661 20th December 2012 09:41 PM

Quote:

Originally Posted by pacificblue (Post 3292971)
They require more volume than one driver, because there are additional chambers between the drivers of each isobaric pair. For each 12" woofer they would at least be 10 l, probably more. The resulting enclosure will not be much smaller than a non-isobaric speaker with two drivers which will also have twice the displacement capability of a single driver. Advantage isobaric, but not as big as expected.

The isobaric version with four drivers has twice the power handling of a non-isobaric speaker with two drivers, but also only half the efficiency. Advantage non-isobaric due to lower power requirements.

A non-isobaric enclosure is easier to design and build and therefore less expensive. Advantage non-isobaric.

Sure, yes, some (tiny compared to cabinet) extra volume will be needed for the driver motor structure etc.

Comparing 2x pair of isobaric vs 2x normal drivers isn't a fair comparison: the former will require half the box size, so of course will be less efficient.

Designing an isobaric cabinet is as easy as a normal one, as is the construction. No advantage either way.


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