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-   -   Please help me understand this subwoofer design (http://www.diyaudio.com/forums/subwoofers/148034-please-help-me-understand-subwoofer-design.html)

Arno 25th July 2009 04:45 PM

Please help me understand this subwoofer design
 
Hello to all,

First post but long time reader, and let me say that this is an awesome forum.

Second I apologize for the poor quality of my cell phone pictures, as well as my poor english which is not my native language.

So, this is about a car subwoofer, specifically, the Porsche Boxster Bose box. The Boxster is a mid rear engine, 2 seats cabriolet, with no room for a decent subwoofer box. Porsche proposed as an option a Bose speaker system, with an enclosure that sits on the shelf on top of the engine, just behind the seats. When the top is down, the roof retracts on top of this box. The box is shallow, about 4".

I don't have picture of the box in the car, but here it is standing alone :

http://i32.tinypic.com/6qhzir.jpg

http://i28.tinypic.com/jin8k6.jpg

When we turn the box over, we can see the two 5.25" woofers :

http://i28.tinypic.com/erj4f5.jpg

When in the car, the woofer faces the engine shelf carpet, and thus there's only about 1/2" between the woofer and the carpet :

http://i26.tinypic.com/20a3mt1.jpg

Inside the box :

http://i30.tinypic.com/2j4yj2c.jpg

http://i26.tinypic.com/dn29tl.jpg

Another view of one of the woofers :

http://i30.tinypic.com/15g8bjq.jpg

I know the basic theory of closed, vented, passband subwoofer enclosures and I understand how they are designed given a speaker's T/S specs.

But here I don't understand how this particular enclosure is working, as the woofers basically face a wall, and I guess only rear waves are heard through the vent. Is this a passband enclosure with a very small front chamber?

Has anyone any clue on how this type of enclosure work?

The box pictured above is a friend's, not mine. I don't have this expensive option in my car, and I'd like to design my own subwoofer enclosure based on a similar design (because there's no room elsewhere in the car), but to do so I need to understand how it works. I guess this subwoofer will probably sounds like crap (the Bose one blurps more than it thumps) but it's a trade-off I'm willing to try.

Last question, would it be worthwhile to change the woofers in my friend's box? I guess he would have to estimate the T/S of the Bose speaker first, don't you think?

Tanks in advanced for your replies.

jbell 25th July 2009 10:53 PM

Looks like a standard vented box to me... I'd ignore the 'flare' as I'm sure it does not do much, but you can model the port and flare in hornresp if you'd like.

I'm assuming your car looks something like this?
http://www.renntech.org/forums/index...0&#entry148543 This guy used focal 5" subs.

If you've got 5" of height, I'd build a sealed box, with the front face at 5" tall, with about 4 of these front facing, not down facing. http://www.parts-express.com/pe/show...number=264-831 You can trim the steel basket top and bottom to save a little height. I know you said 4", but somehow, I'm guessing 5" might be doable?

You'll also need to follow it up with some bass shakers in your seats. (I know, blasphemy) As ear level bass doesn't 'feel' right. With the shakers turned way down, and crossed very low, it'll fool your ear into hearing what you're missing with the top down.

my 2c. I'm sure others will have better suggestions.

ThyDntWntMusic 26th July 2009 03:24 AM

nope, I'm calling that box 6th order bandpass.. series tuned?

there is a port next to each woofer as well, so when the box is put on the carpeted shelf, you get another chamber infront of the woofers, thats vented into the main chamber the woofers are in :)

http://www.diysubwoofers.org/bnd/6thord4.htm

oh... wait.. the ports go the whole way though the box...

so, its just a standard 6th order... :)

Nanook 26th July 2009 04:40 AM

yup, looks like typical schlock
 
from Bose...

it does look like a 6th order bandpass.

Why not stick to something tried and true? Maybe an Isobaric enclosure as found in the Boogieman sub project ? Basically 36" X 24" X 4" using the previously mentioned Tangbands. Stick with 2 of them and you could get that all into 36" X 12" X 4" , close to the space you have...

or have a look at the LAT line array woofs... .

Those could be very interesting and suitable for very small enclosures...

jbell 26th July 2009 02:06 PM

I'm not buying the 6th order bandpass. There is nothing to define a chamber, or a port. If anything, it's a standard vented box, with 'acoustic loading.' Measure the bose driver T/S params, and measure the amount of air in front of the driver, add that mass to your measured driver mass to modify your T/S. (search will turn up a great thread on adding mass to a 55-2421) Considering air is 1.2g/liter approx, and you have less than a liter of air in front of the driver, I suspect that the few hz of lower fs, accomplished by the acoustic loading would be inaudible vs if it were not there.

boogieman isobaric??
isobaric is driver to driver arrangement, the boogieman is standard vented enclosure, not isobaric.

The TB have a reasonable xmax, unlike the bose, so up or down facing would be problematic, and prone to driver damage -- forward facing is the only practical arrangement in my opinion.

Nanook 27th July 2009 03:58 AM

oops, my bad...
 
2 Attachment(s)
I meant to say an Isobaric version of the Boogieman with 2 woofers total....would easily fit and could be made to perform well given the limited space.

chris661 28th July 2009 09:01 PM

I disagree with 6th order bandpass aswell. They are two tuned chambers - one for each side of the driver. Putting the "woofers" to the carpet would give a similar effect to slot loading a driver, which reduces excursion. If the carpet blocks it completely, it's a 4th order bandpass.

TBH, I'd class it as a 4th order bandpass with pass ive radiator. The --unbraced-- plastic cabinet would happily ring along, like a PR.

Joking aside, out of the car, it's a simple, vented box. In the car, it would appear to be either an acoustically damped vented, or 4th order bandpass, dependant on if there's a route for the front of the "woofer" to your ears.

BTW - it's a Porsche Boxster... www.lmgtfy.com/?q=porsche boxster
So space will be at a premium.

Chris

PS - I say woofer with quote marks because there's no way I'd class a speaker that small as a woofer.

ThyDntWntMusic 28th July 2009 09:38 PM

Quote:

Originally posted by chris661


PS - I say woofer with quote marks because there's no way I'd class a speaker that small as a woofer.


of course its a woofer!

I had two 4inch WOOFERS doing 126dB at 44Hz, in car of course, but they really weren't woofers! they had an fs of 100hz!

Nanook 28th July 2009 10:34 PM

DIY subwoofer classes it as a
 
6th order series tuned system .

Regardless, There's gotta be a better way..simple 5" or 6" cardboard tube and a couple of small woofers such as the Tangbands....I'm sure it'd sound a lot better ...

chris661 29th July 2009 01:55 PM

I agree with the Tangbands idea, but I'm not buying the idea of http://www.diysubwoofers.org/bnd/6thord4.htm , because the port fires forwards, the drivers fire downwards, into a carpet.

Something like http://www.parts-express.com/project...oject=Firebird might be fun to try.

Chris


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