Woofer box in conversion van - porting?

[Roy Batty]

The place I want to put the box has a max dimensions of 48" wide,
10.5" high, and 7" deep. So these are the limits of the outside
dimensions of the box. Can't make it any larger without it getting in
the way. It's going to go under the front edge of the rear couch. If
it is any deeper, people's legs will rest on the upper corner.

I had my heart set on four 8" woofers which require 0.55 cu ft each.
Two will be on one side, two on the other, with the enclosure walled
off in between the pairs - the amp driving these four woofers
(Panasonic CY-PA2003U) will have each side driving a pair of woofers,
and a 6x9 (one on each side, mounted in the wall just ahead of the
couch). The front part of the van will have its own amp, same model.

Problem is, when I do the math for a sealed box, I come nowhere near
the 1.1 cu ft per side that each pair of 8's will need. I might get to
0.89 cu ft per side depending on what board thickness I use, but I was
hoping to use 3/4" MDF. Would 1/2" thick MDF be feasible?

So, it comes down to a ported box if I want to make the available
space work. This port calculator page I found seems to require a port
length that would simply not be possible:

http://www.carstereo.com/help/Articles.cfm?id=31

With two 8" woofers, and a tuning freq of 30 hz, the first part gives
me a port diameter of 3.46".

Entering this value into the Vent Length calculator, keeping the
tuning frequency at 30 hz, and the enclosure volume at 0.89 cu ft, I
get a Vent Length of 29.1 inches! Bah!

Is it simply not possible to vent a box that small?

On the other hand, a sealed box that is 20% smaller than it should be
- is that necessarily a deal breaker? Doesn't it provide some measure
of safety against cone overexcursion? Will it really be taking that
much performance out of the woofers? Will it be "good enough" is what
I'm getting at, I guess.

Thanks very much for any insight!

[Matt Ion]

Roy Batty wrote:
> The place I want to put the box has a max dimensions of 48" wide,
> 10.5" high, and 7" deep. So these are the limits of the outside
> dimensions of the box. Can't make it any larger without it getting in
> the way. It's going to go under the front edge of the rear couch. If
> it is any deeper, people's legs will rest on the upper corner.
>
> I had my heart set on four 8" woofers which require 0.55 cu ft each.
> Two will be on one side, two on the other, with the enclosure walled
> off in between the pairs -

Er... so you're going to have two pair of back-to-back drivers, with
each pair sharing the same airspace? I dunno how well that would
work... to my thinking you're asking for cancellation problems at the
very least.

I'd put all four subs in the same side of the box, all facing out the
front or back (I'd aim them back, personally, to avoid passengers
kicking them), with four separate chambers of 10.5x12x7 - that gets you
0.51 cu. ft. each (not including the volume taken by the drivers
themselves), and not accounting for the thickness of the wood, assuming
all these measurements are external.

> the amp driving these four woofers
> (Panasonic CY-PA2003U) will have each side driving a pair of woofers,

I can't find anything that suggests this amp is two-ohm stable, which it
would need to be for you to parallel two speakers on a channel (assuming
these are four-ohm woofers). All the specs on Panasonic's site
(http://tinyurl.com/3as7c6) are at 4 ohms. It does say it will handle
four ohms bridged, so you could bridge it and run all four drivers in a
parallel-series setup.

> and a 6x9 (one on each side, mounted in the wall just ahead of the
> couch).

WRONG.

For one, you have enough problems properly loading this amp with the
subs, without adding another driver.

Second, you'd have to use a passive crossover with that, which will just
muddle things even further and waste what little power that amp has
(despite the name, Panasonic only rates it for 200W/ch RMS).

> The front part of the van will have its own amp, same model.

You'd be better, if you insist on using these amps, to run the subs off
one, and parallel the front and rear speakers off the other. It will
still likely be overloading the front amp, but it won't be as bad as
trying to hammer all the bass through it that way.

> Problem is, when I do the math for a sealed box, I come nowhere near
> the 1.1 cu ft per side that each pair of 8's will need. I might get to
> 0.89 cu ft per side depending on what board thickness I use, but I was
> hoping to use 3/4" MDF. Would 1/2" thick MDF be feasible?

Not recommended unless you want to add significant bracing to it, which
will probably eat up more of your interior volume than just using 3/4"
to begin with.

> So, it comes down to a ported box if I want to make the available
> space work. This port calculator page I found seems to require a port
> length that would simply not be possible:
>
> http://www.carstereo.com/help/Articles.cfm?id=31

Ported boxes for a given sub are typically larger than the sealed design
for that same sub...

> With two 8" woofers, and a tuning freq of 30 hz, the first part gives
> me a port diameter of 3.46".
>
> Entering this value into the Vent Length calculator, keeping the
> tuning frequency at 30 hz, and the enclosure volume at 0.89 cu ft, I
> get a Vent Length of 29.1 inches! Bah!
>
> Is it simply not possible to vent a box that small?

It might not be with those particular subs... perhaps you should look
into different subs?

> On the other hand, a sealed box that is 20% smaller than it should be
> - is that necessarily a deal breaker? Doesn't it provide some measure
> of safety against cone overexcursion? Will it really be taking that
> much performance out of the woofers? Will it be "good enough" is what
> I'm getting at, I guess.

It might. Really depends on the drivers.

Given the amps being used and the space available, you may want to
consider just using a pair of 8s. This will allow you bigger enclosures
for each, which if you want to go ported should allow you smaller port
dimensions.

Other than that, I'd be looking at different drivers that will work in
smaller boxes, and different amps that will handle the loads you want to
throw at them.

[D.Kreft]

On Jun 3, 1:03 pm, Matt Ion <soundy...@gmail.com> wrote:

> Er... so you're going to have two pair of back-to-back drivers, with
> each pair sharing the same airspace? I dunno how well that would
> work... to my thinking you're asking for cancellation problems at the
> very least.

It won't be a problem so long as the woofers are wired properly (so
that all go *out* at the same time). The enclosure is so small that
the drivers will be operating in the "pressure domain" and thus you
don't have to worry about wave mechanics.

> It might not be with those particular subs... perhaps you should look
> into different subs?

Another option would be to isoabarically load them. Sure, it'll reduce
output by 3dB SPL, but when you're in a really tight spot, sometimes
it's the only way to go when you've already got a set of subwoofers on
hand.

I'd recommend the original poster read the JL Audio tutorials on this
topic to see if an iso-loaded setup is right for him:

http://mobile.jlaudio.com/support_pages.php?page_id=146

> Given the amps being used and the space available, you may want to
> consider just using a pair of 8s. This will allow you bigger enclosures
> for each, which if you want to go ported should allow you smaller port
> dimensions.

Or that, too. :-) This might be the best of all options...leave the
two remaining subs in your closet as spares. :-)

-dan

[Matt Ion]

D.Kreft wrote:
> On Jun 3, 1:03 pm, Matt Ion <soundy...@gmail.com> wrote:
>
>> Er... so you're going to have two pair of back-to-back drivers, with
>> each pair sharing the same airspace? I dunno how well that would
>> work... to my thinking you're asking for cancellation problems at the
>> very least.
>
> It won't be a problem so long as the woofers are wired properly (so
> that all go *out* at the same time). The enclosure is so small that
> the drivers will be operating in the "pressure domain" and thus you
> don't have to worry about wave mechanics.

I was thinking more of the waves OUTside the box...

>> It might not be with those particular subs... perhaps you should look
>> into different subs?
>
> Another option would be to isoabarically load them. Sure, it'll reduce
> output by 3dB SPL, but when you're in a really tight spot, sometimes
> it's the only way to go when you've already got a set of subwoofers on
> hand.

Thought of that too, but you still have the issue of proper loading of
the amp.

I've also found (in the distant past, granted) that cheap subs don't
like being loaded in isobaric pairs - they tend to fall apart rather
quickly. I have a sneaking suspicion that we're not dealing with
top-of-the-line here.

>> Given the amps being used and the space available, you may want to
>> consider just using a pair of 8s. This will allow you bigger enclosures
>> for each, which if you want to go ported should allow you smaller port
>> dimensions.
>
> Or that, too. :-) This might be the best of all options...leave the
> two remaining subs in your closet as spares. :-)

Yeah, ready to swap in when one of the others blows up :)

[D.Kreft]

On Jun 3, 10:06 pm, Matt Ion <soundy...@gmail.com> wrote:

> I was thinking more of the waves OUTside the box...

Yeah, that scenario likely wouldn't be ideal, but then again,
considering the wavelengths we're talking about in relation to the
interior dimensions of the vehicle, it probably would have a
negligible deleterious effect.

> I've also found (in the distant past, granted) that cheap subs don't
> like being loaded in isobaric pairs - they tend to fall apart rather
> quickly. I have a sneaking suspicion that we're not dealing with
> top-of-the-line here.

I'm not sure why that would be--I don't *think* there's anything more
stressful about face-to-face isoloading of drivers over conventional
loading. The only thing that does come to mind that will cause major
problems is forgetting in put a 5/8" (or so) spacer ring between the
subs. Forget to do that and the surrounds will rub against each other
leading to failure (i.e. a big hole or a blazing inferno :-).

> Yeah, ready to swap in when one of the others blows up :)

Computer geeks call this "N+1 redundancy"...only this would be "N+2",
so it's even more better. :-)

-dan

[Matt Ion]

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