[Robert Roland]

I want to conserve as much luggage space as possible when installing
my sub, so I have been thinking about this solution:

Install the driver in the rear shelf, just like the 6x9s, but in
addition build a (sealed) box that is hanging under the shelf into the
boot. The box would have a short "neck" the same diameter (or a little
larger) as the driver to reach through the shelf, and be relatively
wide and flat to invade the boo as little as possible.

Is there any reason this is a bad idea?

Would the "neck" cause any problem, like start acting like a bass
reflex vent or something?

Will the wide and flat box sound much different from an "ideal" box?

Will the sub opening inside the car sound worse than if installed in
the boot? I'd guess it'd sound better?
--
RoRo

[The Lull]

I believe there are a few pitfalls to avoid when doing something like
this.

First, in a manual I read at some point it stated that when putting a
sub in a box, it should have at LEAST three inches of clearance in
every direction from the bottom of the sub (inside the box) to operate
nominally. So it sounds like your hanging box has to be taller than
you might want it to be... [I wish I could remember where I read this,
because I'm sure to get flamed for saying I read it somewhere and not
being able to cite it... I've used this rule on several installs,
though, and feel that the logic is very sound (no pun intended)]

Second, if you're planning on making this a ported box, I would
recommend porting into the cab, the same direction that the sub is
firing. Otherwise you're porting into a different space and are
essentially making a very large, very strange bandpass box. If you're
making this a sealed box, then nevermind.

Third, GO FOR IT! I think that this is a kick-ass idea!!! My only
reservation about it, personally, would be that everyone is going to
see your sub...

~The Lull

Robert Roland wrote:
I want to conserve as much luggage space as possible when installing
my sub, so I have been thinking about this solution:

Install the driver in the rear shelf, just like the 6x9s, but in
addition build a (sealed) box that is hanging under the shelf into the
boot. The box would have a short "neck" the same diameter (or a little
larger) as the driver to reach through the shelf, and be relatively
wide and flat to invade the boo as little as possible.

Is there any reason this is a bad idea?

Would the "neck" cause any problem, like start acting like a bass
reflex vent or something?

Will the wide and flat box sound much different from an "ideal" box?

Will the sub opening inside the car sound worse than if installed in
the boot? I'd guess it'd sound better?
--
RoRo

[KU40]

that would be fine to do. If your box is going to be sealed, it doesn't
really matter what shape the box is in. The front wave is separated
from the rear wave (which gets trapped in the box), and that's all that
really counts. Don't really have to worry about standing waves due to
odd corners because they won't be going anywhere anyways.

As for the rearward clearance, Lull aluded to it but wasn't quite
complete. If your subwoofer has a vented pole piece (i.e.- a hole in
the back of the motor), you should allow one length of the diameter of
that hole to the back of the box for the air to escape and spread. so
if your hole is 2" wide, leave 2" between the motor and box. If it's
too close you'll likely get mechanical noise. If there is no vented
pole piece, however, there is no need to leave any room behind the
motor structure.


--
KU40

[D.Kreft]

KU40 wrote:

that would be fine to do. If your box is going to be sealed, it doesn't
really matter what shape the box is in. The front wave is separated
from the rear wave (which gets trapped in the box), and that's all that
really counts. Don't really have to worry about standing waves due to
odd corners because they won't be going anywhere anyways.

You don't have to worry about standing waves in a nominal sub
enclosure, period.

See the section entitled "Enclosure Shape" he

http://mobile.jlaudio.com/support_pages.php?page_id=164

As for the rearward clearance, Lull aluded to it but wasn't quite
complete. If your subwoofer has a vented pole piece (i.e.- a hole in
the back of the motor), you should allow one length of the diameter of
that hole to the back of the box for the air to escape and spread. so
if your hole is 2" wide, leave 2" between the motor and box. If it's
too close you'll likely get mechanical noise. If there is no vented
pole piece, however, there is no need to leave any room behind the
motor structure.

Clearance is *definitely* good and the mo' the mo' betta in high-power
situations, but what you've got here is another rule of thumb that can
be fudged a bit if need be. See any one of the JL Audio stealthboxes
for example. I can think of one in particular (and old design for
Mustangs--fit in the tire well) that had a piece of plexi over the pole
vent of a 10W5 that was definitely *way* closer than the rule of thumb
you cite. And to add heat to injury, it was also designed to house an
amplifier!

-dan

[Brandonb]

He could use a shallow mount sub such as the one from Pioneer or
Elemental Designs. And in the case of the ED sub, a rep from the company
on their forum says it only needs .5" of clearance between sub and box
and could potentially be as low as .25", but is not ideal. So something
like that... very shallow mount, very shallow enclosure, might not be
bad at all. Then just make a decent grill covering or redo the stock one
and good to go.

Brandonb


The Lull wrote:
I believe there are a few pitfalls to avoid when doing something like
this.

First, in a manual I read at some point it stated that when putting a
sub in a box, it should have at LEAST three inches of clearance in
every direction from the bottom of the sub (inside the box) to operate
nominally. So it sounds like your hanging box has to be taller than
you might want it to be... [I wish I could remember where I read this,
because I'm sure to get flamed for saying I read it somewhere and not
being able to cite it... I've used this rule on several installs,
though, and feel that the logic is very sound (no pun intended)]

Second, if you're planning on making this a ported box, I would
recommend porting into the cab, the same direction that the sub is
firing. Otherwise you're porting into a different space and are
essentially making a very large, very strange bandpass box. If you're
making this a sealed box, then nevermind.

Third, GO FOR IT! I think that this is a kick-ass idea!!! My only
reservation about it, personally, would be that everyone is going to
see your sub...

~The Lull

Robert Roland wrote:

I want to conserve as much luggage space as possible when installing
my sub, so I have been thinking about this solution:

Install the driver in the rear shelf, just like the 6x9s, but in
addition build a (sealed) box that is hanging under the shelf into the
boot. The box would have a short "neck" the same diameter (or a little
larger) as the driver to reach through the shelf, and be relatively
wide and flat to invade the boo as little as possible.

Is there any reason this is a bad idea?

Would the "neck" cause any problem, like start acting like a bass
reflex vent or something?

Will the wide and flat box sound much different from an "ideal" box?

Will the sub opening inside the car sound worse than if installed in
the boot? I'd guess it'd sound better?
--
RoRo

[KU40]

ok so maybe that guideline is a bit conservative. in fact, to simulate
what would happen if you cover a pole vent, simply put a woofer flat on
the floor and put some power to it. once enough power is applied, a
mechanical noise is likely to show up. lift one side of the sub off
the floor, and basically the moment you lift the edge off the floor,
the mechanical noise stops. normally not a ton of air moves through
there anyways, but I'd rather be safe than sorry when building an
enclosure if you can. if nothing else, just because it's extra time to
make a new box if by chance there is noise.


--
KU40

[jowens500]

I have done many cars the way you want to do yours, both ported and
sealed and they sound GREAT! If I owned a car, that is the way I would
do it(I have a truck by the way). I have some pics, but they are to
large for this forum, and I must say, I am not very good with
computers, so I don't know how to make them smaller. Sorry for the
stupidity!


--
jowens500

[D.Kreft]

On Oct 11, 6:04 pm, KU40
wrote:

ok so maybe that guideline is a bit conservative. in fact, to simulate
what would happen if you cover a pole vent, simply put a woofer flat on
the floor and put some power to it. once enough power is applied, a
mechanical noise is likely to show up. lift one side of the sub off
the floor, and basically the moment you lift the edge off the floor,
the mechanical noise stops.

I can't help but snicker at this a little bit.

It's not really fair to say "play your speaker in free-air and listen
to the noises it makes as it jumps up and down on your floor--that's
all because the pole vent is blocked." Whenever you play a subwoofer
outside of an enclosure, you're bound to hear noises, plugged vent or
not. The voice coil leads bumping up against the spider (or in some
cases, up against the cone) makes noise. Sometimes the air rushing
around sharp edges on a stamped-steel frame can be perceived as
mechanical noise. Some of that noise may be the voice coil former
violently slamming up against the backplate. And yes, some might be
from air rushing through the permeable material of the spider.

Regardless of where the noise comes from, it's pretty much a moot issue
because subwoofers are not played in free-air and in the absence of the
mids and highs. Of course your average car is going to be making enough
noises of its own to mask noises, too, unless you're really into it and
have Dynamatted the whole vehicle.

In real life, you put a box behind your driver and in that box you
typically stuff it with a bunch of polyfill or other fibrous damping
material which further absorbs those spurrious clicks, whistles and
whirs. Even in iso-loaded configurations (remember those days?) where
you have one or more drivers' baskets exposed to the listening
environment, you don't hear mechanical noises because the rest of the
system does a darn fine job masking that. Even if you put duct tape
over the pole vents (NOT recommended, by the way), you'd have to be a
bloomin' dog to hear mechanical noises.

The only serious concern, really, concerning obstructing the pole vents
is that in doing so, you wind up reducing the voice coil's ability to
dissipate heat, thus making thermal failures more likely.

there anyways, but I'd rather be safe than sorry when building an
enclosure if you can. if nothing else, just because it's extra time to
make a new box if by chance there is noise.

Yes, better safe than sorry, but not because of noise--but because of
thermal concerns.

-dan

[KU40]

I did not say "play your speaker in free-air and listen
to the noises it makes as it jumps up and down on your floor--that's
all because the pole vent is blocked."


--
KU40

[D.Kreft]

On Oct 11, 11:54 pm, KU40
wrote:

I did not say "play your speaker in free-air and listen
to the noises it makes as it jumps up and down on your floor--that's
all because the pole vent is blocked."

http://dictionary.reference.com/search?q=hyperbole

-dan

[KU40]

this is not a hyperbole, you took my quote and changed it. My example
specifically pointed to a pole vent noise, whereas yours could
encompass any noise, from tinsel lead slap to the sub actually bouncing
on the surface from the vibrations. A hyperbole might be "cover your
pole piece vent and your sub will sound like a jackhammer."


--
KU40

[D.Kreft]

On Oct 12, 8:20 am, KU40
wrote:

this is not a hyperbole, you took my quote and changed it.

If I were quoting you directly, I would have done so in typical USENET
style, just as I have done the line above. I was basically boiling down
your post into one line to show how silly the whole thing sounds.

My example specifically pointed to a pole vent noise, whereas yours could
encompass any noise, from tinsel lead slap to the sub actually bouncing
on the surface from the vibrations.

The real issue here is that your experiment was flawed.

If you lift up one corner of the driver, you change the physical
orientation of the driver and thus could have tilted the driver in such
a way as to make the lead wires stop slapping the surround. It's a
classic case of of the Heisenberg uncertainty principle...there's
nothing to say that in the particular scenario you cited, that your
intrusion into the system didn't alter the system itself.

A better way to examine your thesis would be to suspend the driver in
some fashion and put tape over the vent hole and compare that to the
same driver with the tape removed.

And just so we're all on the same page, I'm *not* suggesting that a
free-standing driver *won't* create any noise from a plugged pole vent.

A hyperbole might be "cover your pole piece vent and your sub will sound like a jackhammer."

I think you're missing the point--you're getting too tied up in the
semantics. Missing the forest for the trees, you are. The whole "pole
vent noise" thing is a non-issue in real-world situations. It's not
even worth considering.

-dan

[KU40]

D.Kreft Wrote:


And just so we're all on the same page, I'm *not* suggesting that a
free-standing driver *won't* create any noise from a plugged pole vent.

so maybe my experiment was flawed, but you just stated the point I was
trying to make. if a free standing driver *might* create noise from a
plugged pole vent, what's the difference between that and in a box with
the back panel covering the pole, effectively plugging it the same way
as the free-standing driver was?

If you (Mr. Kreft) had a box with a mounting depth capability equal to
that of the actual driver you wanted to use, so that the back plate was
touching or maybe within 1/8", would you use that box or build a new
one? base your answer as if thermal issues were nullified.


--
KU40

[D.Kreft]

KU40 wrote:

so maybe my experiment was flawed, but you just stated the point I was
trying to make. if a free standing driver *might* create noise from a
plugged pole vent, what's the difference between that and in a box with
the back panel covering the pole, effectively plugging it the same way
as the free-standing driver was?

The difference is simple: audibility.

If you (Mr. Kreft) had a box with a mounting depth capability equal to
that of the actual driver you wanted to use, so that the back plate was
touching or maybe within 1/8", would you use that box or build a new
one? base your answer as if thermal issues were nullified.

I smell a trap, but I'll bite anyway...

**Completely negating thermal concerns**, and assuming that the walls
of the box were sufficiently rigid so as not to vibrate against the
backplate, then then I would absolutely not hesitate to build such an
enclosure if it's all the space I had to work with....or, as is the
case with the JL Audio 18W6 where the recommended enclosure is barely
bigger than the cardboard box the driver came in (1.25 cu.ft., sealed),
if that's all that was needed.

-dan

[Robert Roland]

On 11 Oct 2006 11:25:26 -0700, "The Lull"
wrote:

First, in a manual I read at some point it stated that when putting a
sub in a box, it should have at LEAST three inches of clearance in
every direction from the bottom of the sub (inside the box) to operate
nominally.

Does that mean sideways all around the driver, or just around the
magnet area? The driver will be placed inside a tunnel which is an
inch or two deep, so the magnet will be in quite free air inside the
box.

So it sounds like your hanging box has to be taller than
you might want it to be...

I don't intend to make it extremely shallow, so clearance below the
driver will not be an issue. I haven't done any maths yet, but I
expect a few inches of clearance.

making this a sealed box, then nevermind.

Yes, I am planning on a sealed box.

Third, GO FOR IT! I think that this is a kick-ass idea!!! My only
reservation about it, personally, would be that everyone is going to
see your sub...

Yes, I have thought of that as well. I'll be putting black carpet on
the shelf and a black grill over the sub, so it shouldn't look too
bad. It's only a ten-inch sub, and it is no fancy brand, so theft
should not be too much to worry about, either.

Thanks to everyone who replied. I will start building the box as soon
as the shelf itself is done.
--
RoRo

[D.Kreft]

Robert Roland wrote:

Install the driver in the rear shelf, just like the 6x9s, but in
addition build a (sealed) box that is hanging under the shelf into the
boot. The box would have a short "neck" the same diameter (or a little
larger) as the driver to reach through the shelf, and be relatively
wide and flat to invade the boo as little as possible.

Is there any reason this is a bad idea?

I can forsee two difficulties:

1. Getting the walls of your (presumably wooden) enclosure to
meet-up perfectly all around the top with an uneven metal surface, and
getting that sealed perfectly; and

2. Deck vibration.

I don't know what kind of car you have nor how big your subs are, but
keep in mind that a rear parcel shelf was not designed to hold
substantial weight nor provide a vibration-free platform for
subwoofers. In the world of loudspeaker cabinet design, vibration is
murder because it not only colors the sound of the subwoofer (highly
subjective), but it also creates another source of energy loss--if your
enclosures are jiggling, your subs are not acting in the most efficient
mode.

I'm not saying it can't be done, nor am I saying that it won't sound
good--JL Audio has a couple of Stealthboxes (which aren't reeally boxes
at all, but rather slick mounting pods for their 8" free-air subwoofer:
http://mobile.jlaudio.com/products_s...model_id=1 40)
which I hear work quite well--only that you need to be prepared to deal
with these potential issues. If your car isn't as solidly-constructed
as a BMW, you *may* be in for a rough ride.

Also worth considering is how well the rear deck and your enclosures
will fare on seriously bumpy roads. The last thing you want is to have
the screws rip out of your MDF and have your subs hit the floor of your
"boot".

-dan

[KU40]

D.Kreft Wrote:

I smell a trap, but I'll bite anyway...

**Completely negating thermal concerns**, and assuming that the walls
of the box were sufficiently rigid so as not to vibrate against the
backplate, then then I would absolutely not hesitate to build such an
enclosure if it's all the space I had to work with....or, as is the
case with the JL Audio 18W6 where the recommended enclosure is barely
bigger than the cardboard box the driver came in (1.25 cu.ft.,
sealed),
if that's all that was needed.

-dan

Fair enough.


--
KU40

[Robert Roland]

On 13 Oct 2006 14:16:08 -0700, "D.Kreft" wrote:
1. Getting the walls of your (presumably wooden) enclosure to
meet-up perfectly all around the top with an uneven metal surface, and
getting that sealed perfectly; and

The plan is to cut a hole in the metal, big enough to allow attaching
the box directly to the shelf. I'll still need some way of getting a
good seal between the shelf and box, but I have a couple of ideas.

2. Deck vibration.

I don't know what kind of car you have nor how big your subs are, but
keep in mind that a rear parcel shelf was not designed to hold
substantial weight nor provide a vibration-free platform for
subwoofers.

I am going to chuck out the stock "cardboard" shelf and build a new
one from 16mm MDF.

If your car isn't as solidly-constructed
as a BMW, you *may* be in for a rough ride.

It's an older Audi, so it should not be too bad.

Also worth considering is how well the rear deck and your enclosures
will fare on seriously bumpy roads.

Yes, I have thought about that, too. The driver will rest on top of
the rear shelf, so that part of the weight will be plenty supported.

The problem is going to be the weight of the box itself. I'd like the
box to be removable, so glue is out of the question. I am considering
using threaded rod from the top of the shelf all the way through to
the bottom of the box. That way, the steel rods could also serve as
bracing if nuts and washers are placed at strategic places.
Alternatively, the rods could end inside the box and conventional
bracing be used. The problem, of course, is going to be looks. Steel
nuts visible on the shelf would be seriously un-pretty. Maybe some
nice looking caps over the nuts would be acceptable. I'll se what I
can find or make. I have a metal lathe, so I have options.

Thanks for you comments.
--
RoRo