sealed volume question

[xman]

Ok, first off, thanks to all who helped me with my last question. So I
have decided to replicate my last system(sort of) which was two sony
xplod 12's with a kenwood mono amp. This time it's the 9152D powering
two 12' XS-L123P5B's. The only difference is when I did this in my 2001
cavalier, I had more trunk space and used a ported box. This time it's
a sealed box in my Trans Am since that's all I can fit for my price
range, and the total sealed volume of the box is 3.50' cubed. The subs
each require 1.15' cubed. what effect will this larger than recommended
box have on my sq and spl?

[Cyrus]

In article m>,
"xman" > wrote:

> Ok, first off, thanks to all who helped me with my last question. So I
> have decided to replicate my last system(sort of) which was two sony
> xplod 12's with a kenwood mono amp. This time it's the 9152D powering
> two 12' XS-L123P5B's. The only difference is when I did this in my 2001
> cavalier, I had more trunk space and used a ported box. This time it's
> a sealed box in my Trans Am since that's all I can fit for my price
> range, and the total sealed volume of the box is 3.50' cubed. The subs
> each require 1.15' cubed. what effect will this larger than recommended
> box have on my sq and spl?


The larger airspace will make the system more sensitive. How this
affects your woofers depends on the woofers.

--
-Cyrus

*coughcasaucedoprodigynetcough*

[D.Kreft]

xman wrote:

> Ok, first off, thanks to all who helped me with my last question. So I
> have decided to replicate my last system(sort of) which was two sony
> xplod 12's with a kenwood mono amp. This time it's the 9152D powering
> two 12' XS-L123P5B's. The only difference is when I did this in my 2001
> cavalier, I had more trunk space and used a ported box. This time it's
> a sealed box in my Trans Am since that's all I can fit for my price
> range, and the total sealed volume of the box is 3.50' cubed. The subs
> each require 1.15' cubed. what effect will this larger than recommended
> box have on my sq and spl?

You'll have to model it to see precisely what effect it will have, but
as a general rule of thumb, what will happen is that the larger a
sealed box is, the lower your f3 (your "3dB down point"--the point at
which output drops by 3dB SPL) will be and the flatter the response
will be. A larger box will also increase damping and may make the box
overdamped (it'll sound flat, lifeless and dry) if too large.

The other effect will be reduced mechanical power handling. Think of
the mass of air behind your subwoofer as a spring--the larger the box
is, the looser the spring is and the less control it will exert on the
driver. As this "spring" gets looser, the driver's suspension will have
to bear more of the burden of controlling the driver and it makes it
easier to bottom out against the backplate, especially if the speaker
doesn't have a generously bumped backplate.

If the volume of your box exceeds Vas (volume of the acousstic
suspesion) of the driver, you will officially have an infinite baffle
setup, which ain't gonna be good for your driver's life expectancy if
you've got a lot of power.

Conversely, as the box gets smaller, the f3 climbs, the response gets
more of a bump in it (i.e. is more efficient) just above f3, damping
goes down and mechanical power handling goes up.

If you have an opportunity to model the speaker in the enclosure you're
talking about, do so. Typical sealed box setups sound best with a Qtc
of between 0.8 and 1.1...but this is, of course, *very* driver
dependant and your mileage may vary.

One thing to remember is that your vehicle will substantially boost the
perceived low frequency response of your system, and with the shallow
2nd-order (12db/octave) rolloff below f3, you can get away with a
higher f3 than you would in a sealed box and still have a really
sweet-sounding setup. If you make your f3 too low, your vehicle's
transfer function may make your system sound way too heavy in the
ultra-low limits of human hearing, which will make for an unpleasant
listening experience.

Here's a tutorial that I wrote many moons ago on this very topic (at
least I *think* I wrote this one--I know I did the pictures of the
boxes but I get turned around with some of them as Manville Smith wrote
a few, too, and it *has* been 10 years :-):

http://mobile.jlaudio.com/support_pages.php?page_id=149

If you've never read the JL Audio tutorials, I highly suggest you do
so; they're not all about JL Audio subwoofers and there is a lot of
good stuff in there (if I do say so myself).

-dan

[D.Kreft]

Cyrus wrote:

> The larger airspace will make the system more sensitive. How this
> affects your woofers depends on the woofers.

Careful, Cyrus--that's not really true. What you are perceiving as
increased efficiency is probably the lower f3 and the stretching of the
bandwidth of the box attendant with increasing the enclosure's size.

In the realm of life and engineering especially, you never get
something for nothing. If you want high efficiency, you have to give up
something--in this case the tradeoff is in bandwidth. So, if you want a
system to play really, really loud you'll have to limit the frequencies
it will play. Conversely, if you want to play a wide band of
frequencies, you give up efficiency.

This is where the Qtc or the "Q of the total cabinet" measurement comes
into play. A "high Q" system will be very efficient, but will have
limited bandwidth capabilities. A "low Q" system will have a very flat,
broad response curve, but will be less efficient. For example, a tuning
fork has a super high Q--it resonantes at one and only one frequency no
matter how hard you hit it or with what you hit it; a pillow, on the
other hand, has a very low Q--it doesn't want to vibrate no matter what
you do to it or how hard you shake it.

If you ever get the chance to play around with modeling subwoofers in
single-reflex bandpass enclosures
(http://mobile.jlaudio.com/support_pages.php?page_id=151), you get to
see this very clearly because not only is there a low f3, but there is
another at the higher end of the frequency reponse. It's fun to see
just how you have to balance efficiency and bandwidh--it's possible to
make an enclosure so effficient it'll blow your doors off powered only
by your head unit (that's hyperbole, by the way), but it'll have a
response curve that looks like the Seattle Space Needle and will only
play one note. :-)

The key is to find the sweet spot where the driver is in its Happy
Place--not to simply say "bigger is more efficient" because aside from
being a generalization, it's not technically true. :-)

-dan

[The Lull]

Download WinISD.

It's a free program, that won't 'invade' your PC, that you can enter
all the parameters of your speakers and get a good idea about what's
going to happen in any given box before you even build it. I find it a
very useful tool.

~The Lull
xman wrote:
> Ok, first off, thanks to all who helped me with my last question. So I
> have decided to replicate my last system(sort of) which was two sony
> xplod 12's with a kenwood mono amp. This time it's the 9152D powering
> two 12' XS-L123P5B's. The only difference is when I did this in my 2001
> cavalier, I had more trunk space and used a ported box. This time it's
> a sealed box in my Trans Am since that's all I can fit for my price
> range, and the total sealed volume of the box is 3.50' cubed. The subs
> each require 1.15' cubed. what effect will this larger than recommended
> box have on my sq and spl?

[D.Kreft]

The Lull wrote:

> It's a free program, that won't 'invade' your PC, that you can enter
> all the parameters of your speakers and get a good idea about what's
> going to happen in any given box before you even build it. I find it a
> very useful tool.

Oh yeah, one thing to keep in mind as well--a pretty plot does not
necessarily translate into a good-sounding box. It's very possible
(especially with more complex enclosure designs) to have a frequency
response plot that looks stellar, but the group delay (the time it
takes for the system to respond to incoming signals) plot looks lilke
garbage.

All in all, when first modeling speakers with which you're not
familiar, it'd behoove you to solicit the feedback of those who are
experienced with designing enclosures for your brand of subs....unless,
of course, you like to live on the bleeding edge and don't mind
tinkering. :-)

-dan

[The Lull]

I'm sure you're right. I should qualify the use of WinISD thus: I have
used it to create several ported (simple) boxes with great accuracy.
The program doesn't do ALL the thinking for you, but it helps immensly.
~The Lull
D.Kreft wrote:
> The Lull wrote:
>
> > It's a free program, that won't 'invade' your PC, that you can enter
> > all the parameters of your speakers and get a good idea about what's
> > going to happen in any given box before you even build it. I find it a
> > very useful tool.
>
> Oh yeah, one thing to keep in mind as well--a pretty plot does not
> necessarily translate into a good-sounding box. It's very possible
> (especially with more complex enclosure designs) to have a frequency
> response plot that looks stellar, but the group delay (the time it
> takes for the system to respond to incoming signals) plot looks lilke
> garbage.
>
> All in all, when first modeling speakers with which you're not
> familiar, it'd behoove you to solicit the feedback of those who are
> experienced with designing enclosures for your brand of subs....unless,
> of course, you like to live on the bleeding edge and don't mind
> tinkering. :-)
>
> -dan