My wife and I have been working on our "home theater" and it came time
to replace our 4.6 ft^3 "coffee table" sub. The old sub worked very well
(JLAudio 12W6DVC driver with PE 300-800 sub amp, tuned to 23hz). With the
new decor, however, the obstrusive sub wasn't going to fit. My wife
requested what all wives request -- "can't you build it so that it doesn't
take up any space in the room and I don't see it, but it shakes the room
like nobody's business?" My response, "of course, dear." (I've been
married long enough to know that there is only ONE correct response... and
her question, of course, was rhetorical anyway.)
My first concern was whether or not to use my JL Audio
driver in another enclosure or to purchase a new sub driver. There was no
question that I would continue to use my #300-800 subwoofer
amp from
Parts Express. It's an excellent
amp (although I needed to disable the bass boost filter. If you choose to
use this amp, ask the tech folks at PE for instructions -- it takes about
2 minutes and some wire snips to make the modification.) When I tested the
TS parameters of the JL Audio sub, it appeared obvious to me that I was
not going to get the type of performance I was looking for (sub-25hz
response, high excursion, high SPL) from my current driver. When I checked
around at what was available (Shiva, Titanic 1200, NHT1259, etc.) I
decided on the Dayton
12" DVC because of 1) price and 2) recommendations by others who have
used it and been very happy with it. Also, having used a number of Dayton
speakers in the past, I felt that I could trust this driver to do what
they said it would do. As it turns out, it's actually an even
better HT sub driver than the specs indicate.
A powered subwoofer is not a very complicated device. You have the
driver, the enclosure, and the amp. There is no crossover needed because
that's built into the amp (and usually defeated because of the built-in
crossover used by the HT receiver. Now that the driver and amp were
determined, the only thing left to do was to build the box. Unfortunately,
as I've learned from having built many subwoofers in the past, building the
enclosure for a sub is easily the most sensitive of all enclosure types.
Maybe there aren't as many variables to consider when building a
subwoofer, but these types of enclosures need to have very low box losses
(i.e., they must be well dampened, well sealed, and solidly constructed.)
Also, the alignment and parameters you choose should be tailored to the
driver you select. One of the best aspects of using the Dayton drivers is
that the published T/S parameters are very, very close to what you can
expect when you measure your driver (if you have the capability.) So if
don't have fancy measurement equipment, you will get good results
designing your enclosure around a Dayton subwoofer.
Since I use CLIO
to measure all my drivers, I tested the T/S parameters of the woofer after
breaking it in with a 15hz sine wave @ 2W for about 10 hours.
Using this information, I then designed the enclosure to fit the
requirements that my wife had placed on the design -- the sub had to fit
in the back of the room, behind the couch, under the shelf. This space
gave me all the length I could want (up to 10 feet) but only 14" of width
and about 15" of height (where the power outlets are.) So I had a tough
task. Given the wall thickness, I needed to keep the internal width of the
box to 12". Also, when you put an enclosure in such a confined space, you
will get a very large amount of "room gain effect." This means that the
very lowest frequencies will be amplified because the room, itself, acts
as an extension of the enclosure. If you attempt to use the "perfect"
enclosure suggested by your box design program, you will have a very large
"hump" in the frequency response of the driver under about 30hz. I used
Calsod to model the room gain effects and found that the best alignment
for this enclosure was an Sbb4 tuned to 19hz with a volume of 3.7 ft^3
(104.5L). The internal dimensions of the enclosure were 12"x12"x48" -- 4
cubic feet GROSS volume, which I arrived at by adding the needed box
volume with the volume of the bracing, the port, and the driver. Then I
subtract 7% to account for the "apparent volume increase" that occurs from
the addition of dampening foam on all sides of the enclosure. This shows a
"predicted" response that falls off more quickly than one would expect...
but when room gain is added in, it is flat to about 15hz.
Also, a box that long needs extensive bracing. I factored into the
volume calculations 3 horizontal braces and 1"x1" runners between the
braces. This makes the box very, very solid (and quite heavy). The runners
are covered by the dampening
foam in the picture below, but you can see then overall structure of
the design:
All of the internal seams are sealed with silicon adhesive, including
the inner surfaces of the braces and the seam where the vent tube (4"id x
18" long) is supported by the cross brace. The foam is fixed to the
enclosure walls with 3M
spray adhesive.
Prior to adding
the dampening material and sealing the enclosure, the impedance of the
enclosure was measured to make sure that it was close to the tuning
frequency.
The impedance
curve shows a tuning frequency of just a hair under 20hz (which will
drop due to the "apparent box volume increase" when we add the dampening
material.) We also see that the driver loading and unloading above and
below the resonant frequency of the box (i.e., the reflex action) is
excellent -- you can tell this from the very small difference in the
minimum impedance between the "double-humps" that are typical of a vented
enclosure and the impedance minimum occuring after the double humps (in
this case around 65hz.) Finally, if you look at the shape of the impedance
peaks, you see that they are tall and skinny. For an undamped and unsealed
box, we are doing very good.
The impedance of the finished enclosure shows even lower box losses
(which, btw, forced me to recalculate whether the box size was now
appearing too big to the woofer. It was. But rather than reduce the
volume, I chose to shave a bit off the port to bring Fs back up to the
target.)
When the enclosure was completed (note that no external finish was
required because this box will be completely hidden from view in the room)
it had an extraordinarily deep, transparent bass output that is perfect
for home theater use. We, of course, tried a few of the typical tests --
the footsteps in Godzilla, the THX intro is DTS mode, etc., etc. But where
this sub really shocked us was in the "ghost train" scene from
Ghostbusters II. With out old sub, we could certainly feel/hear the train.
With this sub, you felt as though you were Winston Zedemore and you had
just been slimed. On the other end of the spectrum, we were equally
impressed with how this driver "disappears" -- upper bass notes come
across cleanly and smoothly without the "boom" that you hear from retail,
mass-produced HT subs.
So, if you are debating whether to buy a retail subwoofer or build your
own, there should really be no question -- get yourself a dayton 12" dvc,
the 300-800 amp, and either build an enclosure or buy
this one and add a vent port (the nice folks on the PE Tech Talk board will
be more than happy to help you out, as will I!)
