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Wayne's HT Subwoofer
AKA the Rump
Thumper III
Project,
Text and Images courtesy of Wayne J. of Speakerbuilder.net
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 obtrusive 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.)
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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; 2) recommendations by others who
have used it and been very happy with it; and 3) 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.
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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 have 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 woofer.
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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. Here is the measured result:
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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.
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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.
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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.
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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. Here's what we learn from the impedance
curve:
This 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 occurring
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 undampened 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 our 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!)
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