Parts Express The #1 Internet Source Of Audio, Video and Speaker Building    
Components, For Home, Office, Mobile And Prosound.    
  Call: 800-338-0531 Home
Site Last Updated  


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.)

 

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.

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.

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:

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. 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!)
DISCLAIMER - Parts Express posts the "Project Showcase" as a community service. Projects displayed have not been tested or evaluated by our staff. We make no warranties whatsoever in regard to the projects' fitness for a particular purpose. Claims of performance or fitness are those of the contributor and not of Parts Express. Project re-production for commercial resale may be a violation of United States patent or trademark laws.

Copyright 2001 Parts Express