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Discussion in 'Home Theater Projects' started by Mike SJ, May 20, 2004.
What are your goals for the system? Are you meaning to make two physically separate boxes so that you can have stereo subwoofers? Or is this still going to be one enclosure, with an internal partition (perhaps to strengthen the box)? If it's still going to be just one subwoofer, I can see no real advantage to dividing the volume internally, for a sealed box anyhow. If it was vented, you could tune the two chambers differently to play with the overall response. Your best sealed performance in a single dual woofer box will be from using a pull pull arrangement. This is where one woofer is physically reversed from the other (ie magnet hanging outside the box). Then the drivers are wired electrically out of phase (regardless of being in series or parallel). This will reduce distortion, though how significantly depends on the driver being used. I'm not familiar with the woofers in question, but they have quite a large Xmax, so distortion should be fairly low already.
You may lose some ultimate output capability by going with stereo subwoofers vs a dual driver subwoofer placed in a corner, but that would be strictly as a result of room placement. Stereo subwoofers can result in smoother response, in my experience, and also let you use a higher crossover frequency if so desired (assuming you can place them near their respective satellites).
Hope that answers some of your questions.
Yes, it sounds like your current setup is indeed a push-pull design. It sounds like you have everything you need to easily do a stereo subwoofer setup, so I say go for it! Even if the sound doesn't improve (and I personally am guessing it will), the improved appearance will make it seem better. You could always do laminate on just the sides/top and have the rest painted black, that's not a bad look.
60 liter sealed seems big for that driver. The cab previously being vented accounts for the large size. If you want to go sealed cut the cab volume in half.
I'm not aware of any corelation between surround thickness and enclosure type. There are other factors including total mechanical suspension (surround, spider, weight) along with the electrical parameters.
If you don't post pictures of after the cabinets are made by a professional cabinet maker, you will never get any help from people on this board anymore.
The old rule of thumb was that a vented cabinet requires less woofer excursion for a given output level than sealed. While this is still true, it's also true that a longer linear excursion (Xmax), regardless of cabinet type, will reduce distortion, and give you greater power handling and output capability. A thicker surround comes along with a greater Xmax. The reason you don't often see super high excursion woofers in vented boxes (at least for retail-sold speakers) is the boxes are typically too small to properly house a vent at the required tuning frquency. A tiny 12" cube is a pretty big selling point, and lots of amplifier power is cheap and relatively compact these days, so throw a super high excursion woofer in a tiny box, apply 1000+ watts, equalize the hell out of it, and voila! Some go for passive radiator designs, such as the Definitive Super Cubes, or the Sunfire True subwoofers. A passive radiator operates essentially the same as a vent, but with no vent noise problems, and takes up very little cabinet depth.
So anyhow, to answer your question, you should generally see less movement of the driver if the system is vented. The exception to this is if you feed it information considerably below the tuning frequency, which will generate much more excursion, and if it's at a high enough volume, can actually damage the woofer.
Using your numbers; in a 30 liter box F3 will be ~35Hz. In a 60 liter box F3 is the same, however, the rolloff begins much, much sooner which will result in less overall bass response. Power handling of the 60 liter will be more like that of a vented box. 30 liter Q = .6, 60 liter Q = .72.
A couple of things. Your in room response will be lower than 35 Hz. F3 is the just first -3db point. F6 is 25 Hz. You'll only gain a little lower frequency adding the second driver. What you'll gain is SPL. The small sealed box will be better for music. If you want loud and low for HT then go vented. Your original box size is correct for a vented 2 driver design. A two driver sealed box would be 60L. A vented single box would be 60L. The push-pull design provides better driver control and cancels the mechanical effects of the drivers on the box.
The push pull drivers don't need to be perpendicular, as with the M & K, they can be on the same plane. I believe M & K does this strictly for appearance, and frankly, it's impressive that they do it at all, as nobody else I know of gets away with it in retail speakers. Required box volume will be the same regardless of which plane their are on.