Designing a subwoofer based on a known size.

[Warrant]

Now that my Garnachas are underway I want to add a couple of end table subs to the room. I have a specific size in mind but don't really understand the relationships beyond the most basic ideas.

Is there a formula that allows for volume changes to determine a driver choice?
An example; I would like to build boxes, one with external dimensions of 20"h x 30"w x 36"d and the other 20" x 24" x 36".

Does combining styles such as sealed and ported help or hinder sound quality? If I understand correctly a sealed unit responds more accurately at the expense volume and greater power consumption. Ported can be tuned lower, uses less power and slightly less accurate response in music (rapid transients?)

I seem to remember debate about down firing drivers. has it been determined that down firing shortens the life of the driver?

Its late so I'll stop here.

[Finleyville]

Please note: my comments come with limited amount of experience. More knowledgeable members will correct me and add better info.


While some subs can play equally fine sealed or ported some are designed to work best in one type of enclosure.

For simplicity sake I would recommend the same sub and enclosure type for each table. You usually do not want to "mix" sealed, ported, IB subs in the same install.

Most people with multiple sub setups build identical boxes. Your two box dimensions are different so I am assuming they will have to fit into specific areas of your room. If that is true, then I would just internally reduce the volume of the bigger one to match the smaller one.

The good news is that your planned volume allows for a wide range of driver sizes and choices. 10"-15" at least. (Maybe even 18"?) Plus room for an internal plate amp for each one.

The specific driver and enclosure type dictates the required volume for the total sub. So in a way you are working a little backwards. No problem there. I would choose perhaps the largest driver and enclosure type to closely match the smallest internal volume between the two boxes.

How much volume is needed you ask? That is where a computer program will help immensely. (I use UniBox - it's free!) You will have to enter the specific sub's measurements and choose an enclosure type. From there it will calculate the ideal internal volume needed and port length/size if you go with ported. So some trial and error will be needed on your part to determine the sub you want to match the enclosure. In any case, I would predict you will be using only a portion of the total inner volume of the boxes since they are so large.

As far as downfiring, over time (read: years) gravity will take its toll on any downfiring sub. However, some are designed for this and will hold up better. If you look at the sub pictures thread you will see some of our members make nice furniture out of their enclosures. Try PM'ing them for some additional innfo there.


Good luck!

[fbov]

Most of us use simulators to combine driver properties with box geometry and predict the results. There are some general rules of thumb regarding preferred alignments based on Qtc that might help with candidate driver selection, but modeling is the way to go. I use Unibox, but this site comtains all the tools you'd need to do an XO design.

http://www.pvconsultants.com/audio/frdgroup.htm

Unibox allows you to vary box volume and tuning type (sealed/ported/passive radiator/band pass) so you see the frequency response effect of each. Your understanding is in the right direction, but keep in mind that these are second order effects until you have a specific driver selected. "Combining styles" in the sense of a hybrid is possible, but it's an advanced topic.

Downfiring is fine as long as there's no more than 5% sag, and sag is easy to calculate. Unibox will calculate sag.

The one thing few people talk about is "room gain." As frequency drops, wavelengths become longer than the longest standing wave a room can support in a given direction. That's about 30Hz in my 22' room, but it starts at more like 75Hz with 7.5' ceilings. Here's a link to the application notes for my sub. It shows a variety of alignments, and includes a room gain prediction. You'll note that room gain nicely complements a sealed alignment.

http://www.diycable.com/main/pdf/TX2Apps.pdf

If you look close, you will see differences between this 3 cu ft box sim and my simulation, below. I have a Tempest X, these are for the X2.

Finally, you talk about "boxes." That's a good idea. Subs do excite standing wave patterns within rooms and multiple sources is the only reliable way to reduce the resulting seat-to-seat variance. That's another story.

HAve fun,
Frank

PS +1 on two identical subs
+1 on driver selection criteria
and +1 on Unibox and simulation if that message didn't get through.

[BOBinGA]

If I did the math correctly, those are 8 and 10 cu.ft. boxes. Those are in the territory of 15" and 18" subs or at least 2 or even 3 twelves. Good luck getting spouse approval.

-Bob

[Warrant]

I am considering a pair of Mach5 18.2.2 or even 15.2.2 in a sealed opposed configuration that way they can be used as end tables without the lamps and glasses flying off.

[Warrant]

Ok, where's Unibox for Dummies?