I see tons of posts here every week asking how to tune subwoofer boxes, why they should be tuned to certain frequencies and what affect this has on the way the box sounds. Hopefully, in this thread I can provide a base of information for those who need help with these common questions.
How big should my box be?
Your box should be big enough to allow the subwoofer to reach full excursion without being an infinate baffle (so large that it seems like its not in a box). This number will vary between subwoofers and is directly related to the subwoofer's VAS spec. The manufacturer of your woofer will usually tell you how large to make the box either in the paperwork that came with the woofer, on their website or you can usually obtain this info by calling their tech support line. If all else fails, ask somebody on the forum.
If you cannot provide a sub as much air as is recommended (small trunk, single cab trucks) you probably want to go with a sealed enclosure. Sealed enclosures do not require as much air space.
Subs will play in smaller boxes than normally recommended, but it usually means a sacrifice of bottom end. The box should be accordingly tuned to a higher frequency to compensate for this. If the box is going to be larger than recommended (and is not for SPL purposes) it should be tuned lower.
What frequency should I tune my sub to?
*NOTE* This is just a gerneral speculation. Some woofers are different and won't respond well to low/high tuning. If you're unsure how well your woofer will perform at a given tuning frequency, please consult the manufacturer of the suboofer or ask someone for advice. ***
If you're going for extreme sound quality and tonal accuracy, your best bet is to tune that sucker low. Anywhere from 26-30 hz should provide a fairly flat response curve on most woofers, and will assure that you don't miss any of the low-lows. This also assures that your woofer won't die from over-excursion unless you really abuse it.
If you're an SQL type of person and enjoy a little boom with your quality, you should probably go for a slightly peakier tuning frequency between 31-40hz. This will normally yeild a mild peak at notes within a few hz of the tuning frequency. It will make the bass slightly more boomy at the sacrifice of tonal accuracy. Over-excursion due to playing notes under tuning frequency can be a problem, but as long as you're careful to set the gains right and don't play at high volumes for extended periods, you'll be just fine.
The SPL types will want a high tuning frequency. Most SPL guys who compete tune their boxes in the 50's, 60's and even 70hz range. If you are willing to sacrifice sound quality for boom but don't want to over-do it, I'd suggest going for the 40-45hz range. It will almost always make for a nasty sounding enclosure, but it will peak a lot heavier than a lower tuned box. You also have to be very careful not to blow the subs by playing at high volumes below the tuning frequency.
How do I tune my box?
The formula I use comes from the JL Audio website (click here) and I consider it the gold standard. There are several online calculators like the one at BCAE, and calculators like WinISD, but they will more often than not produce different numbers than you would doing the formula by hand. Do the math twice and make sure that you figure for ALL displacements. Most subwoofer manufacturers tell you how much the woofer will displace, but if you can't find it on their website or in the literature that came with the driver, give them a call. The formula to figure the displacement of a slot port is ((W+T) x H x D), where W = width, H = height, D = depth and T = board thickness.
The rule of KISS (Keep It Simple, Stupid) applies to the design of woofer boxes. First, figure out how much air space the sub needs, then figure the port and add the displacement of the port and woofer to your total volume. Then and only then start figuring the outer dimensions. For instance, lets say my sub needs 3 cubic feet, the sub displaces .1 cubic feet and the port will displace .7 cubic feet. My car will fit a box that is 36" wide, 18" deep and 17" tall. I would make the enclosure 34" wide, 16" tall and 15.5" deep. That would fit inside my trunk nicely with a little room to wiggle it in and also produces a net volume of 3.8 cubic feet (assuming .75" board thickness). A great little calculator to figure the internal volume of square and wedge shaped boxes can be found here. DO NOT USE THEIR TUNING CALCULATOR.
Any other general advice?
Take your time and measure every thing twice. In fact, do all the math twice too. A screw up on paper isn't as hard to fix as one on wood. Not nearly as costly either.
Make sure your enclosure is sealed tight. The port should be the only part of the box that lets air escape. The easiest way to check for gaps is to take the box into a dark room and shine a flashlight inside it. If you can see any pinholes of light between the pieces of wood, you'll have to seal it better. I suggest running a bead of caulk around the inside of the box after it's finished just to be safe.
Never assume anything. If you can't prove it in writing, with math or from a trusted source, it's probably not correct. Taking shortcuts is never okay. Remember what our pal Ben Franklin once said: "A stitch in time saves nine".
If you are unsure of your mathematical skill or just don't like doing math, you should probably leave enclosure design to those with experience. Several people including myself offer box design services for a minimal fee. If you feel like you just need a little help or aren't sure if you're doing something right, never be afraid to ask. Trust me, it doesn't make you look dumb to ask questions about complex math formulas. In fact, people will respect you that much more for making the effort.
Donn, why are you so **** cool?
I dunno man, I just am.
If you guys have any other questions you'd like me to address, please feel free to ask and I'll edit this post as needed.