Exterior to Net Internal Dimensions
HOW TO QUICKLY DETERMINE NET INTERNAL VOLUME USING EXTERNAL DIMENSIONS.This section will show you how to quickly determine the net internal volume you have with three maximum external dimensions. The following step-by-step instructions, along with examples should prove to be easy to learn, memorize and duplicate time after time. The most important thing we want to point out is that this formula works and will produce high-energy enclosures every time. Note: this formula only works for "DD enclosures" using our port & bracing recommendations. Enclosures that use far less port area will come up with a Net Internal Volume that will not match our formula.
1st: K.I.S.S. - Keep It Simple Stupid. Despite whatever makes some of us try to account for the displacement of a single spec of MDF dust that we could not get out of the box; the K.I.S.S. method will work for street beating enclosures way better than we few want to admit. We want to believe all the high-tech, mumbo-jumbo that we feed our customers so much that we actually practice the precise, displacement accounting techniques. With the information below and a little bit of your time plotting out about 10 mock boxes of your own made-up dimensions, you'll find this method to ring true. We want to warn those types of folks that the truth may be too hard to swallow and ask that you read no further in case of cranial-flatulent symptoms!
We first have to measure the trunk for our maximum external dimensions. Note that just because the box may fit IN the trunk does not mean that you can fit the assembled box THROUGH the front or rear trunk openings. Once we have determined the proper Width, Height and Depth, we can use the following simple formula to get a pretty accurate net internal volume that accounts for bracing, subwoofer and port displacements.
Let's use the external dimensions of 32" for Width, 18" for Depth and 14" for Height. If you multiply them together, you get what is commonly referred to as the Gross External Volume of 8064 cubic inches. After finding the Gross External Volume, you now need to figure out the Gross Internal Volume. Our formula is based on the use of the industry standard ¾" MDF. The Gross Internal Volume is the amount of volume left after the thickness of the externals walls is accounted for. To find this number, you simply need to multiply the Gross External value by 0.77. We determined that this number is the one to use after mocking up roughly 20 enclosure sizes (5 cubic feet or less), with all sorts of external dimensions. It works every time; trust us and read on if your head is not hurting yet. The magic number you should have come up with is 6209 with the remainder cut off. Now you need to know what the Net Internal Volume is so that you can calculate your proper port size. How do you do this without knowing how much volume the port occupies first? Check it out! You multiply 6209 by 0.82 to get the Net Internal Volume. This 0.82 will account for the subwoofer, bracing and port displacement; again, all determined by the common use of 3/4" MDF. This was the number we came up with after taking those 20 or so boxes and applying our port recommendations, different bracing techniques and different series DD subwoofers which all have different displacement volumes. 0.82 came up as the average. The number you should have come up with is 5092 cubic inches, rounded up. Divide 5092 cubic inches by the ever-popular 1728 to determine the Net Internal Volume in cubic feet. That value in this particular instance is roughly 2.95 cubic feet. If you refer to our chart of boxes for high efficiency, you'll find that 2.95 cubic feet is great for a pair of 10's, three or four 8's or in some instances, a single 15" subwoofer. Which sub or subs you choose depends on many factors such as budget, appearance, weight etc. Let's do another one real quick without all the words.
W=28, D=16.5, H=13.5
28 x 16.5 x 13.5 = 6237
6237 x 0.77 = 4802
4802 x 0.82 = 3938
3938 / 1728 = 2.28 Net Internal Volume in cubic feet.
Now to find the port area, using our recommended formula, you multiply the Net Internal Volume of 2.28 by 16 to get 36.5 square inches. Since we know the internal height of the enclosure and subsequently the port is both 12", we can divide 36.5 by 12 to get the port width of 3.04". None of us want to go through the hassle of measuring out that 0.04", so you just drop it. If you built both the rounded-off version and the anally-accounted-for version and listened to them with 200 different songs in your car, you would not be able to hear the difference between the two.
All of these numbers easily come within 10% of the exact calculated values and due to the non-computerized way the human ear perceives auditory stimuli, we cannot easily determine a difference at all. That is the rule of thumb followed with this formula. As long as you come up with a variance of 10% or less, your ear cannot differentiate. To test the numbers, we'll revert back to the original box dimensions.
32 x 18 x 14 = 8064
8064 x 0.77 = 6209
6209 x 0.82 = 5092
5092 / 1728 = 2.95 Net Internal Volume in cubic feet
2.95 x 16 = 47 square inches of port area
Take 1.5" off of each external measurement to get the "real" internal dimensions of 30.5 x 16.5 x 12.5 and multiply them together to get a Gross Internal Volume of 6290 cubic inches. We are off by 81 cubic inches as compared to the formula. Now you have to take out the volume of the port
which we know is 47.2 square inches plus the cross-sectional area of the
internal port wall which equals 9.375 square inches to equal a gross port
cross-sectional area of 56.575 square inches x our recommended length of 16
inches minus the .75" thickness of the front baffle to come up with a product of
719.8 cubic inches of displacement within the enclosure. Subtract 719.8 from
6290 and you should have 5570.2 cubic inches. Now we have to determine which
sub(s) to use so that we know how much volume they occupy and how much volume to
add to the final Net Internal Volume by accounting for the negative displacement
of the speaker cutouts in the front baffle. Let's make it simple and call it a
day with a pair of 12's that have a cutout of 11" diameter each. If you do the
math, the holes add 142.55 cubic inches to the enclosure to bring it back up to
5427.65 cubic inches of Net Internal Volume. Let's say that the subwoofers we
chose each have a displacement of .125 cubic feet each. The pair of subs would
displace 432 cubic inches. Now we have to subtract 432 from our 5427.65 to come
up with 4995.65 cubic inches. To find the final Net Internal Volume, we have to
determine what the exact displacement of our chosen internal bracing is. Let's
use two 4" wide strips of 3/4" MDF; one from top to bottom and one from front to
back to keep the enclosure from flexing. After doing the math, you'll find that
the two braces displace exactly 87 cubic inches. 4908.65 cubic inches should be
the number you get. Divide that by 1728 to get the Net Internal Volume of 2.84
cubic feet. As you may have noticed, our formula's 2.95 net cubic feet and the
exact volume of 2.84 are well within that 10% margin where the human ear could
not differentiate the two. When you get closer to the 5 net cubic feet mark, the
formula may work out to be less volume than the exact calculated volume. As port
lengths vary, such as tuning lower by extending them, the formula can easily be
modified by changing the percentages just a tad. The more you work with the
enclosures and modify them slightly here and there, the more familiar you will
become with which way to modify the formula.
If you are stuck on doing 2 hours of math for a box that should be quickly calculated and built within 45 minutes of you seeing the car, we highly recommend that you take the time to build your chosen enclosure by exact math and another by the formula and then compare the two