In order to get the most performance from these non-optimum applications we have taken a different design approach. Vehicle interiors have limited space, so we started our research on making the most bass energy from a specific amount of space. The key is to extract maximum energy from a limited resource, cargo space.

We engineered a high efficiency type of ported box, then set about engineering a speaker to optimize the box energy. We keep the moving mass low, energy high and add a suspension with high restoring force to help control cone motion. This means that the subs will not have to rely on the box for their loading.

The vehicle itself will offer some loading to the system, and the transfer function will make the system play an octave lower than the tuning freq. So, if you could find some speakers that could stay in control below the box tuning, subs that are really tuned to the box loading plus the car loading, you could design a system to play low, loud and do it in a relatively small box. That is the DDbox.

Each Sub Series we make is designed to work in the DDbox system. Each size sub in the series is designed for the same airspace and same response. They all get the same motor, the same spiders, suspension throw and power handling. The 10 inch size will play just like the 15, but do it in a much smaller space.


^ft = cubic feet | sq” = square inch

All volumes listed are after vent, and subwoofer displacement; a.k.a. NET volume This chart is for tuning boxes in the 35-40Hz range.

Volume Number & Size of Drivers Port Area & Length
0.35^ft (1) 6.5″ 5.25 sq” vent x 24.5″ long
0.50^ft (1-2) 6.5″ 7.0 sq” vent x 24″ long
0.75^ft (2) 6.5″ or (1) 8″ 11.25 sq” vent x 23.5″ long
1.00^ft (2-3) 6.5″ or (1) 8″ 15.0 sq” vent x 23″ long
1.25^ft (3-4) 6.5″ or (1) 8″ or (1) 10″ 20.0 sq” vent x 29″ long
1.50^ft (3-4) 6.5″ or (2) 8″ or (1) 10″ 24.0 sq” vent x 28.5″ long
1.75^ft (4) 6.5″ or (2) 8″ or (1) 12″ 28.0 sq” vent x 28″ long
2.00^ft (4) 6.5″ or (2) 8″ or (1) 12″ 32.0 sq” vent x 27.5″ long
2.25^ft (2-3) 8″ or (1) 12″ 36.0 sq” vent x 27″ long
2.50^ft (3) 8″ or (2) 10″ or (1) 12″ 40.0 sq” vent x 26.5″ long
2.75^ft (3) 8″ or (2) 10″ or (1) 12″ 44.0 sq” vent x 26.5″ long
3.00^ft (3-4) 8″ or (2) 10″ or (1) 15″ 48.0 sq” vent x 26″ long
3.25^ft (3-4) 8″ or (1) 15″ 52.0 sq” vent x 25.5″ long
3.50^ft (3-4) 8″ or (2) 12″ or (1) 15″ 56.0 sq” vent x 25.5″ long
3.75^ft (3-4) 8″ or (3) 10″ or (2) 12″ or (1) 15″ 60.0 sq” vent x 25″ long
4.00^ft (4) 8″ or (3) 10″ or (2) 12″ or (1) 15″ 64.0 sq” vent x 24.5″ long
4.25^ft (4) 8″ or (3) 10″ or (2) 12″ or (1) 15″ 68.0 sq” vent x 24.5″ long
4.50^ft (4) 8″ or (3) 10″ or (2) 12″ or (1) 15″ 72.0 sq” vent x 24″ long
4.75^ft (4) 8″ or (2) 12″ or (1) 15″ 76.0 sq” vent x 23.5″ long
5.00^ft (4) 8″ or 10″ or (2) 12″ or (1) 15″ or 18″ 80.0 sq” vent x 23.5″ long
5.25^ft (4) 10″ or (2-3) 12″ or (1) 18″ 84.0 sq” vent x 23″ long
5.50^ft (4) 10″ or (2-3) 12″ or (1) 18″ 88.0 sq” vent x 23″ long
5.75^ft (4) 10″ or (3) 12″ or (1) 18″ 92.0 sq” vent x 22.5″ long
6.00^ft (4) 10″ or (3) 12″ or (2) 15″ or (1) 18″ 96.0 sq” vent x 22″ long
 ……  ……  ……
8.00^ft (3-4) 12″ or (2) 15″ or (1) 21″ 128.0 sq” vent x 20″ long
  1. Any available space larger than above chart, simply use the chart to determine what multiples of subwoofers you can choose from.
  2. The above info and chart works with all DD Series woofers.
  3. We recommend to allow at least several weeks of playing time to break in DD Woofers, full break in may take many months. DO NOT play subwoofers at max amplifier power in the first couple of days, give the suspensions a chance to loosen up. As the suspension breaks in, the subs will play lower and louder.
  4. After you give the woofers a chance to break in (you’ll know when this happens because the woofers will be playing lower and louder) you can start to look at port changes to alter the sound to your personal taste and your vehicles acoustic loading conditions.


The DDport was developed with the DDbox to increase the overall efficiency of the system and broaden the power response curve as discussed previously. In order to keep a 3-1 ratio of cone area to port area, use the following formula:

16 square inches of port area per cubic foot of box volume, make the port 18-20 inches deep. The port is tuned to the box volume, not the subs.

This generally tunes the box to the upper 30Hz area, with the vehicle transfer function extending the frequency another 10-12Hz or so. This is the most popular all purpose tuning.

If the system is primarily playing bass tracks and electronically massaged music, extend the port length in the 24-28 inch length. The box will lose a little upper frequency response while adding extending the low frequencies.

Once you determine how much space you have available, divide the sub(s) into that space. Don’t cram the space, its better to error on the big side than the small side. A 10″ sub will outperform a 12″ sub if the space isn’t big enough.

The proper combination of a vented enclosure coupled to a driver that is suited for this application can give a substantial increase in output for a given input power and yield outstanding SQ. A vented enclosure has increased output due to the fact that it has an increased effective radiating area.


Getting Started

This 4 step process will show how to design a subwoofer system for your vehicle. For many, this seems like a daunting challenge for mathematicians and alchemists, blending the acoustic voodoo arts with the Pythagorean Theorem. Its really not that hard, and before you bail on the idea in favor of calling on someone to do the work for you, read on a bit and have some fun designing your own system.

The benefit of doing the design yourself is you get to call all the shots; you get the system done like you really want. We have spent years working on the acoustic/enclosure design of the equation, leaving you to enjoy the creative side of your system design.

Our DDBox system takes all the guesswork out of the sound produced, just follow the size/port charts and formulas. Keep in mind that airspace is what makes the bass. Follow the hyperlinks for more in-depth tech sections.

As discussed in the DDBox tech page, DD woofers are built specifically for duty in the DDBox system, the woofers and boxes were developed together to extract the highest efficiency from a given box volume.

Don’t start the process with a predetermined idea of what sub size is best, just because your cousin heard a system when he was on vacation at his Uncle Lewey’s house and caught a ride with a guy who heard a system once that blew the rear tire off a car with a 6×9 in a cigar box, doesn’t mean 6x9s are the only speaker to design your system around (not cigar boxes).

Let the size and shape of the box determine the quantity and size of the subs.


Step 1: Measuring for the Box

You will need to determine where in the car you can put the subwoofer enclosure and the general amount of space you are willing to give up for the enclosure. Everyone hauls around different stuff, and your type of stuff should be taken into consideration as the system is layed out.

Once you have picked the perfect sub spot, you’ll need to determine the height(H), width(W) and depth(D) of the available space. These three dimensions will determine what sub possibilities can be used.

If you measured out a box, say, 13 high, 32 wide and 10 inches deep, you can pretty much rule out the use of 18″ and 15″ subwoofers. This is known as the Karloffsenson Paradox, the famous Swedish engineer who determined the optimum meatball size for a given cooking pot while desiring a much larger meatball, he went on to invent meatloaf and his paradox subsequently goes pretty much unnoticed. But, the points still remain that once you define your space, the subs geometry must fit the box and bigger woofers don’t make bigger sound if the box volume is not correct.

Now, in the self-important minority of the world, we use a measuring system based on an old dude’s foot, divided into 12 equal pieces, called inches, because it didn’t make sense to divide things by 10. It was decided to then further chop these inches in half, and half again, and half again until the numbers get uncomfortably large, or small, however you look at it. This is known as the Imperial System, implying a very important foot was measured and worthy then of unquestioning blind support for centuries to come.

If you are from the self-important majority of the world, you might be using a measuring system developed in France, if you really need more reason to stick with dividing some guys foot into fractions of pieces……, if not, then you’ve decided to measure in metres divided my millies and orders of magnitude therein. To convert the superior imperial measurements to Vulcan like metrics, multiply inches by 2.54 for centimeters and for cubic feet, 28.3 liters per cubic foot.


Step 2: Calculate the Airspace

Space is a three dimensional thing, so we need three dimensions H x W x D, multiplied to calculate volume.

Step 2.1: 

We want to know the airspace inside of the box, because that is the amount of air that will be connected to the subwoofers. The box will be made out of some kind of structural material, commonly from wood because it is a renewable resource and we are environmentally responsible. It is also a fairly cheap resource due to many governmental subsidies and controversial forestry regulations which are responsible for all the forests of neatly rowed trees making trekking life easier on all the furry animals.

We must un-calculate the wall thickness from the outer dimensions, 3/4″( or three quarters of an inch for the metric thinkers. The little dash dashes are the symbol for an inch, not misplaced accent marks to make the pronunciation of the vowel A into a different A sound. To make the math easier on a calculator, we convert the fractions of an inch to a decimal equivalent, the true superiority of the Imperial system starts to show, dividing 3 by 4 to get — 0.75″) wood being the most common thickness for enclosures. Our above box, 13H x 32W x 10D would need 1.5″ taken out per dimension, leaving 11.5H x 30.5W x 8.5D, representing the dimensions of the airspace residing in the enclosure.

We now multiply, 11.5″ x 30.5″ x 8.5″ = 2,981.375, to come up with cubic inches.

Step 2.2:

We want to convert cubic inches to cubic feet because this lets us use a smaller number, and smaller numbers are easier to use. A cubic foot, as defined by the Mendenhall Order in 1893, is 12″H x 12″W x 12″D.

Multiplying, 12″x12″x12″= 1728 cubic inches, which is the number of coinhabiting cubic inches that reside inside a box with aforementioned dimensions. If you are an advocate or begrudging user of the Imperial system, this 1728 number is worth remembering. It gets used a lot.

Now, divide our inner box volume by 1728 to gain the cubic foot equivalent;

2981.375/1728=1.72533275. Remember, accuracy to 8 decimal places is excruciatingly unnecessary, 2 places will do nicely. If there are metric guys still following along, you can use all 8 if you want to.

Our box has 1.73 cubic feet of airspace residing inside the enclosure walls.


Step 3: Converting Gross to Net Volume After Speaker and Port Displacements

Net volume is the amount of airspace after subtracting the volume of air occupied by the woofer and port. This is the amount of airspace with which the port is calculated. How is this done without knowing the volume of the port and sub(s)? Is this another, which came first, the chicken or the egg? It is, unlike estimating the timing sequence of evolutionary or whimsical spontaneous creation, we can estimate occupied volumes based on many cases of fixed calculations and come up with a pretty close percentage.

The occupied volume of the port and woofer, for 40Hz tuning and 35Hz tuning, are approximately 18% and 23% respectively. The larger 23% number for 35Hz tuning representing the longer port length for lower tuning, the longer port takes up more of the inner volume.

Multiplying the numbers from our box:
For 40 Hz tuning, 1.73cuft x 0.82 = 1.42 cubic feet of net volume.
For 35 Hz tuning: 1.73cuft x .77 = 1.33 cubic feet of net volume

Port area for a DDBox is 16″ of port area, per cubic foot. See “Why Go Ported”.

Using the 40 Hz tuning example, 1.42cuft x 16″ = 22.72 square inches of port area

Keep in mind that a 10% variation in port area will not be audible so there is a tolerance of plus or minus 2 square inches.


Step 4: Port Length

The sound of the system is somewhat tunable via changes to the port length. As we learned in the Why Go Ported section, the port area needs to be in proper ratio to the cone area, so the DDBox system uses variations to the port length as primary means of tuning frequency mods.

Refer to the DDBox site chart to 23″ long. This will enable a DDBox/sub combo to have effective bass response in the car, down into the low 30s. The DDBox plays all types of music very well and is a perfect tune to begin advanced dial in of your system after the subs break in.

The “break-in” period refers to the time it takes for the speaker’s spiders to loosen up, allowing more excursion. We build speakers with very strong suspensions so they can be enjoyed for years, as they loosen, the bass will become deeper and louder.

After the break-in, the tuning frequency can be lowered by adding length to the port, try 4 inch increments. Conversely, the tuning can be raised by shortening the port.

Step 4.1: Variations on tuning

Some users have specific, frequency enhanced, types of music they like to play. Some even like the way body panels, windshields and non-cartilage types of body parts flex to the beat, some cars offer a complete beat down to those occupants. These applications might skip straight to a longer port length, adding 6 inches to the tuning frequency charts. Some upper frequency response in the 60-80Hz range may be affected.

This added port length also works well for applications where there is poor acoustic loading in the vehicle, or in open room applications like DJ systems and home applications. The increased port mass helps to enhance the low frequency extension.

Some users would like to cut down on box volume knowing their DD system exceeds their dynamic range expectations. The port area can be reduced by 25%, to 12 square inches per cubic foot and using the lower range of DDBox sizes for a given subwoofer diameter. There is a slight sacrifice in maximum output but some very compact designs can sound amazing if calculated correctly.


The Compact Box Recommendation is for when a DD user has limited space, due to physical dimension limitations. If room for life has to be made along with the necessity for serious bass, the Compact Box Recommendation is the one for you. Remember, these drawings are just designs to base your enclosure on. These will work in 95% of even small sedan trunks.

These boxes are absolutely great for all types of music playback with plenty of low end response to give your tracks a more than live feel. Bass guitar, keyboards and drums will bring dramatic impact to the daily drive.

This box is a little less efficient than our Full Size Box; where the lowest octave plays stronger with low frequency enhanced bass. There is no substitute for cubic inches when it comes to making body flex demos. If you are after max energy, you are probably not looking for single sub applications anyway.

For standard to metric conversion, simply multiply dimensions by 2.54 to get units of centimeters. To convert cubic feet to litres, multiply volume by 28.316.


The Full Size Box Recommendation is for when a DD user’s system output takes priority over cargo capacity. The Full Size Box is more efficient down where modern Hip Hop and Bass CDs are going. Those that enjoy the virtues of singular sine-wave sinus cavitation experiments will benefit from the full size cans. If you’ve got the room and are willing to spare it, this is the size we recommend. A cool bonus is that you’ll draw a little less current from your amp…..call it…..”the green box”. Leave a smaller carbon footprint and all that good stuff.

For standard to metric conversion, simply multiply dimensions by 2.54 to get units of centimeters. To convert cubic feet to liters, multiply volume by 28.316.


DD Audio
4025 NW 36th Street
Oklahoma City, OK 73112
(405) 239-2800 | Fax: (405) 239-7100