Tech Talk | WRITTEN BY AARON TRIMBLE | December 2017

Sure, that pair of 18’s will totally fit in your Honda Del Sol! Of course you can fit a 9512 under the seat in your Ford Ranger! Don’t worry, we’ll cut the box for your 15 in half to fit it in the trunk of your new Camaro! Yeah, some of these examples might be a bit hyperbolic, but the DD Audio Tech Support team does get some doozies. Today we’re going to look at what you can do to fit the biggest woofers in your car without sacrificing performance by forcing a woofer to go in an enclosure that’s just too small.

We’re going to assume that you want to maximize the performance of the space you have in your vehicle. After all, if you’re just looking for a little bass in your Tahoe, a pair of 8’s is still going to fit without having to sweat the numbers. With the handy dandy DD Box Size Graph we can determine what will fit.

DD BOX SIZES FOR HIGH EFFICIENCY SUBWOOFER SYSTEMS

Volume	Number and size of Drivers	Port Area and Length
0.75^ft	(1) 8″	11.25 sq” vent x 22″ long
1.00^ft	(1) 8″	15.0 sq” vent x 23″ long
1.25^ft	(1) 8″ or (1) 10″	20.0 sq” vent x 23″ long
1.50^ft	(2) 8″ or (1) 10″	24.0 sq” vent x 22″ long
1.75^ft	(2) 8″ or (1) 12″	28.0 sq” vent x 21.5″ long
2.00^ft	(2) 8″ or (1) 12″	32.0 sq” vent x 21″ long
2.25^ft	(2-3) 8″ or (1) 12″	36.0 sq” vent x 21″ long
2.50^ft	(3) 8″ or (2) 10″ or (1) 12″	40.0 sq” vent x 21″ long
2.75^ft	(3) 8″ or (2) 10″ or (1) 12″	44.0 sq” vent x 20.5″ long
3.00^ft	(3-4) 8″ or (2) 10″ or (1) 15″	48.0 sq” vent x 20″ long
3.25^ft	(4) 8″ or (2) 10″ or (1) 15″	52.0 sq” vent x 20″ long
3.50^ft	(4) 8″ or (2) 12″ or (1) 15″	56.0 sq” vent x 20″ long
3.75^ft	(3) 10″ or (2) 12″ or (1) 15″	60.0 sq” vent x 19.5″ long
4.00^ft	(3) 10″ or (2) 12″ or (1) 15″	64.0 sq” vent x 19″ long
4.25^ft	(3) 10″ or (2) 12″ or (1) 15″	68.0 sq” vent x 19″ long
4.50^ft	(3) 10″ or (2) 12″ or (1) 15″	72.0 sq” vent x 19″ long
4.75^ft	(3) 10″ or (2) 12″ or (1) 15″	76.0 sq” vent x 18.5″ long
5.00^ft	(3) 10″ or (2) 12″ or (1) 15″ or 18″	80.0 sq” vent x 18″ long

Source: http://ddaudio.com/support/dd-box-design/

We’ve gone through and outlined three DD employee vehicles with unique challenges to show what will fit in a car. For the sake of simplicity, these builds will assume that the car will remain stock and nothing in the vehicle will be removed.

2015 Subaru Impreza Sport Wagon

This guy is a basshead, he’s totally cool with giving up his whole cargo area to feed the beast, as long as it fits under the cargo cover. Now, there’s two ways to look at this vehicle. From wheel well to wheel well the trunk width is 42”, but you can only maintain that width for 14” of depth because the rear of the trunk tapers down to 36.5” at the hatch opening. If we only build the enclosure to be 36.5” wide we can increase the depth of the enclosure to 22” in depth. For either approach, we can build the enclosure as high as the cargo cover which is 15.5”. We’re going to construct this enclosure out of ¾” MDF, so we’re going to subtract our material thickness from all sides to get a realistic gross volume to work with.

Option 1:
External Dimensions – 42.0” x 14.0” x 15.5”
Internal Dimensions – 40.5” x 12.5” x 14.0” = 7087.5 cubic inches or 4.10 cubic feet

Option 2:
External Dimensions – 36.5” x 22.0” x 15.5”
Internal Dimensions – 35.0” x 20.5” x 14.0” = 10045.0 cubic inches of 5.81 cubic feet

Option 2 in this case allows us more airspace and is a dimension that allows the enclosure to be slid into the cargo area without having to fold down the seat and wrestle with it like Option 1 would. The other main difference with Option 2 is that we will be forced to fire the woofers up and the port up or to the side, vs being able to run everything firing back like we would be able to with option one due to the proximity of the enclosure to the rear hatch. So, 5.81 cubic feet is our gross airspace, from this we’ll need to subtract bracing, woofer/s, and port displacement. Those of us who’ve been on the DD Support pages will remember our easy displacement values, if not, you can read all about them on the DD Box Design Page. Those values are .82 and .77, the values are not absolutes when dealing with strange dimension enclosures, large bracing or excessively low tuning, but they’ll get us in the ballpark. So, if we multiply our 5.81 cubic feet by .77, we get an estimated 4.47 cubic feet. Now that we have loosely estimated our net volume we can use the Box Size Graph to figure out what woofers to put in our Subaru.

4.47 cubic feet is good for:
4 x 8” Subs
3 x 10” Subs
2 x 12” Subs
1 x 15” Subs

Selecting subs is as much preference as it can be performance. Yes, three 10” woofers is the most cone area, but if you don’t want to spend the money on three woofers and instead want to buy a single higher powered 15”, that’s a great option too. The problem arises when you look at your boot and say, “I want two 15”s in there!” Let’s look at another vehicle.

2017 Mazda CX-8

The CX-8 is a 7 seat, third row SUV. With the third row up, there’s less than 10” in depth. However, the owner of the Mazda says they want to really get some wind moving back there. So, we’re going to do our measurements with the third row seat down, which extends the depth to up to 36” deep! In this example we will again be using ¾” MDF for construction, and just like the Subaru, this car has a couple different ways we can approach the space. The space between the wheel wells is 40” and the opening at the hatch of the vehicle is 42”, so we’ll work with a fixed 40” width. The height and depth are our variables here. If we want to go for the full depth of the boot, we can make the enclosure 36” deep and 13” high before its corners will begin to hit the hatch when it’s closed. Alternatively we can decrease the depth to 32” and increase the height to the top of the second row seat (16 inches) which also allows us to fire the port back without and port loading issues.

Option 1:
External Dimensions – 40.0” x 36.0” x 13.0”
Internal Dimensions – 38.5” x 34.5” x 11.5” = 15274.9 cubic inches or 8.84 cubic feet

Option 2:
External Dimensions – 40.0” x 32.0” x 16.0”
Internal Dimension – 38.5” x 30.5” x 14.5” = 17026.6 cubic inches or 9.85 cubic feet

Option 2 is going to be the best option for us here since it’s larger and still allows us to fire woofer(s) up and the port back in a classic SUV style enclosure. Using our .77 to give an estimated space occupation for woofer(s) and port, 9.85 cubic feet gross will become 7.58 cubic feet net.

7.58 cubic feet is good for:
6 x 10”
4 x 12”
2 x 15”
1 x 18”

With this much space there’s a lot of different combinations of woofers that can be made. For example, DD 12’s work well in enclosures from 1.75 to 2.75 cubic foot enclosures depending on model and applied power. So, in 7.5 cubic feet, depending on performance goals you could do three or four 12” subs. If the debate was between three or four 12” subs, there may be some mitigating factors to consider. If you’re going to be using an M3c (3800 watts at 1 ohm), you could best capitalize on this power with four 2500 series 12’s (3200 watts RMS) or three 3500 series 12’s (3600 watts RMS). The 2500’s will save you $100 or so and have more cone area, but the 3500’s have larger coils and can come with ESP upgrades, so it’s a matter of weighing what is important to you. Let’s examine one last vehicle.

2007 Honda Civic

The Civic is a little more challenging. The owner is fine with taking up a fair amount of the space in the trunk, but would like to retain some space for groceries. So, in this vehicle after a little discussion we’ve determined that the enclosure needs to leave at least 12” of space towards the bumper. This means that there is a maximum of 16” of depth in this install. The wheel wells are 40” but the trunk tapers at the rear to 32” to pass through to the cabin when the rear seat is folded down. If the enclosure is 32” wide it can fit inside the taper and take advantage of the full 16” depth, it will however be limited to 14” in height. If the enclosure is 40” wide, it will lose 3” of depth, but gain 2” in height. This enclosure has a lot more variables.

Option 1:
External Dimensions – 32.0” x 16.0” x 14.0”
Internal Dimensions – 30.5” x 14.5” x 12.5” = 5528.1 cubic inches or 3.2 cubic feet

Option 2:
External Dimensions – 40.0” x 13.0” x 16.0”
Internal Dimensions – 38.5” x 11.5” x 14.5” = 6419.9 cubic inches or 3.72 cubic feet

In this application, because of the depth limitation, there isn’t a significant difference in overall volume. When we look at estimated net volumes we’re comparing 2.46 and 2.86 cubic feet respectively, which is about a 14% difference in size. 14% in this case isn’t going to allow us to do a 15” woofer in either option, nor is it going to allow us to go from using two 10” subs to three or anything like that. Because the volume differences do not provide any significant improvements in cone area potential or in listenable performance we can opt for the more convenient enclosure; both to construct and for the owner to use and move around. In this case we’ll use Option 1 because it will be both a more efficient use of wood and it will easily slide into and out of the trunk without unnecessary maneuvers like having to slide it in sideways then right it once it’s in the trunk.

2.46 cubic feet is good for:
3 x 8”
2 x 10”
1 x 12”

The Civic is a prime example of a vehicle where someone might get over zealous. Yes, there is more room available in the trunk and if owner of the vehicle wasn’t concerned about being able to still use some of their trunk, we’d have had almost 5.85 cubic feet of estimated net volume. At that point you could squeeze in an 18”. However, an 18” would only be able to fit on the top of the enclosure which would be about an 1.5” from the top without accommodating for a double baffle that a woofer of that stature would necessitate. It becomes impractical. Knowing what will fit in a vehicle, and what should fit in a vehicle can mean the difference between boinky, hollow bass and something that can play deep and sound full like it should. So remember, check your space before you buy your bass.