Ported Speaker Enclosures

So recently I have been tinkering with the idea of how well a ported box that is only 14 inches square on the outside would sound. I have done the calculations and the math works but I am not sure of how it will sound. I am thinking of making it tonight to see what I come up with, I have the specs for it in an excel spreadsheet 14InchBox.xls. The port is a two turn serpentine design inside the rear of the box, I will upload pics for it later.

The Subwoofer Code
by John Brown

This document is intended to explain a lot of what I know about sound and car electronics. I have found that the net leaves much to be desired in this category. I have held a certification in car electronics installation for 6 years. In that time I have worked on more than 1000+ cars and trucks ranging from new BMWs to 1960s muscle cars. I have put alarms in RVs and Stereos in Boats.

What I have found is that most vehicles out there are unique and common depending on what you are doing. We will start with a subject that I find to be the most mystical and yet the most fun and experimental. The last 5 years have seen me working on creating the perfect ported sub box. The term sub box refers to a wood enclosure which a speaker designed for low sounds mounts to and has a large affect on the sound of that speaker.

The ported sub box (or subwoofer) is a needed part of all audio. With no bass, you get no feeling. Any system can be made better with the right amount of bass. One of the most efficient ways to produce good bass with a little power and good design is a ported subwoofer.

The ported box consists of 3 parts, the Port Area, Port Depth, and Box Volume. The port area and port depth are different and must remain that way. All calculations for box frequency rely on the independence of these variables.

VB = volume of box (this is the interior size (minus the displacement of the speaker) not including the port. If your box is 10.75*10.75*10.75 (not including port) outside and your wood is 3/4 thick, your interior volume is 10*10*10 or 1000 cubic inches. If you need metric, there are online converters.

AV = Port Area. This is the size of your port. If circular remember A=2*pi*r

LV = Port Length. This is the length (end to end) of your port. Longer Ports make lower frequencies.

FB = Box frequency. This is the frequency that your box and sub will make the most sound at naturally. (I will always recommend a value between 38 and 42).

The porting formula which I use (thank you www.jlaudio.com) is as follows:

FB = .159*sqrt((AV(1.84*10^8))/(VB(LV+.823*sqrt(AV))))

Good values for the FB are between 38 and 42 for 12 inch speakers, 36 to 40 are good for 10inch, you can go lower or higher, it just depends on the space and sound you want. For example a Suzuki Esteem sounds good with a ported 12 tuned to 39, but my open air jeep does better with a 37. The volume in the jeep is lower because the sound escapes faster and in more places.

Wood: I prefer to use 3/4 MDF. Medium density fiberboard because it has no grain to affect the sound, it glues together fairly easily and it is plenty stiff. It does not stand up well to wet conditions so be careful to cover it or seal it using a urethane. Many builders tell you to use some advanced groove for the corners, that is up to your time and interest, I use square corners and Elmer´┐Żs wood glue, each piece is held together with clamps for a few minutes while I use a finish nailer to make sure a corner holds.

Example porting: Say you have a Honda Civic and you want a box in your trunk. You want the most bass for your buck so you can show off on the cruise line. Trick is that the height of your trunk in that civic is only about 15.5 inches without getting in the way of proper trunk closing. The width is fine, about 36″ inside fender to fender. You want to use up all that space to keep the box from moving around. Now we have the basis of our box, we know that the width is 36 and the height is 15.5. These are both outside measurements, using 3/4 MDF makes our interior measurements 1.5″ smaller so we get 34.5 and 14 (good, that 12″ sub will fit). Next we need a depth to this box, before we get to this step, remember that the port has to go somewhere, I would suggest facing the port to the same side as the sub, this means it will take up some of your volume inside your box.

Since we have decided to use square ports, and made them 2″ wide, we are going to guess at losing about 3.5″ (port + wood) from the width of the box. Now we are down to 31 wide and 14 tall. For that port to be fairly short, we should try to get at least 14 inches of depth exterior (12.5 interior). This puts our interior box volume at 5425 cubic inches.

VB = Length * Width * Depth = 31 * 14 * 12.5 = 5425 cubic inches

Remember how we decided on that 2″ port width? Since the interior height of the box is 14″ Our AV is 28″.

AV = port area = 14 * 2 = 28 square inches

We have chosen a FB of 40 so all we need now is the port length. The ugly formula up top would work but you would have to know your algebra.

LV = ((AV*4.65*10^6)/(VB*FB^2))-.823*sqrt(AV) <- the “better” formula

LV = ((28*4.65*10^6)/(5425*40^2))-.823*sqrt(28) = 10.66 inches.

This means that our port will be 10.66 inches from end to end, our interior box depth should allow for 2 inches around the corner at the back of the box. If we had made the box any less deep, our port would sound funky because of the smaller corner. That is all there is to it. Enjoy your new box.

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