There seems to be a lot of confusion about how to design and build a subwoofer enclosure with multiple subwoofers in the same airspace. This blog post is going to clear it up.
A Single Subwoofer in one enclosure
Let's start off with something simple, a single subwoofer in a 2 cubic foot enclosure tuned to 28 hertz. In this example the port has been drawn outside of the enclosure for easier comparison. The port is 1"X12" and is 17 3/4" in length. Before you go out an build this enclosure keep in mind that a 1"X12" port is probably not large enough to prevent port noise. This size was chosen just as an illustration.
If you wish to add a second subwoofer then you will need to increase the size of the enclosure. If you increase the size of the enclosure you will need to make a shorter port. This is because you tune the port to the airspace in the box, not the number of subwoofers in the enclosure. So here is a 4 cubic foot enclosure with a cut out for a single driver. We can see that the port is a lot shorter.
Two Subwoofers in a Single Enclosure
Now Let's add a second woofer cut out and we see that the port is exactly the same.
However, now we have created a new problem. Now we have two subwoofers, so we are moving twice the air. That means the port needs to do twice the work. So the port cross section needs to be larger or else the air speed velocity in the port will create turbulence in the port. This will lead to audible port noise, also known as "chuffing." We need to make the port opening larger. As described in this blog post, when the port opening becomes larger the port needs to be longer. So let's double the port area. This image shows the same enclosure with a 2"X12" slot port, and now the port is longer.
This final design is perfectly acceptable, it may be a good idea to make the port area a bit larger or flare the port to minimize chuffing, beyond that all that is needed is to fold the port into the enclosure and adjust the enclosure size in order to account for the volume that the port takes up.
Dual Ports
Some people might prefer the look of an enclosure with two ports instead of one. How do we design that? Nothing like you might expect. When you add a second port each port will tune half of the enclosure. In this example, where we have doubled the enclosure volume, and double the number of subwoofers, an enclosure with two ports will look exactly like two enclosures with just one subwoofer. You could divide this enclose into two equal sized chambers and get the same frequency response (but 6 db louder since you will have double the cone area and double the power).
In fact that is how most pre-fab subwoofer enclosures are designed. After browsing Amazon for some examples I found it difficult to locate an enclosure that had one chamber with two drivers, and even more difficult to find one that had one chamber with two ports. The typical dual subwoofer enclosure available on Amazon has two chambers with a single port down the middle such as this one (click on the pic for details):
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| Dual Chamber enclosure with a single port. |
For this type of enclosure the port is typically half-sized on each chamber and then merges into a single, larger chamber. I imagine this is done so that the ports serve as a brace.
If you are looking for a good subwoofer then you should check out my list of 10 little-known but totally awesome subwoofers. To learn more about port tuning here is a great video over on the DIY Audio Guy YouTube Channel:
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