It seems like this would be an easy decision. Just pick the box that best fits your design goal. But did you know that there are two differing rules of thumbs on what type of box a driver would best be utilized in? Seriously. One of them uses the QTS of the driver, while the other uses a formula called Efficiency Bandwidth Product. Let’s talk about both for a minute.
Efficiency Bandwidth Product
This is a concept talked about in Vance Dickason’s Loudspeaker Design Cookbook. If you aren’t familiar with it, it is worth picking up and adding it to your library. Back to the point, this uses a formula to come up with a drivers efficiency bandwidth product. Once you figure out this number, it should help you decide what type of box is best for that particular driver. The basic formula is
EBP=FS (resonate frequency of a driver)/QES
Don’t worry about remembering that, I have a handy calculator down below. The most important thing is to come up with the EBP. Once you do it will give you a number anywhere from 1-100. The closer that number is to 50 the better it is for a sealed box. The closer it is to 100, the better it is for a ported box. And something in between, might work well in either type of enclosure.
The Q of a Driver
The second way to do this is much easier. It only looks at the total Q of a driver. This is found in the drivers T/S (thiele/small) parameters. You are looking for a number labeled QTS. If this number is 0.4 or below it is typically best suited for a ported enclosure. Anything above 0.4 to 0.7 is typically best suited for sealed. And a number above 0.7 is typically good for free air or infinite baffle. Although, there are some people that really try to look for a drive with a Q above 1 for these applications.
Problems with These Calculations
There aren’t any inherent problems with these calculations, but keep in mind they do not always correlate to the same answer. Meaning you may calculate EBP and it indicate it should be a sealed, while QTS will indicate ported. What you need to keep in mind is that both of these are rules of thumb…and we all know what that means. Rules are meant to be broken.
There are rule breakers. The most common among home theater enthusiast is the 18″ Dayton Ultimax subwoofer. If we look, it has a Q of 0.53, indicating it is better suited for a sealed box. It’s the same when we look at the EBP. If you don’t believe me check for yourself. The driver has an FS of 19.5 and a QES of 0.62. That leaves us with and EBP of 31.45. Once again, telling us this driver is better suited for sealed. An yet, this is one of the most common drivers to be ported in Home Theater. There are threads just dedicated to it’s ported design – the Marty. In fact this driver has even been used in infinite baffle setups. This should just remind you that it is best to model it for yourself. For that I recommend pickup WinISD for free and checking out the tutorial videos.