The 12-inch subwoofer is an extremely popular size because it offers the performance needed to hit deep notes but the 12" driver size (or in some cases 13") keeps the overall dimensions manageable, especially with sealed models. Of course, those seeking a smaller sub can always opt for 8-inch or 10-inch models, and bass addicts have plenty of 15-inch options as well larger but rarer 18-inch and larger. But 12-inch is a so-called "Goldilocks" size for many shoppers, and that's why there are so many options.
Subwoofer Designs: Sealed, Dual-Opposed, Ported & Passive Radiator
Although there are more than four types of subwoofers, the following designs are the most common among consumer models. Each has its plusses and minuses, but the biggest divide is between sealed designs and subwoofers that utilize some sort of tuning (port or passive radiator or horn-loaded) to increase their output at the bottom end of the frequency response curve. To be sure, there are other subwoofer types beyond this list, but the following are the ones that you encounter when shopping for consumer subwoofers.
Sealed: The simplest subwoofer design. It's basically a driver in a sealed box with an amplifier. Modern sealed subwoofers use DSP programming to tailor the driver's response and output, as well as how it interacts with the amplifier, to maximize output and fidelity.
Sealed subwoofers are compact and comparatively affordable. However, ported models at similar price points, while often larger, tend to offer more bass output near the port tuning frequency.
Dual-Opposed: This type uses two of the same driver at opposite ends of the cabinet. The result is "force cancellation," where each subwoofer driver's momentum cancels out the other. As a result, dual-opposed subwoofer designs are free of driver vibration.
A dual opposed design allows for a small enclosure with a large driver surface area. It produces more output than a single driver in a similar-sized cabinet. A bigger amp and two active drivers are required to overcome the pressure differential in a small enclosure. This is an unusual design, but it works well and is probably the best way to make a compact subwoofer with high performance.
Ported: A ported sub increases output near the tuning frequency by using one or more tuned ports. While this demands a larger cabinet for a given driver size, the added output is often considered to be worth the trade-off, at least in installations with room to spare.
Because ported subwoofers are the most prevalent type, there are a plethora of various types to choose from. The most important thing to understand about corded subwoofers is that they typically roll off abruptly below the port tuning frequency, in contrast to sealed subwoofers, which roll off more gradually.
Passive Radiator: Passive radiators operate in a manner, not unlike tuned ports. The passive radiator is a driver that is housed in the same cabinet as the active radiator but does not have a motor structure. Rather, the diaphragm of the passive radiator is tuned according to its weight and size in relation to the main driver.
The disadvantage of a passive radiator is that it is more expensive to implement than a tuned port, but the advantage is that it is immune to the port distortion sounds associated with some vented cabinet systems.
About Subwoofer Frequency Response Specs
Although response specs can help with choosing a sub, be aware it's not always an apples-to-apples comparison. With subwoofers, it's hard to really compare the response spec, because it is not tied to the output level. It is often the case that a sub will start to have trouble—running out of power or out of cone excursion—with the lowest (deepest) notes. The point is, you can use the response specification to compare bass extension among subs made by the same brand, but comparing between brands is tricky unless both provide more in-depth and verified specifications than is typical.