Subwoofer specifications explained
Subwoofer specifications may be difficult to understand. In addition, manufacturers don't necessarily follow the same standards. That means this guide may not help you compare between subwoofers, but it will at least help you understand what any given spec sheet can and can't tell you.
Frequency response (How deep can it play)
One of the main questions many ask themselves when looking at subwoofers, is how low will it go. What is the deepest bass notes (or explosions) the subwoofer can reproduce? This is actually surprisingly hard to find out, as this may be specified very differently.
Here is a typical specification:
The first is the frequency range which the subwoofer can play. From the deepest to the highest frequency. So we are looking for the first number to be as low as possible.
But what does the "(-6dB)" mean? A subwoofer will typically aim to be "flat" through the entire frequency range, which means all notes the subwoofer can play are of equal volume. But in practice any subwoofer will have a natural (or artificially enforced) roll-off towards the lower frequencies. The specifications often indicate this with "-3dB" or "-6dB". In our example, the specs indicate that the subwoofer has started rolling off significantly at 25hz, and at this frequency, the volume will be 6dB lower (half as loud) compared to the main frequency range where the subwoofer has a flat response. So if two subwoofers both are specified with the same frequency range, but one is -6dB and the other is -3dB, the latter will go deeper.
Sometimes the "-x dB" isn't specified at all, and then it's anyone's guess. In addition it can be measured either in a anechoic chamber, or in-room. Then the difference can be up to 10hz. So in general it's hard to compare between manufacturers.
And how low is low enough? Most music rarely dip below 40hz, and very few tracks dip below 30hz. So if the subwoofer response is in the 20s, you're probably good for most music. If you want to be sure that you have a subwoofer capable of reproducing any and all music including room ambience from things like live recordings, you should be looking for a subwoofer that can reach at least 25hz. If you are looking for a subwoofer for movies, you may want it to go even deeper. But then you are typically looking at quite expensive subwoofers, and/or relatively large ones.
A final problem with the frequency response specification is that often isn't linked to SPL (how loud it can play). So the specifications may imply that it can play 20hz, but it doesn't say how loud it can reproduce those frequencies, and that's pretty essential. So you may come across very small subwoofers that claim to go very deep, but in practice these frequencies will be inaudible.
SPL / dB (Sound pressure)
Not all brands will include this specification, and if they do, it's often unclear exactly what the number means. Sound pressure is typically measured in decibel, and a specifiaction can look something like this:
Max SPL: 100dB
This still leaves us wondering about a few things. At what distance was this measured? Typical distances are 1 or 2 meters. 100dB at 1 meter, equals 94dB at 2m. At which frequencies? Lower frequencies are progressively harder to reproduce at high volumes. If a subwoofer can reproduce 100dB at 30hz, that same number may be down to 90dB or even less at 20hz. Finally, at what distortion level? Is it reproducing this sound pressure level quite comfortably, or is the subwoofer about to bottom out?
If you want to have any chance at comparing this between subwoofers, you may try looking for something called CEA2010, which is a measurement standard for determining maximum SPL levels. It typically includes information about the distance, frequency or frequencies, and distortion level. Without this information, it's hard to compare SPL across brands.
A small note is that even with a CEA2010 measurement, we only learn about the final "breaking point" of the subwoofer, where the distortion gets too high. It does not say anything about the distortion at moderate playback level. So two subwoofers with similar CEA2010 results, may show very different distortion levels at moderate levels. This is often what separates a more expensive subwoofer from its cheaper cousin.
The type and number of drivers
To be able to make sound, a subwoofer needs one or several subwoofer drivers. The size of the driver is typically specified in inches. So you will see something like 12" driver - indicating that it has, not surprisingly, a 12 inch driver.
In theory, a bigger driver will be able to reproduce the same SPL (sound pressure) with less distortion. The same is true if the subwoofer has multiple drivers. As you may suspect, in practice it's a bit more complicated than that. The quality and abilities of a driver is determined by a host of different parameters. So two different, equally sized drivers may have completely different qualities depending on how they are built. Which is why you may find a 12" subwoofer for anything between 100 and 10,000USD. So the size and number of drivers gives you an indication of the subwoofer capabilities, but it's not the whole truth.
Cabinet / Enclosure type and size
There are a number of different cabinet types, but for this guide we will stick to the two most common. Sealed and ported. As a general rule, sealed subwoofers will be more precise and is often preferred for more music-oriented subwoofers. Ported subwoofers on the other hand, may be more effective - and is often the preferred design for cinema-oriented subwoofers. Ported subwoofers are also larger than sealed. Note that expensive, ported subwoofers with low port tuning may rival a sealed subwoofers in sound quality - but will still typically be significantly larger.
So if size is a factor, a sealed subwoofer is probably the way to go. If you want as much sound pressure as possible and don't care about the size, ported might be best. If you want your cake and eat it too (have a small, precise and high capacity subwoofer) it will be expensive.
And finally, Power ratings!
I saved this for last because contrary to popular belief, this is actually the least important specification. The power required by a subwoofer is heavily influenced by the cabinet size and the driver parameters. This means that two 500 watt subwoofers may have a maximum sound pressure level and overall capabilities that are very different. It has also become somewhat of a marketing fad to boast impressive numbers. If the power figure sounds too good to be true, it probably is. The easiest advice here is to simply ignore this part, and trust that the manufacturer has fitted the subwoofer with an amplifier that is the right size (very likely).
Hope this helps, and happy subwoofer hunting!