So what happens when you want to go beyond what you can get out of one or two fairly conventional subs? If you're buying from the conventional hifi retailers, the price goes up quickly as you start to move into 15" or 18" subs. You pay a lot more but do you actually get that much more?
In over a decade now of going into rooms and testing how they perform, one thing has become clear. You can do a lot better, when you know your room. Once you do understand your room acoustics and how the sub will perform in different positions in your room, this gives rise to some different goals. Many people assume that all subs in a system should be the same so that one doesn't bottleneck the system. In reality, the opposite is often true. When you have more than one sub, often they are not all doing the same job.
The heavy lifter
In some rooms, you may have one position that is ideally suited to very low bass. It's usually due to having the best room gain in the bottom octave. It could also be because this particular position allows a larger sub than other locations, which might be limited by things like windows, doors, speakers, furniture. In a multi sub set up this sub doesn't necessarily need to cover the upper bass. This type of sub suits a ported, bandpass or horn alignment. Where this type of sub is used, normally it will handle the lower bass without any help from the other subs. Hence, it will allow any other subs to be smaller.
The infill sub
If this is the second sub to be paired with a heavy lifter, it won't need to cover the low end. That means generally it can be more compact. Typically an infill sub will cover 40 - 80 Hz. It might be a conventional sub with a standard hifi driver in a sealed box. Its purpose is to fill in dips caused by room modes, where we can't find a single position that avoids all dips.
This is the name that I give to a sub that functions as an infill but that is designed for higher output. We can achieve this with a bandpass design, giving up some extension to get higher sensitivity. This is the case with a recently designed sub that I cal "lil thumper." It has 92 dB sensitivity. We can also achieve this using pro drivers, where the sensitivity might range from 93 - 97 dB.
The stealth infill sub
Sometimes we need an infill sub to fill in some dips, but we might need that sub in a position that might localise. This can happen where the sub is located around the back of the room and quite close to the listening position. If you aren't careful, you may be able to tell where the sub is located and that can be distracting. For this type of sub we have to be careful to avoid any features that would give away the location of the sub. This can come from air leaks, turbulent ports that chuff, ports with port resonances above the passband or horns with ringing above the passband. Even a sealed sub, which by design avoids all these things, can still localise.
This type of sub is tricky, especially where it must fill in dips in the mid and upper bass. Extra care must be taken.
Does this all sound confusing? Watch this space, as I will be adding some example that will make the use of these specialist subs more clear.