I’m going to put this plainly: audio systems of all kinds need subwoofers—in the plural. The reasons for this I’ll get into later on, but understand that I’m talking about more than the wallop of home theater. I’m talking music.
Subwoofers are infamous for their inability to integrate into a 2-channel system. But is that a fault of the subwoofer or of the approach? Many people use subwoofers to extend the lower frequency spectrum, add more impact and weight, and obtain some texture that their conventional loudspeakers can’t portray. Often times this zeal creates an impressive sound that throws great punches, but calls attention to itself.
So I offer a different approach. Subwoofers actually should NOT be used to increase bass output of the audio system.
It’s all about loading from the bottom up. If you’ve ever played with port tuning of a speaker cabinet you know how dramatically the sound will change when you properly load the driver in the cabinet. This is precisely what a subwoofer is for. It loads the entire room. And that means you should be building from the bottom up, not the top down.
Rather than begin an in-depth look at room acoustics we’ll keep it simple. Constructive and deconstructive interference causes peaks and dips in the frequency response in a particular room. Because the wavelengths in the bass region are so long they tend to overlap wavelengths of higher frequencies. This is why bass trapping not only helps alleviate bass anomalies in a room, but also clears up phasing issues further up the spectrum equally as well.
Subwoofers do a similar trick. With proper placement of the subwoofer in the room you have a more even distribution of bass. Frequencies below 120hz are basically omni-directional, so even if the woofer driver isn’t right next to your speakers it will still disappear when properly placed in a room. The key is proper placement and tuning.
Using multiple subwoofers in a room allows you to eliminate the issues with room modes by having a more evenly distributed bass response. This also allows the subs to properly pressurize the room.
When you read guides about subwoofer placement often times they describe placement either in a corner or slightly out from a corner. The intention is to take advantage of rear waves bouncing off the corners and creating constructive interference with the front wave to make the bass louder and more intense. The problem with this approach is that this interference is out of phase with the musical signal. So bass gets boosted in unintentional places. Not good. Further out from the wall gives more time before this interference occurs and presents a more even bass response from a single subwoofer, though perhaps with the perception of less bass.
The lower frequencies are used to create the foundation of the music. This is both in terms of down beat, but staging as well. When there’s distortion in the low frequencies or uneven response the music loses out at the foundation.
A single subwoofer often times does more harm than good. It creates a localized source for bass coming from a single point in the room. This is why it becomes difficult to integrate a sub. Even below 120hz where the subwoofer is omnidirectional because all phase information comes from a single spot we start to detect subtle timing anomalies in the music.
Dividing bass content among different channels is the best option. First this helps to create a more even bass response in the room by having two sources outputting low frequency information it helps to eliminate much of the uncontrolled interference in a room caused by early and late reflections. Second it gives you more control over phasing between the channels. The phasing information between right and left as well as timing information for driver integration is what calls attention to a subwoofer.
Deep (often inaudible) bass is the foundation to feeling the music as well as the support structure for the sound stage.
A single sub creates phase anomalies from it’s location in the room as well as for driver integration. This calls attention to the sub.
Multiple subs have a more even bass response and require less power for the same output, this means lower distortion and a flatter response in your room.
Multiple subs isolate phase between right and left and give a more balanced loading of the room. This allows the subs to disappear and creates a more balanced stage and output potential without clogging the music.
Placement in the room depends on the room, but keeping subs away from close walls aids in their disappearing act by avoiding constructive interference that may call attention to them.
These are points that should be considered for all systems. Proper bass response in a room can make the difference between an entry-level and high-end system. Before spending your whole budget on speakers, consider chopping the budget and considering a cheaper set of speakers and better subs. This will give you better control of your in-room response without having to deal with costly room treatments. Ultimately your room will play one of the largest parts in the sound you hear, so why not put those variables into the equation so that they can be taken out?