An Introductory Guide to Subwoofers 
The Basics 
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Location, location, location. Yes, it's that important!

Location (placement), the single most important factor for getting good sounding bass in your room.

First we need to start with just a little bit of physics. As we know air is a fluid, and as such sound moving through air does so in waves.

 The sound waves from your subwoofer interact with the geometry (dimensions) of the room. This interaction creates areas in the room where frequencies are boosted (these are called peaks), and other areas where frequencies are cancelled (these are called nulls).

 Understanding the interactions that create these peaks and nulls will enable you to obtain the optimal performance from your subwoofer.

As illustrated by the animation below, when sound waves are 'in-phase' (aligned crest to crest) they create areas of higher output (peaks). When sound waves are 'out-of-phase' (aligned crest to trough) they create areas of zero output (nulls). 

This illustrates why it's important to have your subwoofer operating 'in-phase' with the main speakers.

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How to I check to see if my speakers are in-phase?

Take a AA battery. Apply a wire from the (-) terminal on the speaker to the (-) terminal of the battery. Then touch a wire running from the (+) terminal on the speaker to the (+) terminal of the battery. The speaker cone should move forward. If any cones move backward, the speaker is wired out of phase. The single exception to this rule is in some instances midrange drivers are deliberately wired out of phase with their woofers. Our test is intended to check the phase of woofers and subwoofers.
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When sound waves traveling in opposite directions interact the result is a standing wave. This is illustrated by the animation below. 

This illustrates why there are areas in the listening room where there are bass peaks, and other areas where there are bass nulls

Animations courtesy of Dr. Dan Russell, Kettering University

 Later we'll go into detail about the implications of peaks and nulls created by standing waves, and what the homeowner can do to influence the impact of these effects using acoustic treatments (bass traps), and equalization.

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The effects sound waves interacting with the walls, floor and ceiling (boundaries) of the room create what are called 'room modes'. There are 3 basic types of modes, axial, tangential, and oblique. Examples of each are shown below.

Axial 

Tangential 

Oblique

Illustrations courtesy of McSquared.com

The McSquared website (link above) contains a java script calculator for those wanting to see the effects of changing the wavelength (frequency).

The location of your sub in the room creates these modes. And the modes are what determine whether your listening position gets great sounding bass or has bad sounding bass.

For those computer minded folks there are computer programs that will model the interaction of the sound waves with the boundaries of the room. Unfortunately most people don't have unlimited options with regard to placement, so the benefit from one of these programs is limited. If one is building a dedicated listening room, then one should certainly take advantage of the information these programs offer.

Cara 2.x, RPG Acoustics Room Optimizer, and the Excel spreadsheet based freeware Room Response Calculator will  let you model room response characteristics.
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So what do all those peaks and nulls mean to me?

They mean the placement of the sub in the room is extremely important. 

Got too much boom? Move the sub. 

Don't have enough boom? Move the sub.
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I have an equalizer, why can't I use it to solve my room problems?

An equalizer will solve some problems, primarily those related to having peaks in the response. A null is an entirely different animal. Think of it as a drain. No amount of water can fill a drain, no amount of boost can fill a true room induced null. Lots more on this later....
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How about bass traps, won't they solve my problems?

The answer to this is a definite maybe.......

 For those that don't want read the entire Linkwitz webpage I linked to below, here's a quick synopsis. Sound waves 200Hz and higher are easy to control with relatively modest room treatments, since they don't create discrete resonances that dominate the room. Frequencies lower than 200Hz create discrete resonances and those resonances are difficult to control. It can be done with bass traps. More on bass traps later.....
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NOTE, neither bass traps or equalization alone are a cure for room related issues. When possible a combination of traps and equalization will provide optional performance. The usual approach is to start with acoustical treatments, then use EQ to fine-tune everything.
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For those wanting lots more technical information about room acoustics, I know of no better on-line resource than that provided by Siegfried Linkwitz.

For those wanting more room/acoustic information, but can't quite wade through the Linkwitz website, MarkTAW.com has several web pages that are a good primer. 

The ultimate resource for acoustics is "The Master Handbook of Acoustics" by F. Alton Everest
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