Is one subwoofer sufficient or are two the right number? I only ask because somewhere I've seen monitors with two subwoofers.
How do you define what "sufficient" is? Depending on what you listen to response down to 40 or even 50 Hz is sufficient, which many speakers can do by themselves. I'd first educate your ear and what various deep bass frequencies sound like. Then I'd read Floyd Toole's book "Sound Reproduction" to learn how bass behaves in-room and the advantages of 'distributed bass' (multiple subs or as Duke LeJeune from AudioKinesis here on Audio Circle, terms his offering as a 'swarm' of subs). Bottom line, (3) 10 inch subs are sufficient for me, my chosen musical genres, listening habits, and 8ft x 13ft x 21ft room, so in AV nomenclature I have a 2.3 stereo system.
The value of multiple subs to minimize bass peaks and dips that occur at different frequencies and locations in-room. Think of moving your hand along the length of a bathtub with 4 inches of water in it. Waves would be generated that run parallel to the length of the tub. As they hit the end of the tub they bounce back and hit the next wave where they will add, cancel, or something in between. Those adds are peaks, the cancels are dips. That's how bass behaves in-room. Intelligently applying equalization (or DSP) can only solve the problem for a single listening location. Poorly applied, it can overload the amp and destroy drivers. Multiple sources reduce the peaks and dips across the room.
Distributed subs are just that. Best to place them rather randomly around the room and out of line with the main speakers, since they produce bass too. In fact frequencies below the so called Schroeder or transition frequency (roughly between 120 and 200 Hz, room dependent) behave as waves and higher frequencies behave like rays. The upper range where subs are desirable is roughly 80 Hz (the THX standard) because any higher and the bass sources can be located by ear and interfere with imaging. One advantage of multiple subs is that smaller subs that blend in with room gain to produce deep bass can be used.
When trying to provide the best overall sound in-room it's best to start with the room. Bigger is better. Avoid cubes (Toole speaks to that in his book and gives a range of what are considered good room dimensional ratios). Using a near-field or mid-field setup with speakers/listener away from walls reduces room effects. Acoustic isolation is valuable and warrants a whole thread/circle. The next step is room treatment which is often confused with isolation and generally breaks down to absorption and diffusion (refer to the acoustics circle). Finally to provide the icing on the cake is equalization (nowadays commonly in the form of DSP). Some try skipping the one or both of the first two steps but can never achieve the best results.
