Jim Asks:
I see in your articles that two subwoofers are better than one. Can you add a second different make, or are you better off buying two
identical subwoofers?

You can certainly add a second sub, and ideally four subs are the recommended number based on research by companies like Harman International.

It is advisable to use the same identical subwoofers to ensure the same performance consistency and overall characteristics.

I’ll second what Rob said, and I would also like to respectfully suggest that one really good $1000 subwoofer will usually sound better than two subwoofers that cost $500 or four subs for $250.  Of course, this is a very general statement, and it depends on a ton of other factors, but I believe that it holds for the majority of cases. 

I would also like to bring up the wife acceptance factor as most of us married guys have a hard time getting one subwoofer into a room, never mind 2 or 4.  If this is a factor, I would suggest one bigger subwoofer is easier to get approval for than 2 or 4 smaller ones.  Custom millwork helps a lot, as if the sub(s) are out of sight it certainly helps with the décor factor, but does limit which subs you can put into said enclosure, and obviously where they go in a room.  I had our carpenter build a shelf unit that had space for two subwoofers.  I also made the enclosures big enough that I could put in some fairly substantial subwoofers down the road if budget would allow. 

Interestingly enough, I have never ended up keeping a second subwoofer in the second spot.  I started with one 10 inch sub, and did eventually buy a second unit.  The second sub did not give the impact I was hoping for, so I returned it for a 15 inch sub that nearly doubled my budget (thank you Visa).  While I tried both subs for a while, it was total overkill, so I sold the original 10 inch sub, and kept my system with the single 15 inch sub. 

I have often wondered what sort of structural damage I could do with two 15 inch subwoofers in that room, but so far I have resisted temptation.

What is the advantage of having more than one subwoofer in a room? I thought a subwoofer’s sound was low enough that the direction was undetectable? If that’s the case then why would one need more than one? Is it just to have more punch from more amps.

The advantage of multiple subs is not merely more umph. Rather it is for quality as much as quantity. It is certainly easy to have too much bass, even with one sub (Have you calibrated your system lately?). If done correctly (positioning, level matching, etc), multiple subs provide more accurate bass, less affected by room nodes, over a larger listening area than is possible from a single sub. CEA/CEDIA Bulletin CEB-22 Home Theater Recommended Practice: Audio Design, a collaborative work by many of our industry’s most knowledgable minds utlizing the most up to date research, says that at least two subwoofers (preferably four) should be used at different (specified) locations.

Achieving great bass is not simple and cannot be guaranteed just by using great subs. Unfortunately, physics does not take WAF into account. That is why Triad and others make great InWall & InCeiling subs.