[Wayne A. Pflughaupt]

I hope someone appreciates this, because it cost me dinner and a movie for my wife to create these charts for me! (Sonnie, maybe you can tell me why they look so bad? We modified the charts for our subwoofer review, but they didn’t look like this.)

Okay, first we have to address a few “givens” here. The first is that two subs’ measured response in a given room will be very much the same, except at the lowest frequencies. This is because most of a sub’s in-room response is determined by the room itself - it’s modes, cancellations, etc. The reason subs will deviate most in the lowest frequencies is obviously because the most prodigious subs will have more output down there.

You can see what I’m talking about in this graph from my subwoofer review:





As the graph plainly shows, the Infinity and Velodyne subs had better extension than the JBL, while the SVS sub handily bested all the others in this regard. Sure, there are some deviations in the upper frequencies, but they are minor. They are minor enough, in fact, that you probably won’t hear the difference. I certainly could not tell a difference in these subs’ performance in the 38-70 Hz region when I listened to them.

The other given is that if you place two subs in the same location - we’ll use a corner for sake of argument - there will be about a 6 dB gain in output over their combined operating response. Notice I said “combined” operating response. Obviously at frequencies where one sub is deficient, you won’t get the full 6-dB gain.

So, let’s take the case of two theoretical mismatched subs.





As you can see, Sub 1 has good output all the way down to, and below, 20 Hz. Sub 2, on the other hand, rolls out pretty hard below 30 Hz or so.

(Sub 2’s response had been shifted downward 2 dB, for the sake of clarity. For the sake of this presentation, both subs have the same maximum output. As noted in the first chart, the two subs’ response won’t be as identical as this. It was just easier for the sake of this presentation [read preparing graphs] to have them mirror each other.)

Okay, what do we get when we stack the two subs in the same corner? Something like this:





As you can see, we get a 6-dB gain in output over the “combined” operating range. In this case, that means everything above 30 Hz is up 6 dB compared to either sub by itself (remember in the first chart we shifted Sub 2’s response only for the sake of clarity). Notice what happens below 30 Hz, where Sub 2’s response fall out. At those frequencies Sub 1 is carrying the mail all by itself, so everything is 6 dB lower compared to above 30 Hz.

So what do we have? Sub 1 by itself gave us nice smooth response, with great extension. But when you combine the mismatched subs, overall response suffers considerably below 30 Hz. You can be that the problem will be audible!

Trying to fix the problem is “iffy,” even with equalization. Boosting either sub’s ultra-low frequency response to take up the slack may overdrive it, but you might get away with it if you have plenty of headroom. However, the best thing to do is simply to ditch the lesser sub and either go with the one good one, or if you really need the output, make your Sub 2 another Sub 1.

Regards,
Wayne