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Stereo Subs - Do they need to be identical?

I'm thinking of getting a couple of subwoofers to mate with my MBL 121 monitors. Generally, I've seen setups where people use two subs of the same brand and size. I'm wondering whether it can make sense to use two different sizes. For example - a Venoyne DD12 and a DD15. I'm likely going to go with Velodyne DD series or JL Audio.

In particular, I note that the big Velodyne 1812 uses an 18 inch and 12 inch woofer, with suppsed benefits in terms of speed (12 inch) and extension and depth (18 inch).
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Showing 2 responses by audiokinesis

audiokinesis
2,724 posts
 
I would say that... it depends.

In my opinion the goal is to get a smooth in-room response in the bass region, and a smooth (perceptually seamless) blend with the main speakers. Depending mainly on the frequency response characteristics of the subs and their low-pass filter, their positioning, and the rolloff characteristics of the main speakers (whether or not they are high-pass filtered), it's possible that in some situations identical subs would give a better blend, and in some dissimilar subs would give a better blend. In general, I would expect dissimilar subs to be a bit smoother. Let me explain why:

In my opinion, most subs are not designed with room gain correctly taken into account. Here's a link to a graph that I think was generated by Martin Colloms, depicting "typical" room gain (roughly 3 dB per octave below 100 Hz):

http://www.speakerbuilding.com/content/1020/rge.gif

So if you take a sub that's "flat" to 25 Hz (anechoic), in-room it may well be +6 dB at 25 Hz - which will sound heavy and boomy.

Now suppose you have two subs, one of which is flat to 40 Hz, and the other flat to 25 Hz. The summed response of these two subs may well come fairly close to approximating the inverse of room gain.

If you start out with subs whose anechoic frequency response is approximately the inverse of room gain, then I think two such subs would be smoother in-room than two dissimilar ones.

Another factor to consider is where the subs will be placed, and the slope of their low-pass filter. Placing the subs far apart (but not in corners) is usually beneficial if smooth bass is the goal, but the farther the subs are from the main speakers the more important it is that they have a steep-slope lowpass filter so that lower midrange energy doesn't come through loud enough to give away their location. I personally favor highly asymmetrical placement, but if the crossover is high and/or the slope shallow that might not work well.

Duke
audiokinesis
2,724 posts
 
In some cases, the quarter-wave cancellation from the bounce off the rear wall can be taken advantage of to tame a peak.

As for asymmetrical placement, here's a link to a brief informal paper by one of the leading researchers in psychoacoustics, Earl Geddes. He's comparing four subs in corners vs four subs asymmetrical, so it's not exactly what's being discussed here, but might be worth taking a look at as it illustrates one of the advantages of asymmetrical placement. The thing to look at is how much the peaks and valleys are reduced for each of the three listening positions in the asymmetrical case (represented by different color lines):

http://www.gedlee.com/downloads/sub%20study%20.pdf

Duke
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