We want to use the -3dB down point specified by the speaker manufacturer only as a starting point to turn the low pass filter up to – not as a rock hard rule

Paul McGowan writes:

In a recent post we covered with good detail the steps to achieving low bass with your subwoofers.  By keeping the frequency lower than you need to match your main speakers you can focus your efforts entirely on getting the very low bass right.  Now it’s time to match up to the mains.

Matching the mains should be done by simply turning up the low pass filter control on the subwoofer until you get a seamless presentation, but it turns out not to be all that easy every time.  The tendency I’ve had with this adjustment is to get it too high – because a high setting on the low pass filter tends to give us a warmer, richer sound.  That is very pleasing but also unnatural and, over time, you’ll get bugged by it.

The first thing I would do is look at your main speaker’s specifications.  They will usually list the lowest frequency it supports by specifying what we call the -3dB down point.  This means that the bottom end of your main speaker is “flat” to a point where its response drops off fairly quickly and as soon as the measurement device meter goes from the flat reference to -3dB (half power) that becomes the lowest the speaker goes.

A couple of things to remember about this setting: -3dB ain’t flat and your main speakers are probably not in the proper place in the room to give you a flat response at your listening position anyway.  So what’s an important take away here is that even though your speaker may measure “flat” to 80Hz, for example, that measurement was made in an anechoic area 1 meter away from the speaker – and that is NOT at your listening position.  The response at your listening position is more than likely not the same as the measured response.

We want to use the -3dB down point specified by the speaker manufacturer only as a starting point to turn the low pass filter up to – not as a rock hard rule.  Remember also that when your subwoofer’s low pass filter control says its cutoff is 80Hz, that too is the same 3dB point.  In other words, 80Hz on the subwoofer is also 3dB down, not flat.  So if your main speaker is 3dB down at the same point your subwoofer is 3dB down you’ll still have a missing area at that frequency – and that’s only right in front of the main speaker, not at your listening position.

So what you’re going to wind up with is a bit of overlap and really, without measurement equipment, you’re going to have to do this by ear and trial and error.  Take your best shot and then use Paul’s Dirty Dozen technique to dial it in.  This means have a dozen or so reference pieces of music that are all over the place in type and character.  Play all of them and see if something continues to be common among them – there really shouldn’t be anything common between all these discs – and this helps you dial it in.

Tomorrow will be the last in our series on subwoofers and we’ll cover matching the sub to the main speaker in terms of speed.

Paul McGowan