I was curious about why the soundstage was now so ridiculously wide. Was it because the subwoofers are now spaced far apart and in the corners? Was it because the speakers are far from the side walls and the reflections are delayed?
So I made two filters - one without subs, and the control with subs. The first, shown below in red/green (upper curve) is a normal filter with the system configured as a 4 way, with subs in the mix. There are two versions of this, one with 50Hz as the XO point, the other with 80Hz. The second filter (blue/brown) has the subwoofers turned off, with the woofers extending all the way down to 20Hz. Once overall room correction is applied, the curves look almost the same, but with the 3 way version without subwoofer showing slightly earlier bass roll-off and some dips in the FR:
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Result: not even close. The version with the subwoofer has an enormous sense of scale and the feeling that the walls aren't there, even at low volumes. The 3 way version sounds like a normal speaker in a room. It's good, but nothing special. The version with the sub really is something special. Precision and size of the center image is the same with both filters, which is what you would expect given that everything from 500Hz up is exactly the same. And then there is the impact of the version with subwoofers, there is an authority that the 3 way version does not have. I could confirm this subjective impression by taking sweeps at different volumes, and I am 100% sure I will see the bass dropping off in the 3 way version, but there is no need to since the effect is so powerful.
David Griesinger talks about "bassiousness" - the idea that subwoofers set up in stereo and spaced widely apart contributes to subjective spaciousness. There is a debate somewhere on ASR about this, and it linked to
this article. The article mentions that the spatial enhancement of low freqs has a lower limit - with some debate whether it is 50Hz or 80Hz (hence the motivation for trying two different XO points). In this system, and in this room, the 80Hz version seems to create more spaciousness than the 50Hz version, although the effect is pretty subtle.
That article makes it seem as if you need to make a choice between a setup for stereo bass or for proper bass management, and seems to forget that you can have both if you have DSP. I am now a believer - spacing your subs wide apart really helps.
On another note, I did an unplanned experiment yesterday. I washed my laundry and hung it up outside to dry. As is typical for Melbourne, one moment it is sunny and the next moment severe thunderstorms roll in. So I had to take the laundry rack with wet clothes into the house. Here it is, placed in front of the right subwoofer:
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I realized that I had made myself a bass trap with wet clothes, so I turned on the system and had a listen before the clothes had a chance to dry. The impression was that the room felt a little smaller with the wet clothes in situ and I was getting ready to move the clothes rack around the room to experiment with various positions when suddenly the power cut out -
300,000 homes were left without power as the thunderstorm damaged critical infrastructure, and my home was one of them. So no more listening and dinner was biscuits, canned fish, and cheese by candle light because all the local restaurants were closed and I could not cook.
I have power and internet back now. It was a crippling feeling, as if you are jailed in your own home.