I've been reading up on bass arrays the last few weeks. For indoors home listening the concept of exploiting a "plane wave" seems very good. Unfortunately for me and my current PC room layout I cannot have anything useful on my back wall, so the excludes double bass array (subs on the back wall) and single bass array (I cannot have thick enough sound deadening on the back wall). So I've just started dabbling with virtual bass array like previously discussed in this thread, and practical applications have been briefly discussed here:
Room equalization through inverse delayed and attenuated bass signals.
In that thread OCA links a video showing a method using a custom impulse response and convolution to output the delayed cancellation signal.
I also hypothesised a method that would duplicate the sound signal, then you would delay and filter the signal as required for the cancellation effect, and then merge that signal back in. Whilst I haven't tried it,
I recently came across another user doing basically the same thing, which suggests it can work. Doing this method with a single subwoofer would probably work best with it in the middle of the front wall?
Also lately I happen to have been watching some vids on bass simulation for concerts etc (in open air spaces), and how the pro scene has simulation software for the task,
like I briefly discussed here. This has shown that there are a few different ways to arrange 2 subwoofers so that they project more bass forward than they do backward, which sounds sort of like the basics of a plane wave. So the 3 arrangements I've noticed so far are (and I hope this is what they are called);
- End Fire - where there is 2 subs in a line, and the rear sub is delayed. This helps project more sound forward, but you still get a fair bit at the back. I think they said the affect is quite consistent across all frequencies.
- Cordioid - still 2 subs in line, but the rear sub is polarity inverted and delayed, which has ends up giving and even better front vs rear projection ratio. Again I think the effect is quite consistent across frequencies.
- Inverted Stack - here the subs are on top of each other, one firing forward, and the other backward. The front vs rear isn't as good as cordioid, and also apparently it is quite frequency dependant based upon the distance between drivers.
So yer, some interesting arrangements there. 2 subs in a line isn't particularly practical for small rooms, whereas 2 on top of each other is. And if getting the subs near the middle of the room vertically is important, then stacking has a natural advantage.
It seems the defining feature of the virtual/pseudo bass array is the delayed cancellation signal coming from the source driver, but with these 2 subwoofer arrangements there is a bit more choice, because the signal could be output from the front or rear subwoofer, or both. Perhaps even the original signal could be output cordioid style, and after processing the cancellation signal could be output end fire style? And one other possibility I thought of is simply adding a third subwoofer to the inverted stack, solely for outputting the delayed cancellation signal - how would that go?! Because one thing about the VBA methods mentioned previously (convolution method or copying in a second signal method), both suggest the subwoofer needs to output even more frequencies at one time than it would just normally playing the song, which might cause a problem with "dynamics"? Or possibly when adding the cancellation signal to the live signal it just cancels the signal all together, and then the speaker does nothing, and thus the in room signal doesn't get actively cancelled at all and just keeps decaying as usual?
So there is much to think about. Before starting to write this post I spent a few minutes in REW Room Calc for 2 "stacked" subwoofers, and it seems plausible. Here's my actual room and listening position (
and these are without a cancellation signal factored in);

It has gone from ~108dB at ~43Hz (which is room length peak) and ~88dB at 60Hz, for a 20dB difference
estimate down to ~90dB at ~43Hz peak .............................................. and ~74dB at 60Hz, for a 16dB difference (and 75dB is the target in REW Room Sim, so whilst the peak to valley difference isn't remarkebly better, the overall room gain seems more controlled).
Or I just now came up with this one using delay, and both 43Hz and 60Hz are flat at 79dB!
