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"within inches of the main speaker" - I think you are overstating it




If you use two subs in a stereo configuration, I think there is more flexibility than you think.

The two subs need to be located in the front half of the room and placed symmetrically with respect to the listening position and speakers. And the two subs must be reasonably far apart so that if you draw a line from the listening position to one speaker and another line from the listening position to the corresponding sub, the angle between the lines should be no more than 10 deg in the horizontal plane. Futher, if the distance from the listening position to the sub is significantly greater or less than the distance from the listening position to the speaker, then time alignment is also required. Based on my own experience, I think if you follow these guidelines you will have no problems with imaging in the bass range.

The picture illustrates the two subwoofer placement options which work best in my downstairs room (please note the picture is only notional, things are not to scale). At one point in my system's evolution, I was using a 100 Hz 2nd order LPF on the subs and no HPF on the small standmount speakers, based on the idea that a minimal/low order crossover would give the best transient response across the bass range. I used multiple bands of parametric EQ and shelf filters on the subs only, in order to flatten the combined response of speaker + sub in the overlap region of 70-120 Hz and then cut the subs off quickly at 120 Hz.

With the subs in position 1 (next to the speakers), they suffered from a deep quarter wave null in the mid-bass caused by the front wall reflection, same as the main speakers do. With the subs in position 2 (near the front wall) they do not suffer from a mid-bass null, but time alignment is necessary due to the extra distance to the listening position. With capability for time alignment, the second position was objectively and subjectively superior, by more than a little bit, and even imaging of low frequency notes seemed better than in position 1. Without time alignment, an interference null is introduced with the subs in position 2 and the transient response suffers a bit, so it becomes a toss-up which position is better.

My experiments did not involve crossover frequencies as high as 150 Hz, but the same principles should apply. There are manufacturers who have used this approach with even higher crossover frequencies.

But I also have to admit that I'm not using that configuration any longer, and although I'd like to try again it probably won't be soon. My present components (network audio player and integrated amp) offer other sonic advantages that are more important to me now than optimizing subwoofer integration.

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