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I would like to suggest an alternative factor or explanation to your recent ULF/midbass findings.

When you were chronicling the design and implementation of your midbass horns I was curious as to the effect of the very close placement of the two horns.

When two loudspeaker drivers are placed close together they become acoustically coupled and so more efficient than when spaced well apart. On in-phase signals, two closely placed drivers would play louder in the bandwidth where they are acoustically coupled but have significant destructive interference for out-of-phase signals. I think this might well apply to the mid-bass horns.

In addition to these effects, on in-phase signals the pair of midbass horns might also function something like a two-sector horn where the combined mouth area determines the low frequency cut-off. The low frequency cutoff would be lower for the two horns running in-phase signals than for a single horn.

I am not sure if you ever tested for any of these effects although I remember your pleasure at how the useful range of the midbass horns was lower than a simple formulaic analysis would indicate. To test for these effects you can’t test one channel at a time. You would have to run a frequency response test for both horns running simultaneously and compare the response to in-phase vs. out-of-phase signals.

Now the same issues and analysis applies to your woofer towers and with the new placement of the towers some of these effects may have become exposed.

If these effects are significant than your new wider spaced ULF towers might actually be functioning more effectively in their upper range than the midbass horns in their lower range on stereo recordings with phase differences between the channels in this frequency range.

Because of the low-pass roll-off on the ULF you may have found that a significantly elevated gain for the ULF is improving the music reproduction where the mid-bass horns have unwanted cancellation effects. But this elevated gain does not hurt the reproduction of the ambient/environmental sounds intended to be reproduced by the ULF towers because these sounds are much less sensitive to output level than the musical signal.

How significant these effects are in practice are less clear. Stereo phono records usually become quite monaural in the low frequency range to facilitate both cutting the master and playback tracking. CDs and other modern digital sources may have enough out-of-phase bass for the coupling/cancellation effects to be noticeable.

If you have found or suspected that your LPs and monaural source material have a somewhat richer or fuller sound in the bass than your digital sources this may be a clue.

Regards,
Robert

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