A sort of Infinite Baffle question

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Not exactly, but there is no better way to title this.
In another thread someone was using a PE sub plate amp with a 12in driver in a sealed box. The thead was about his mains, but at one point he had gotten around to actually measuring the amount of signal getting to his 12in driver through his PE amp and discovered significant output above what he'd been setting the cutoff at. He said he'd applied some expensive filtering hardware he had setting around and it improved things enormously.

This brings me back to a question I had years ago about sub placement. Frequencies below 100hz or so travel almost unimpeded through the average wall/floor. If a sub going to be used strictly for music, and one wishes to cut off lower than 80hz, or even lower, could the subwoofer be mounted in the joists below the listening room floor? No aperture between. Only the low freq energy that is able to propogate through the floor is filling in the bottom end of the mains. Any of the higher frequency (trash in this case) is naturally blocked. Obviously a good sized amp would be needed to get something of a reasonable match in efficiency with the mains.

I no only as much about room loading, wall/floor/ceiling reflections, phase nulls, etc as I've gained in these forums from casual reading, but this seems like this arragement might even address a few of those problems as well.
 
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Frequencies below 100hz or so travel almost unimpeded through the average wall/floor.

"almost", I work in car audio, and deal with a lot of issues related to having the subwoofer in one compartment (boot) and the listener+rest of the speakers being in the other part, this is comparable to your suggestion of having the sub mounted below the listening room floor.

In every sutiation where the boot is well sealed off from the cabin space, I see significant improvement in bass quality and volume by directly venting/ducting the subwoofer output into the cabin.

Sound doesn't just travel through a wooden floor (unless it has holes/leaks), it transfers energy to the floor, which then vibrates and transmits that energy to the air in the room, you're essentially turning the floor into a passive driver, wooden floors are bad enough in the way they normally behave with bass, let alone turning them into a sub 🙂
 
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