I have an older RM-1 built from a kit I ordered in 2001. The PR foam was getting deteriorated and a bit stiff, so I thought about replacing the foam surround. Instead I decided to try a mod, and if it didn't work I would order new refoam kits. I already had the white PVA glue from a previous refoam kit. I spread a bead of PVA glue all around the foam with the PR facing up, just enough for a thin coat across the foam roll, then quickly brushed it evenly over the foam. After it dried, I flipped the PR face down and did the inside of the foam roll, but did it in four sections as this side is harder to access. Now when the PVA glue dries clear, you will notice that the foam is stiffer than it was, and there is some hysteresis effect when you work the suspension. This is a bit like a resistive flow vent or a well stuffed transmission line. However, resonant frequency will be raised, so you must add considerable more weight to the cone. I used TWO heavy coats of the wood glue mod that is already common for the RM-1. It seemed to return the cone to a similar resonant frequency than before the mod. The worst thing you can do with this mod is use too much PVA glue when coating the foam. You might want to do only one side and see how that goes.
My bass is now extremely tight and well extended. When setting my meter to zero on the 90dB scale and using a correlated pink noise, at my listening position I get -1dB at 40Hz, -3dB at 31.5Hz, and -6dB at 25Hz. With the meter set to zero on uncorrelated pink noise I get +1dB at 40Hz, -1dB at 31.5Hz and -4dB at 25Hz. This is a slower rolloff than I get with my Vandersteen 2Ci speakers, which are flat at 40Hz, -6dB at 31.5Hz, and fall on their face at 25Hz. On top of that, the RM-1 has a higher efficiency as well more bass extension and higher power handling capability without sound deterioration. Would be nice to compare an unmodded and modded RM-1 side by side. The PVA glue should help extend the life of the foam if done when the foam is new, and it would be less stiff than my older foam. You could also change to a rubber surround and get twice the lifespan of foam, at the least. BUT.....and this is important, PVA glue does NOT stick to rubber surrounds. Neither does PU glue. Leave the old glue on the metal frame when replacing foam, in most cases, as long as it is sticking tightly. Old foam can usualy be rubbed off as it is literally falling apart.
I also refoamed my original woofers and used rubber surrounds, but that was a much more involved project and another long post. The heavier rubber surrounds will slightly lower your resonant frequency and help drive this heavier PR mod. If doing the PR foam coating mod, expect something more like tight transmission line bass rather than any bass extension. You are adding hysteresis to the suspension, much like adding more acoustic dampening stuffing in a T-Line.
