Westmassguy's Idea: Employing Damping to Mitigate Comb Filtering and Lobing

Polk Folks:

I have a pair of 1980 vintage RTA-12's that I have been modding for months. I had done most all the standard Polk Forum recommended improvements, the latest being a complete Westmassguy crossover rebuild with Sonicaps/Mills resistors (see David's post "RTA-12 Original Project" in the DIY, Mods & Tweaks category). One job remaining was to install Sonic Barrier or Blackhole 5 inside the cabinet. Before starting David suggested that we experiment with damping material. David explained that since these speakers (and other Polk models) have two drivers side by side, producing the same frequencies, problems can occur such as lobing and comb filtering. He explained that this can affect the virtual point source created by the tweeter and woofers, and that smearing and loss of detail are common problems. Since later versions of the RTA-12 had the outer woofers rolled off by the crossover at 600 Hz, David suggested that we try overdamping the outer woofers and use the standard four-inch strip behind the inner woofers. To accomplish this we needed to extend the original 4x9 inch brace from the top of the cabinet down between the woofers to the exact bottom of the woofer openings. Basically, as David described we needed to create a three-sided box around the outer woofers so all three inner sides could be completely covered with damping material. David explained that deep bass frequencies would not be affected, but upper bass and midrange would be altered on the outer drivers, so less comb filtering and lobing would occur resulting in a more precise image between the two speakers.

The results of this were more than successful. These speakers, which already performed quite well, now have much improved imaging, even greater detail/resolution, improved clarity and they disappear behind a nice wide soundstage. Bass is huge, if the recording goes low, I can really feel it. I did replace the gaskets on the drivers and PRs in this latest project so there is definitely no air leakage. The damping material has also eliminated this speaker's tendency to become "shouty" at high volume (Nightfall has noted this as well in other postings). Also, the relative balance of midrange, bass, and high frequencies is the most realistic I have ever heard from these speakers (midrange was a bit much previously from all that backwave rolling around in the cabinet?).

If you have a Polk model with the dual drivers producing the same frequency this mod will take them to the next level.

The most difficult aspect of this job is cutting and installing the extension to the existing brace. Using a table saw I cut the appropriate amount of 5/8 plywood in an "L" shape to within 1/16 tolerance (and of course each speaker is a bit different...), so that I could wedge the extension in there and epoxy it without fasteners or clamps. You want it bombproof. I used PC Woody wood epoxy as it is a putty, has great gap filling properties, does not drip/run, and does not cure by the evaporation of solvent. I employed a small detail sander that could access the woofer openings to smooth the joints before installing the damping. I used the thickest version of Sonic Barrier. Because of all the flat bracing on the sides and back of in the cabinet, there was very little flat surface, so I had to cut and fit the Sonic Barrier in like the tiles on the space shuttle. I did my cuts with a band saw, which worked very well with next to no waste. The poly batting was reinstalled behind the drivers up above the PR area.

Sorry about the lack of pictures. I was so intent on getting this finished I forgot to photograph what I was doing.

All the credit for this mod goes to Westmassguy (thank you David!). His idea, his genius, his patience, his time and effort to think it through and explain it all. We're the ones that get to sit back end enjoy his fine work (our new crossovers) and his advice.