RM-1 Acousta-Stuf damping mod

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7x57

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RM-1 Acousta-Stuf damping mod
« on: 14 Sep 2012, 02:51 pm »
Acousta-Stuf has improved every speaker I have used it in, even Marshall guitar amp cabinets (but not as much as trashing the cheap original Chinese Celestions and installing a good USA Eminence or English Celestion speaker). Deeper and cleaner bass is the result, cheaper than open cell foam pads, no itchy fiberglass to deal with when going back into the speaker later.

While the manufacturer recommends a half pound per cubic foot for most sealed systems, I stuff to about one pound per cubic foot in an audiophile speaker. That prevents the need to use 3M 77 adhesive to stick it to the cabinet walls. In a sealed guitar amp cabinet, I use 1/2 pound per cubic foot and attach to the walls with 3M 77 spray adhesive.  One pound per cubic foot is the transmission line stuffing recommendation by the manufacturer, and T-lines have tight bass when done right, and the goal of a PR system is to remove the port noise and unloading of the woofers below port resonance that bass reflex speakers have. I have stuffed the ports with resistive dampers on every bass reflex speaker I now own, but the addition of Acousta-Stuf improves the sound further.

Parts Express sells Acousta-Stuf at $45 for a 5-pound bag. That is the right amount to stuff a pair of RM-1 speakers, in my opinion. For some reason, my latest bag came without the "fluff it out" admonition that a previous bag came with. There are many clumps that need to be fluffed out and the material is highly compressed when you pull it out of the shipping box. I spent about 2 hours per speaker taking handfulls of the material and fluffing it out as much as possible. Lay the speaker on its back and remove the drivers. Throw in the puffballs to form an even amount throughout the speaker. If you have done the job right, there will need to be only a slight amount of compression of the Acousta-Stuf. It is trilobal and has much more surface area than regular polyfill, so you stuff to a lower weight for the same damping results. You might not think you have enough when you get the 5-pound bag, but its enough, and I doubt that those who stuff to the 1.5+ pound per cubic foot level are doing the time-consuming fluff operation.

The puffballs will work themselves in against the woofer and PR cones under pressure of their weight. Take a thin sheet of fiberglass batting cut to the size of the cabinet bottom to sit over the PR and form a shield. Do the same for the space behind the woofers, cut to size to fit from midrange enclosure to cabinet bottom. This slight amount of fiberglass batting will have minimal effect on the Acousta-Stuf performance. Under normal use, the Acousta-Stuf is supposed to expand slightly.

You have spent about $50 and an afternoon of your time so far if you do this mod. If this were done at VMPS, I suspect the fluff job would add much more to the cost than the actual cost of the polyfill material. Your bass performance should be better than fiberglass damping material if you did the job right. You may have to change your weight on the PR. I added weight by putting on another heavy layer of wood glue, plus I live at 3300 foot altitude, so my damping is different than sea level users. I have useful bass response down to 25Hz at my listening position.

Phil