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Posts: 6659
Joined: April 4, 2000
Contributor
Since: March 1, 1999
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You wrote:
Bracing inside cabinet per your suggestions.
Golden Ratio, or 1/3 spacing?
Neither.
Looking at a picture of the Polk SDA1b, I looked for what appears to be the weakest areas of the baffle for the front to back brace's.
For a single front to back brace, that would be either inbetween the bottom tweeter and the top two woofers, or inbetween the bottom two woofers and the passive radiator. If there is no brace at either location, and you only want to use one, then the latter would be advised.
If there are some internal braces that run across the rear of the baffle itself, then adjust the location accordingly.
For side to side braces, for top to bottom spacing, I would use a 1/3 ratio and use two, but shade them a little towards the middle, say putting the edges of the top braces at the 1/3 point and the rest of the wood going towards the center of the sides. Make the locations not exactly the same in terms of the "1/3" points, place the bottom one offset from the 1/3 point (arbitrarily) about 1" closer in towards the center of the side panels, same thing with how the rest of the wood in the brace is located..
I would then shade the upper most one towards the rear of the cabinet, and the bottom shaded more towards the front. Of course, you have to make sure they do not touch the other cross braces, or interfere with the drivers in any way, such as a woofer vent in the magnet, etc.
Edges and corners of a wooden box are stiffer than the open portion of the panels, and on cabinets where the baffle is recessed, this tends to stiffen the side wall much more in that area. The Polks do not appear to have a recessed baffle, so in your case that aspect is not in play.
You wrote:
Dampening behind mid range, and sda array, thinking of sides too.
I do not want to change volume but minimally.
Maybe dampen the mid and sda chassis with hot glue or rope caulking.
What do yo think?
Most hot melt is going to cool to a fairly hard consistency, so unless you can get some that stays "soft" or has a rubbery aspect to it, I would not use hot melt.
Rope caulking would work great for damping, except that it has a tendency to fall off over a long period of time, and so for that reason, I would not recommend it. It does stick to (clean) metal better than wood, so the actual driver frames might be OK, but I wouldn't trust it to hold to the wood panels long term.
If you do use rope caulk, condition it enough to get it good and sticky, place it on the driver frames, and then use a heat gun (at a distance) to warm everything up (careful not to damage the surrounds or cones, etc.) so that it actually "melts" a little bit onto the metal, this should help make it stick, as long as you push it back into place after the outer layer cools a bit. Use Latex gloves while handling it, unless the tackyness does not bother you. I would suggest a store of rag cloths to wipe your hands on in the latter case.
Alternatively, you could warm up the driver very gently by placing it a warm car just before the application of the rope caulk. Avoid direct sunlight, and do not forget the drivers and leave them in the heat too long, etc. A warm garage might do just as well, just so they are not at "room" temperature to start with.
Jon Risch
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