Before I share the results of my testing on damping material for the X-LS Encore, I thought it might be useful to know the following:
- I try to be as consistent as possible and only change one thing at time as I measure.
- Unless otherwise specified, I do secure crossover boards with pan head screws and the underside of the board gets generous strips of butyl rubber. I also use butyl rubber to minimize potential component/wire vibration on the boards. So, while my intent is to keep the crossover adding any mechanical noise, the side benefit is changing the resonant modes of any panel where the crossover is mounted. For this speaker, the boards are mounted on the bottom and the lower inside of the right side panel.
- While an impedance sweep is currently my best tool for identifying resonances, it may miss some and identify others that are potentially inaudible. Even if the designer publishes his curves, it is not always clear they are fully representative of flaws in the design. In my experience, the designer's may just post sim graphs or a golden measure. A really clean graph usually indicates a really well done implementation but may just an overly ideal one too. As we have seen with some well built commercial speakers, small resonances are not all that uncommon.
Here are my test cases based on materials on hand. For most cases, I only applied the material to the largest exposed areas (the top, the tops of the sides and the open part of the bottom):
- 1/4" F13 felt (originally obtained to do diffraction reduction duty on front baffles)
- 5/8" foam sheets (from Madisound and used in my Zaph kits)
- Acousta-stuf fiber fill (2 ounces from a larger Parts Express bag) applied only in lower back of cabinet
- Both 2 and 3
Results for Test #1 (Felt):
The felt notably blunted the ugliest of the resonances and indicates that they were likely due to standing waves formations. Still a bit of an issue at 275 Hz and a slightly greater one at 600 Hz.
Results for Test #2 (foam sheets):
Not much to say, although expected to be better than felt, looks about the same to me.
Test #3 (Acousta-stuf):
Here it seems less is more. While 2 ounces does not seem like much, it fills the back lower quarter of the cabinet. Best result so far, but maybe more is better?
Test result #4 (Foam sheet plus Acousta-Stuf)
My call is this is a bit of a draw. The 275 Hz blip seems slightly worse and the 600 Hz one is slightly better.
Hope you found this useful. My conclusion would be that Acousta-Stuf is sufficient for now. I want to see if I can isolate the remaining resonances, but probably at a point of diminishing returns. Another check I did was to see if the Bassbox sim indicates a port resonance, but the posted params for the woofer did not produce a good correlation with the real thing. So will have to measure the port directly. That is another test setup and will share later.
Finally, my brief listening tests were generally positive. My wife would probably tell you that I never met a speaker I did not like though.