Hmmm. It would be awesome if someone could get to the bottom of this mystery and figure out what causes the issue. Is it the dampening pads, the sliders, something else, etc?
Yes, it would be awesome. My intuition tells me it's air rushing
between the sliders and the housing. This seems plausible since the tolerances are supposedly tighter in the Type-S; that's why nothing similar is seen on standard HHKBs. This does not, however, account for why it tends to be in the middle of the board. Maybe the case mold produces slightly smaller holes in the center?? This is definitely a problem where keyboard science could help; however, it is not my place to tear this Type-S apart since it's going to be returned to EK. But yes, I think someone who's willing to dig into their board and methodically test different variables could get to the bottom of it. Better yet, PFU should get to the bottom of it.
Ohh and intelli, did you use actual landing pads (o-ring style) or did you use little donuts shaped cutouts from a silicon/rubber sheet like some others did?
OK, so there are a few different options to distinguish between
1. Bona fide soft landing pads
http://elitekeyboards.com/products.php?sub=access,slpads&pid=sl120_cs2. Dental bands
3. O rings
4. Silicone/rubber punched out of sheet
I have done (1) on an FC660C. I ironed the pads down to <0.5mm. It was extremely effective at dampening sound. It made the board so quiet that it was boring and I eventually reversed it!
I have also done (2) on a HHKB, as per the video shown above. The result was not nearly as quiet, but it definitely takes the edge off the upstroke and the sound is lovely!
Dental bands are much smaller and thinner, in general, than o rings. Couldn't say how they compare in thickness to the silicone sheet, I assume that just depends on what you source.
Personally, I think (1) (2) and maybe (4) are the viable options. Most o rings I've seen would be so thick that they'd screw up tactility. The dental bands I used didn't.