If you pump air from one tube into a wider tube, the pressure will drop when it enters the larger chamber. Simple physics. By adding wadding the pressure waves will be diffused far better than adding rubber or foam though. Something similar to acoustic wadding or felt would work well, closed cell foam or rubber doesn't actually give the pressure wave far to go.
Because of the differences in the way airsoft works compared to real firearms you need to stop thinking about how something like a real suppressor works and more like how you dampen a speaker enclosure or an exhaust muffler.
More to the point, as mentioned above - if we're talking about a can on an AEG, what comes out the front is way less of a consideration than what comes from the gearbox and piston assembly.