Quote:
Originally Posted by JouCalle 
The plastic (reflector) moves along with lower frequences but cannot move as fast as high f so they bounce of, maybe?
I need to understand this! :P
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I'm no scientist, but...
Just imagine a large sheet of super thin plastic only (like plastic food wrap), hanging in front of a door opening. Regardless of how it may vibrate, I don't see how it could effectively absorb low-end. If it does, it would be such a minimal amount that it would not be audibly detectable.
But since the plastic does have a bit of mass technically (very small amount), it does reflect something... the very highest of highs... whatever super short frequency wave-length it's small mass / thickness is able to interrupt. This is something that CAN be detected by ear... I know because I've tried it. If you cover an open room frame with plastic only and step inside, you WILL hear very high frequency reflections.
As far as using plastic in a bass trap where the plastic is covered with at least 3/4" of fluffy batting material, I don't see how the plastic will reflect anything because, let's say the plastic would have reflected 10kHz and up, the 3/4" batting would absorb the 10kHz and up before it even hit the plastic, and everything below 10kHz will just go right through the batting and plastic like it wasn't even there... the ultra thin low mass plastic cannot interrupt longer waves.
Then just make sure to have some good dense absorption material behind the batting and plastic to eat up the longer waves / lower frequencies.
Anyway, this is all just per my understanding, I am not a scientist and am not proclaiming that any of this is 100% correct. I am however building some custom traps based on these ideas that I will soon be putting to the test and will report back. I may not do any scientific measuring, but will test
musically... and if it SOUNDS good, it IS good.