Thx for all your inputs.

So more info asked.. allright:
Im building an anti-sound module - Im going to stop noise with noise (as in V8 engine noise (until 250 Hz, rest is dampened with passive isolation). I allready managed to produce a working module with round speakers. But indeed, as mentioned, the next module has far less space (height) available. So oval shaped speakers would be great to use, but only when their output is pure.
The round speakers cone material was PP ( 6" )
btw. Bert, I know what ya mean » Me is Dutch too.

ColdPlays