Check out the active noise control faq:
ftp://rtfm.mit.edu/pub/usenet/sci.answers/active-noise-control-faq
-Adam

> WF wrote:
>
>     Hi,
>
>     A student is studyng how to invert some wave...
>
>     He intends to NULL some produced sound...in the factory that he works have
> some TEXTIL machines and intends to reduce the noise produced by
> them...applyng a inverted wave...
>
>     Does someone have some suggestions about this? Formula, site, etc...
>
>     I think that is too hard!
>
>     Miguel Wisintainer
>