I used some noise cancelling headsets yesterday on a B757-200 on 2 two hour sectors, this was as a test on behalf of the company.
1 The background noise reduction was dramatic, I had not realised before how noisy the FD was.
2 For best effect both ears need to be covered by ear pieces and the FD intercom used.
3 After a while I began to feel nausious, although my fellow pilot was fine. Has anybody else experienced this?