CCC - the cap simply adds an extra layer between your skull and the pressure wave coming through from above, and helps to absorb some of the low frequency waves. It makes quite a difference, and if you try with and without irrespective of whether on top or under the headset, you will immediately notice the difference. Then try and put something else on top of that such as a hood or a newspaper and it gets a lot quieter again. The idea is that the tinnitus is caused by the low frequency pressure waves effect caused by the rotor directly above the head, and anything that cushions/absorbs this between the source and skull will reduce the effect. I am in no doubt as to the validity of this theory.
Droopystop - you are quite right, and as far as I am aware the full benefits of a helmet have yet to be investigated, and therefore I am yet to be fully convinced myself. What we do know is that the levels are unacceptably high. What I am certainly advocating is a much discussion and trial as possible and finding where the solution lies and not just dismissing options for whatever reason. There are multiple factors, as we are all aware, and it is not just noise, but also various types of vibration, and it will probably require a combination of fixes, such as a helmet for vibration and low frequency and ANR/DC or good earplugs for higher frequency. Perhaps there are alternatives to the helmet that offer more comfort such as some form of fitted hood made from a material that will absorb the vibration. This however might upset that fashion conscious amongst us as it may give the impression of a wannabe Russian cosmonaut in the cockpit, and of course we wouldn't want to confuse the passengers!
