Isn't the area that we really want to be low in decibels the area right at
our ears?  That would seem to be best solvable with active noise reduction
headsets, avoiding the weight, hassle and cost of cabin sound reduction, and
yielding far greater perceived noise reduction.
Perhaps the only real reason to be concerned about noise not better
addressable by headsets is it suggests air leakage which suggests air-flow
disruption which suggests increased drag. Just an unenlightened thought.
Wendell Durr