Originally Posted by
tdracer
It's mainly ECS - I've even heard Boeing test pilots comment on how much quieter the Airbus ECS is compared to Boeing and wondered how they did it.
Hopefully the ECS noise is being addressed on the 777X.
If this is so, then the solutions are basic and well known from ground installations. Ductwork of optimum size for the airflow. No sharp corners or square sections in the ductwork. Fans with assymetric blades to reduce harmonic noise. It's all stuff that Boeing must surely be aware of, and it's been an issue since the 777 first came along.