I dunno. I flew helicopters for 15 years and never used ear plugs. I only used the issued helmet. The noise levels were supposed to be in the damaging range. I did try them a few times, but couldn't hear the turbine wind up/down, etc, and decided that flying nights without hearing the engine was more hazardous than risking hi freq hearing loss. I had my ears checked every year and never had a drop up up until I retired in 1995.

I think that part of the noise factor is where you are around the plane. I always get to the other side of the muffler before going WOT. I throttle up near wide open, but not for long.