I just flew NH126 last week and we sat in the middle seats of the last row of business. It was very quiet and I don't think we ever heard any noise from premium economy behind us. This may be different on a daytime flight, though, since I think most people on the plane were trying to sleep for the majority of the flight. Also, not sure if this has to do with where we were in the plane or our particular flights, but we noticed the cabin temperature was much cooler toward the back of the cabin compared to our flight out to Tokyo a couple of weeks prior where we sat in row 3.