Answer to Question 1
The industry uses a noise criteria system to designate the level of noise in the area where by the registers will be used. Each NC curve is assigned a number, in 5 decibel increments. The number corresponds to the decibel reading at approximately 1500 Hz.
Answer to Question 2
Black iron is the natural iron finish. Galvanized is dipped in zinc to make it more resistant to corrosion.