Sound becomes noise when it is unwanted, bothersome or painful.
Sound is measured on a dB scale. The scale ranges from zero, the average least perceptible level of sound, to about 130, the average pain level. Many researchers consider the sound pressure of 70dBA to approximate a quiet ambient condition. AI ranges from 0.0 to 1.0, with a higher value indicating better communication. The FAA considers an AI of between 0.25 and 0.3 acceptable for communication between crew members who are approximately three feet apart.