In general, noise refers to unwanted sound, but in audio systems it is the low-level hiss or buzz that intrudes on quiet passages that is of most interest. All recordings will contain some background noise that was picked up by microphones, such as the rumble of air conditioning, or the shuffling of an audience, but in addition to this, every piece of equipment which the recorded signal subsequently passes through will add a certain amount of electronic noise, which ideally should be so low as possible to contribute insignificantly to what is heard.