Background noise may be defined as background sound that is apparent in the room, stemming from many possible sources such as the heating, ventilations, and air-conditioning (HVAC) system, other equipment in the room, exterior sources such as traffic or airplane noise, or neighboring spaces.
Currently there are two noise criteria that are recommended in ANSI standard S12.2-1995: Balanced Noise Criteria (NCB) and Room Criteria (RC). However, many more have been proposed in recent years, including Room Noise Criteria (RNC) and RC Mark-II. Research is needed to validate these various rating sytems with actual subjective testing data, and to develop criteria which can characterize the effects of discrete tones and time-varying fluctuations.
The calculations of many of the noise criteria methods listed above involve first measuring the sound levels in octave bands across a certain range and comparing them to certain developed curves.
Optimum values of background noise for concert hall settings are quite low (NC-20 or lower, RC-20(N) or lower).
To meet low noise criteria rating, one should carefully design the HVAC systems: use low air velocities, appropriately NC-rated diffusers, and well-balanced duct designs. Self-generated noise can be a problem in these systems too. To reduce this type of noise, encourage smooth flow throughout the system (with turning vanes and smooth transitions between ducts), and place dampers as upstream as possible from diffusers.