Interior Noise Level Design Criteria

When specifying interior noise level objectives for different areas of a building, many factors have to be considered. In general, the ventilation system noise should be low enough so as not to interfere with speech or other communicative sounds. In determining the acceptability of the air‐conditioning system noise, it is necessary not only to aim for a particular noise level or loudness but also to consider its relationship with any other noise which exists in the area. An attempt to achieve a good balance between the two is important. In particular, other sounds which may be important in deciding upon a particular noise level include:

1. Necessary sounds, such as speech, music, audible warning signals, etc. Here the HVAC system noise level should be kept low enough to ensure it does not mask such sounds.
2. Sounds which should not be heard, such as speech from adjacent areas. In this case, the ventilation system noise can be used to mask the unwanted sounds.
3. General ambient noise, both from outside and within the building. In this case, the ventilation system noise created should be kept just below the general ambient noise level.

Beranek was one of the first to suggest suitable noise criteria [21, 22]. Table 6.4 in Chapter 6 shows acceptable ranges of background noise levels produced by air‐conditioning systems in various types of buildings, evaluated with the NCB curve approach suggested by Beranek [23, 24].

The lower NCB value is usually used only in situations where the general ambient level is very low and where good noise reduction techniques are employed. The upper end of the scale is used in buildings with a relatively high ambient noise level caused by other sources. It should be noted that these ranges are based on the assumption that the ventilation system noise is steady and has a broadband frequency spectrum. If this is not the case, then more stringent limits should be specified (i.e. for impulsive or pure tone sounds). The NCB curves can be used to evaluate the acceptability of the air‐conditioning noise.