The annoyance due to a given noise source is perceived very differently from person to person, and is also dependent upon many non-acoustic factors such as the prominence of the source, its importance to the listener's economy and his or her personal opinion of the source. For many years, acousticians have attempted to quantify this to enable objective assessment of noise nuisances and implement acceptable noise limits. When large numbers of people are involved, reactions tend to be distributed around a mean, and the Rating Level (Lr) parameter has been developed in an attempt to put a numerical value on a noise to quantify its annoyance in relation to the general population.
The Rating Level is defined in the ISO1996-2 standard (see section on International Standards). It is basically a measure of the noise exposure corrected for factors known to increase annoyance. It is used to compare measured levels with noise limits that usually vary depending on the use of the property under investigation (see section on Assessment). The basic parameter is the A-weighted equivalent continuous sound pressure level or LAeq.
The formula for the Rating Level is (in general terms):
Lr = LAeq + KI + KT + KR + KS
KI is a penalty for impulses
KT is a penalty for tone and information content
KR is a penalty for time of day
KS is a penalty (positive or negative) for certain sources and situations