For a variety of reasons, some not completely understood, there is significant variability in the responses of individuals to noise. There is a general increase in annoyance with increasing dB(A) level as illustrated in the figure below which shows the proportion of the population highly annoyed by noise from air, road and rail transport. Similar curves have been produced for low and medium annoyance.
The noise level measure used here is the Lden which is the 24-hour daily equivalent noise level but with 5 dB(A) added to evening noise level and 10 dB(A) added to the night time noise level, in line with increased disturbance in the evening and at night. These curves are sometimes referred to as Schultz curves and are based on the synthesis of social surveys on noise annoyance by T.H.J. Schultz in 1978. The Lden has been adopted by the European Union as an appropriate metric to assess annoyance from transport noise.
Acceptable limits for noise generally align to the levels that would, if imposed on the whole population, be considered to be highly annoying by 10% of the population. This level of annoyance does not occur because people will tend to reside away from what is personally considered to be a source of annoyance.