lden

Lden, short for Day–Evening–Night Level, is a metric used to quantify environmental noise exposure over a 24-hour period. It is designed to reflect that people are more sensitive to noise during evening and night hours by applying penalties to those periods. Lden is based on three period-specific A-weighted equivalent sound levels: L_day for the daytime portion, L_evening for the evening period, and L_night for the night period.

The standard way to compute Lden combines these period levels with time-based penalties. The common formula is:

Lden = 10 log10 [ (12/24)·10^(L_day/10) + (4/24)·10^((L_evening+5)/10) + (8/24)·10^((L_night+10)/10) ].

This corresponds to 12 hours of day, 4 hours of evening with a 5 dB penalty, and 8 hours of night with a 10 dB penalty, all using A-weighted levels. In practice, L_day, L_evening, and L_night may be measured or modeled for a location.

Lden is widely used in environmental noise assessment and planning, particularly in the European Union. It features prominently in noise maps and regulatory frameworks to evaluate and compare the impact of different noise sources, such as roads, railways, and airports. It is analogous to the Day–Night Level (DNL) used in other regions, though the penalties and interval definitions differ. Limitations include its reliance on fixed time intervals and its inability to capture short-term peak events or non-typical activity patterns.