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Lday, Levening, Lnight, and Lden

The Environmental Noise Directive established the noise indicators L_den and L_night for the preparation and revision of strategic noise mapping (thus, these indicators are called “Strategic indicators” in some forums). Nevertheless, the Member States may use supplementary noise indicators for other purposes – acoustical planning, noise zoning or to assess specific interventions.

The directive defines:

• L_day (day noise indicator) shall mean the noise indicator for annoyance during the day period. It is the A-weighted long-term average sound level as defined in ISO 1996-2: 1987, determined over all day periods of one year;
• L_evening is an indicator as L_day, but for the evening period;
• L_night is an indicator as L_day, but for the night period.

The day period lasts 12 hours, evening – 4 hours and night – 8 hours. The start of the day (and, consequently, the start of the evening and the night) can be chosen by the country. Currently, not all of the EU countries are using the same exact times per period.

Therefore, L_day, L_evening and L_night are LA_eq indicators determined over the mentioned periods.

Finally, the L_den noise metric is used to reflect a person's cumulative exposure to sound over a 24-hour period, determined over all days of a year. The L_den noise metric provides a mechanism to describe the effects of environmental noise in a simple and unified way. L_den is similar to LA_eq but they differ in how noise is treated during the evening and night-time. L_den is calculated using L_day, L_evening and L_night, however, to take into account the annoyance induced by the noise during the sleeping and the evening periods, a penalty (additional dB) of 5 dB is added for the evening period (usually from 19:00 to 23:00) and 10 dB for the night period (usually from 23:00 to 7:00).

The graph below represents the meaning of penalties graphically. Orange bars represent the real hourly LA_eq – those are used for the day period. Blue bars represent the penalty that has to be added in the formula during the night period (to take into account the annoyance induced by the noise during sleeping). Green bars represent the penalty that has to be added in the formula during the evening period (to take into account the annoyance induced by the noise during the resting time).

It is important to highlight that it is difficult to relate noise annoyance to objective noise metrics – human auditory perception does not function in the same way as a calibrated sound level meter – that is why it was decided to use 10 dB penalty for the night period or why 5dB is used for the evening. This creates challenges when attempting to relate noise exposure to annoyance response. For this reason, a variety of indicators have been studied over the years.

When using prediction models to generate noise contours instead of measuring directly, the L_den reflects a person's cumulative exposure to sound over a 24-hour period, expressed as the noise level for the average day of the year based on annual aircraft operations.

Most of the airports in the United States (except Californian airports) use a slightly different version of L_den. It is called DLN or Ldn, and the only difference is that this indicator does not have a penalty for the evening period. Just the same penalty of 10dB for the night period.

Another standard indicator is L24, which again is a 24-hour indicator throughout a year, however, there are no penalties with this indicator.

Many times, when non-expert users try to find information on different websites about noise levels, they can confuse all those indicators (L24, DNL and L_den) and may make inappropiate comparisons between the values of different indicators.

In order to get an idea about the magnitude order of the values of each indicator (L_den, DNL, and L24) applied to the same noise measurement, the graph below is an example that shows four different hourly LA_eq series coming from four different locations to the same airport. The orange line is the noise monitor, which is closest to the head of the runway. The dark blue line is the noise monitor furthest from the runway.

L_den indicator is always higher than the rest, but the difference is higher when the difference between night/evening and day levels is higher.

Conclusion

It is important for the public to know the main indicators and the differences between them to compare airports and noise interventions correctly. It is essential to use appropriate indicators for the intended purpose and avoid comparisons between indicators that are not the same.