Ldn - Day and Night Sound Level
The EPA has defined an A-weighted sound level day and night equivalent for a 24 hour period.
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The Environmental Protection Agency - EPA - has adopted a system of four "sound descriptors"
- A-weighted sound level
- Sound Exposure Level - Ls
- Equivalent Sound Level - Leq
- Day-Night Sound Level - Ldn
to summarize how sound is heard and measured, and to determine the impact of noise on health and welfare.
Day-Night Sound Level - Ldn
is the A-weighted equivalent sound level for a 24 hour period with an additional 10 dB imposed on the equivalent sound levels for night time hours of 10 p.m. to 7 am.
Examples of outdoor day-night average sound levels in dB measured at various locations:
| Location | Ldn (dBA) |
| Apartment next to freeway | 87 |
| 3/4 mile from runway at major airport | 86 |
| Downtown with construction activity | 79 |
| Old urban residential area | 59 |
| Wooded residential | 52 |
| Agricultural crop land | 44 |
| Rural residential | 39 |
| Wilderness ambient | 35 |
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