聲級

概述

The Day-Night Sound Level (Ldn) is an EPA metric that summarizes the A-weighted equivalent sound level over a 24-hour period. Nighttime hours (10 PM to 7 AM) receive a +10 dB penalty to account for increased human sensitivity to noise during sleep. This descriptor is widely used in environmental noise assessments, land-use planning, and regulatory compliance.

關键公式

Day-Night Sound Level

$L_{dn} = 10 \log_{10}\left(\frac{1}{24}\left(15 \cdot 10^{L_d/10} + 9 \cdot 10^{(L_n + 10)/10}\right)\right)$

Where 15 hours are daytime and 9 hours are nighttime.

變量

Typical Outdoor Sound Levels

計算器

單位換算器

The source page includes a Unit Converter heading. Sound levels are logarithmic, so this converter works with sound-pressure ratios, intensity ratios, and dB differences rather than treating dB as a linear unit.

計算示例

With daytime level $L_d = 70 \, \text{dBA}$ and nighttime level $L_n = 50 \, \text{dBA}$:

$L_{dn} = 10 \log_{10}\left(\frac{15 \times 10^{7} + 9 \times 10^{6}}{24}\right) = 68.2 \, \text{dBA}$

The 10 dB nighttime penalty effectively treats the 50 dBA nighttime environment as 60 dBA when weighted.

Sound Descriptors

The EPA uses four sound descriptors to evaluate noise impact on health and welfare:

• A-weighted sound level – instantaneous, frequency-weighted measurement
• Sound Exposure Level (Ls) – total sound energy in a single event, normalized to 1 second
• Equivalent Sound Level (Leq) – constant level with the same energy as the varying sound over a given period
• Day-Night Sound Level (Ldn) – 24-hour Leq with a 10 dB nighttime penalty

还原的原始源表

The following tables are restored from the original source page to preserve the complete reference data.

Typical Day and Night Sound Level vs. Location

工程要點

• A 10 dB increase in Ldn roughly doubles the perceived loudness; Ldn values above 55 dBA are generally considered incompatible with residential tranquility.
• The +10 dB nighttime penalty reflects the EPA's weighting of sleep disturbance, not a physical measurement change.
• Ldn is community-averaged and does not capture single-event extremes like aircraft flyovers; for those, Sound Exposure Level (Ls) or Maximum Sound Level (Lmax) may be more appropriate.
• Measurement positions, ground effects, and meteorological conditions can shift outdoor Ldn readings by several dB; standards such as ANSI S12.9 govern measurement procedures.

參考资料

• Original Source – Engineering ToolBox