Which of the following indices represents the background level of noise…

Concept

A percentile noise level Ln is the A-weighted sound level equalled or exceeded for n% of the measurement period. A larger exceedance percentage corresponds to a level that persists for more of the observation time, while a smaller percentage emphasizes intermittent higher levels.

Application

The question asks for the background level. L90 is exceeded for 90% of the measurement period, so it represents the persistent lower-level acoustic environment and is used as the background-noise indicator.

Cross-check and contrast

• L10 describes the level exceeded for 10% of the period and emphasizes the higher-noise portion.

• L25 describes the level exceeded for 25% of the period and summarizes an upper-quarter exceedance level.

• L50 is the median noise level, exceeded for half of the period.

• L90 describes the level exceeded for 90% of the period and tracks the persistent lower-level environment.

Therefore, the background level of noise pollution is represented by L90.