Which of the following indices represents the background level of noise pollution?

The L90 index gives the sound level exceeded for ninety per cent of the time, so it represents the background level of noise.

The L10 index gives the level exceeded only ten per cent of the time, marking the peaks rather than the background.

The L25 index reflects a fairly high level exceeded a quarter of the time.

The L50 index is the median level exceeded half the time, close to the average.

Since the background is the quiet base that persists most of the time, L90 captures it.

These percentile indices are used to describe how noise levels vary through a period.