Which of the following is represented by Noise Index L90?

The correct option is Background noise level.

To understand why this is correct, we can look at how environmental noise is measured and analyzed. Noise levels fluctuate over time, so acousticians use percentile-exceeded sound levels to describe the noise environment. These indices are generally represented as:
$L_N$
where $L_N$ is the noise level (measured in decibels, dBA) that is exceeded for $N$% of the total measurement duration.

Let's break down the significance of different percentile indices:
1. Peak Noise Level ($L_{10}$): This is the sound level exceeded for only 10% of the monitoring period. Since it represents a level that is rarely reached, it captures the louder, transient events like passing vehicles, sirens, or intermittent loud sounds.
2. Median Noise Level ($L_{50}$): This is the level exceeded for 50% of the monitoring time, representing the average or middle point of the noise distribution.
3. Background Noise Level ($L_{90}$): This is the sound level exceeded for 90% of the monitoring period. Because the ambient sound level is higher than this value for nearly the entire duration (90% of the time), it represents the steady, underlying quietest levels of the environment when temporary or peak noises are absent.

Therefore, the index $L_{90}$ is the standard metric used internationally to define and evaluate the ambient Background noise level.