Why is sound wave of intensity `10^-12(W)/(m^2)` and frequency 1000 Hz taken as the standard for expressing the intensity level of all other sound waves?

Explanation

The human ear is most sensitive so intensity variation at 1000 Hz. At this frequency, the intesity of just audible sound is `10^-12(W)/(m^2)`. This is why the intensity of level of a sound wave of frequency 1000 Hz and intensity `10^-12(W)/(m^2)` is taken as the reference level for expressing intensity.