Question

how does loudness of sound vary with amplitude

Answer 1

The amplitude of a sound wave determines its loudness or volume. A larger amplitude means a louder sound, and a smaller amplitude means a softer sound. The frequency of a sound wave is what your ear understands as pitch. A higher frequency sound has a higher pitch, and a lower frequency sound has a lower pitch
The human ear can detect a wide range of frequencies. Frequencies from 20 to 20 000 Hz are audible to the human ear