Loudness of sound wave is ditectly proportional to [[1]]  amplitude.

The amplitude of vibration of the vibrating object determines the loudness of the sound. The sound will be louder if the vibration amplitude is greater. When a Sitar string is lightly plucked, it vibrates with a small amplitude, resulting in a faint or feeble sound. A sitar string vibrates with a large amplitude and produces a very loud sound when it is plucked hard.

The square of the amplitude of vibration is directly proportional to the loudness of sound.