Every sound our ear-brain system processes, it receives the following qualities of sound signal through ear.
- Pitch - frequency of sound.
- Loudness - the intensity of a sound.
- Distance - perception of how near or far away a sound source is.
- Timbre - tone quality.
Intensity of sound energy as received by the ear-drum is perceived as Loudness. Another term which describes loudness is amplitude of the sound wave. For a sound wave following figure describe its amplitude
As the sound waves impinge on the ear drum, their amplitude is converted into the amplitude of its vibrations. Higher is the amplitude of sound wave, higher is the pressure created and transmitted, via air, on the ear drum. Which results in higher amplitude of vibrations of the ear drum. These vibrations are transmitted to the brain through the other parts of ear and analyzed there.
It is to be remembered that sound loses energy as it travels away from its source so it gets gradually less loud, or its amplitude decreases.
This is how humans perceive the intensity of sound.