Psychoacoustics is the branch of psychophysics involving the scientific study of sound perception and audiology—how humans perceive various sounds. (including noise, speech, and music)
Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they produce. Psychophysics has been described as the scientific study of the relation between stimulus and sensation.
In conclusion, Psychoacoustics is sound, how we hear things and what we hear. Psychophysics is how we process the sound and what we do to react to it. It is also what we feel, physically. It is how we react to things (such as sound.)
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