psycho acoustics
Psychoacoustics is the science of human perception of sound. It deals with the hearing characteristics, the hearing threshold and the phenomena influencing them. Various evaluation criteria and units of measurement have been derived from the findings of psychoacoustics. For example, loudness given in Sone, roughness given in Asper, sharpness with evaluation in Acum, and Vacil for fluctuation strength.
Since many psychoacoustic phenomena are based on subjective perception, the empirically determined dependencies of frequencies and loudnesses are used in audio compression for data reduction. One of the most important effects is the masking of sounds with different loudness or the temporary masking, the masking.
During the conversion, the listener is deprived of certain sounds that he cannot perceive. Likewise, the change in hearing threshold caused by loud and soft sounds is taken into account in the data reduction, since it makes no sense to transmit sounds that are below the hearing threshold. Also, the human ear combines adjacent sound frequencies into a single sound.
In addition, speech and music can be compressed to different degrees due to their different frequency and dynamic ranges.