As Wikipedia puts it: “Psychoacoustics is the scientific study of sound perception”. While there’s a lot of theoretical research on the topic, one of the main application of psychoacoustics is lossy audio coding. One of the first codecs to make use of psychoacoustic tricks — long before MP3 was born — is the G.711 (u-law/A-law) codec. In general, lossy audio codecs attempt to reduce the bitrate by coding the audio signal with just enough accuracy to avoid the distortion being audible.
What you can get away with
There are many types of distortion that can be inflicted on an audio signal without causing too much audible degradation. Here are some examples below.
The human ear is almost completely insensitive to the phase of signals. For example, we can’t distinguish between a waveform and its inverted version (the only reason loudspeakers have a red and a black connector is to avoid wiring them 180 out-of-phase with each other and getting cancellation effects). As long as the phase distortion is constant (or nearly constant) in time and that the variation in group delay across frequencies isn’t enough to cause temporal smearing, then the phase can take a lot of …
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