Transparency
In psychoacoustics, transparency is the ideal result of lossy data compression. If a lossily compressed result is perceptually indistinguishible from the uncompressed input, then the result can be declared to be transparent. In other words, transparency is the situation where artifacts are nonexistant or imperceptible.
Transparency, like sound quality, is subjective. Many listeners are trapped by placebo effect, in which subjective like/dislike of a certain compression methodology emotionally influences his/her judgment.
The only way to scientifically prove whether a certain compression methodology is transparent is to perform a double-blind test using the ABX method. This also depends on the ability of the listener's ear to properly discern the slight, sometimes very slight, differences between two soundclips.
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