This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
Good thinking! As the sound quality mainly depends on the encoder
implementation, it is true that there is no such thing as a "Layer-N"-
quality. So we definitely only know the performance of the reference
codecs used during the international tests. Who knows what will happen in
the future? What we do know now, is:
Today, in MPEG-1 and MPEG-2, Layer-3 provides the best sound quality
at low bitrates, by far better than Layer-2.
Tomorrow, both Layers may improve. Layer-2 has been designed as a
trade-off between quality and complexity, so the bitstream format allows
only limited innovations. In contrast, even the current reference Layer-3-
codec does not exploit all of the powerful mechanisms inside the Layer-3
bitstream format.
 
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