Since I Started college 10 years ago I have been verg strict when listening to music. I used to listen and download new music each day, couldn’t listen to a mp3 @128kbps and I could easily tell the difference between 128 and 320 Easily but nowdays I can’t. Tried replacing my cheap Z312 logitec speakers with Beyerdynamics DT 770 PRO but couldn’t notice difference neither Did MP3 encoding just got better over the years or my ears are not so good like they used to be?
The encoders are a moving target. The output bitrate is a hard limit on the output rate of the encoding, but there are many choices that encoders can make on how to represent the audio at that rate. For instance, there is compression involved, which is sensitive to minute details of the data encoding and trivial changes to data can compress differently. There’s a byte reservoir that allows use of more bytes temporarily if particular part of music needs it, but you have to pay that “loan” back by using less bytes later. The psychoacoustic models may have become better, and increased CPU power can also be used to perform more exhaustive search in the encoded audio space for the best representation.