Unrelated (except the title): When MP3 was a new thing (1994, I think?), I had a pair of not-so-good speakers, that made it very hard to tell a 128Kb/s MP3 enc

Unrelated (except the title): When MP3 was a new thing (1994, I think?), I had a pair of not-so-good speakers, that made it very hard to tell a 128Kb/s MP3 encoding from a CD; with reasonable speakers, it was quite easy at the time, because (a) 128Kb/s is not so good quality, and (b) the encoders at the time were all simple. However, one day I noticed that there IS a very simple way to tell: Turn on the noisy fan. CD music was mostly robust to the noise - it sounded like music+noise. The MP3s, however, were much more significantly degraded by the fan noise. I suspect whatever redundancy the MP3 encoding process removed was useful to mitigate the fan noise.