[makmanalp]

It's really amazingly impressive that it also holds well across the different musical pieces and different frequencies / timbres. Is this yet another order of magnitude improvement in audio file size?

I wonder why the MP3 at low bitrate sounds a bit like a lowpass filter.

[jmvalin]

At low bit-rate, lame applies a low-pass and resamples to a lower sampling rate. If it didn't to that, MP3 would end up completely starved of bits (trying to code all frequencies) and have really atrocious artefacts. Based on personal experience and the results from a few listening tests I saw, I think you can assume that Opus can now give you about the same quality as MP3 for about 60% of the bits. For example, 76 kb/s Opus should be around the same quality as 128 kb/s MP3.

[Avery3R]

Because it basically is a low-pass filter. http://i.imgur.com/OqEks7A.jpg