High-Efficiency Advanced Audio Coding (AAC+) is a standard for encoding audio data that far exceeds existing formats for quality and space, including mp3 and ogg.

Here’s a quote about people’s general perception of quality of AAC+ from the wikipedia article:

Scientific testing by the European Broadcasting Union has indicated that HE-AAC at 48 kbit/s was ranked as “Excellent” quality using the MUSHRA scale. [7]. MP3 in the same testing received a score less than half that of HE-AAC and was ranked “Poor” using the MUSHRA scale. Data from this testing also indicated that some individuals confused 48 kbit/s encoded material with an uncompressed original.

Try out this 32kbps example:

HE-AAC+v2 (44100Hz Stereo@32kbps) 1.1 MB (http://teknoraver.net/software/mp4tools/)

That example is about 4:30 in length and weighs in at 1.1mb. My 192kbps songs of that length come in at 6mb+. A 6x savings is downright impressive.

Audiophiles will surely detect difficiencies in the audio. I feel I can if I really listen. But just listening for enjoyment and considering the space savings, this is a real miracle of an encoding format. I say this because only a few years ago a 64kbps mp3 stream was worse than am radio quality.