Part of the MPEG-2 Part 7 Standard (1997) which is likely to be patent free in 2018. T166024
It enjoy a wide industry and hardware support.
What about HE-AAC
This format adds backwards compatible features to the AAC encoder for supporting low bit-rates. HE-AAC is typically used for bit-rates 32–80 kbps and HE-AAC v2 for 16–40 kbps.