SBR

SBR stands for Spectral Bandwidth Replication is an audio enhancement technology designed especially for low bitrate audio and speech codecs. It improves the perceived audio quality by increasing the bandwidth at a low bitrate, and by increasing the coding efficiency of the underlying codec by limiting its bandwidth.

SBR method can be used with any codec capable of producing an acceptable audio quality at the given level. The codec transmits the lower frequencies of the spectrum, and the SBR algorithm takes care of the higher frequencies. The SBR decoder generates the frequencies mostly by analyzing the lower frequencies, with a little help from low-datarate guidance information.

SBR has applications wherever bandwidth is limited. For example in Internet radio broadcasts, and mobile audio communications. It can be used for mono, stereo, or even multi-channel audio.

You can find a very informative description of SBR at Coding Technologies