SBR (Spectral Band Replication) is the bandwidth-extension tool that makes HE-AAC efficient at low bitrates. Rather than spend bits coding the full spectrum, the encoder codes only the lower band with AAC-LC and adds a small stream of guidance — envelopes and noise/tonal parameters — telling the decoder how to rebuild the high frequencies by transposing and shaping the low band it already has. Because high frequencies are perceptually forgiving, this reconstruction is convincing at a fraction of the data. SBR is also used in mp3PRO and other codecs. Its weakness is that aggressive settings can make highs sound smeared or 'watery', a hallmark of very low-bitrate AAC+.
