1. An apparatus for enhancing a source decoder, the source decoder generating a decoded signal by decoding an encoded signal obtained by source encoding of an original signal, the original signal having a low band portion and a high band portion, the encoded signal including the low band portion of the original signal and not including the high band portion of the original signal, wherein the decoded signal is used for a high-frequency reconstruction to obtain a high-frequency reconstructed signal including a reconstructed high band portion of the original signal, the apparatus comprising: a high-frequency reconstructor for generating the reconstructed high band portion of the original signal from the decoded signal; a noise adder for generating the high-frequency reconstructed signal having a noise content similar to the noise content of the original signal by adaptively adding noise to the reconstructed high band portion of the original signal; and a complex low delay filter bank for synthesizing an output audio signal from a combination of the decoded signal and the high-frequency reconstructed signal.