A method of decoding an encoded audio bitstream at a decoder, the method comprising: receiving the audio bitstream, the audio bitstream comprising a low band bitstream; decoding the low band bitstream to get low band coefficients in a frequency domain; copying a plurality of the low band coefficients to a high frequency band location to generate high band coefficients; processing the high band coefficients to form processed high band coefficients, processing comprising modifying an energy envelope of the high band coefficients, modifying comprising multiplying modification gains to flatten or smooth the high band coefficients (111, 112), and applying (108, 112) a received spectral envelope to the high band coefficients, the received spectral envelope being decoded from the received audio bitstream; and inverse-transforming the low band coefficients and the processed high band coefficients to a time domain to obtain a time domain output signal; wherein the method further comprises evaluating modification gains, evaluation comprising analyzing and modifying the high band coefficients copied from the low band coefficients; wherein the evaluation of the modification gains comprises evaluating the following equation: Gain k = C 0 + C 1 ⋅ Mean_HB / F_energy_dec k, k = Start_HB,..., End_HB − 1, where {Gain(k), k = Start_HB,...,End_HB-1} are the modification gains, F _ energy _ dec[k] is an energy distribution at each frequency location index k of a copied high band, Start_HB and End_HB define a high band range, C0 and C1 satisfying C0 + C1 = 1 are pre-determined constants, and Mean_HB is a mean energy value obtained by averaging energies of the high band coefficients. receiving the audio bitstream, the audio bitstream comprising a low band bitstream; decoding the low band bitstream to get low band coefficients in a frequency domain; copying a plurality of the low band coefficients to a high frequency band location to generate high band coefficients; processing the high band coefficients to form processed high band coefficients, processing comprising modifying an energy envelope of the high band coefficients, modifying comprising multiplying modification gains to flatten or smooth the high band coefficients (111, 112), and applying (108, 112) a received spectral envelope to the high band coefficients, the received spectral envelope being decoded from the received audio bitstream; and modifying an energy envelope of the high band coefficients, modifying comprising multiplying modification gains to flatten or smooth the high band coefficients (111, 112), and applying (108, 112) a received spectral envelope to the high band coefficients, the received spectral envelope being decoded from the received audio bitstream; and inverse-transforming the low band coefficients and the processed high band coefficients to a time domain to obtain a time domain output signal; wherein the method further comprises evaluating modification gains, evaluation comprising analyzing and modifying the high band coefficients copied from the low band coefficients; wherein the evaluation of the modification gains comprises evaluating the following equation: Gain k = C 0 + C 1 ⋅ Mean_HB / F_energy_dec k, k = Start_HB,..., End_HB − 1, where {Gain(k), k = Start_HB,...,End_HB-1} are the modification gains, F _ energy _ dec[k] is an energy distribution at each frequency location index k of a copied high band, Start_HB and End_HB define a high band range, C0 and C1 satisfying C0 + C1 = 1 are pre-determined constants, and Mean_HB is a mean energy value obtained by averaging energies of the high band coefficients.