Method for extending the frequency band of an audio frequency signal during a decoding or improvement process comprising a step of obtaining the decoded signal in a first frequency band called low band, the method comprising the following steps carried out in the frequency domain: - extension (E401a) on at least a second frequency band higher than the first frequency band of the decoded low band signal to form an extended decoded low band signal U HB1; - extraction of tonal components and an ambient signal from the signal from the extended low band signal; - combination of the tonal components and the ambient signal by adaptive mixing using energy level control factors to obtain a combined signal; and - synthesis (E404b) of an audio signal to bring back into the time domain a signal from the combined signal, and according to which the energy level control factors include an ambient level control factor, Γ, the method being characterized in that: - the extraction step of tonal components and an ambient signal is carried out in the 6000-8000Hz band; - the energy level control factors further include an energy level control factor, fac, calculated as a function of the total energy of the extended decoded low-band signal and the tonal components, the total energy of the signal extended decoded low band ener HB being calculated according to: ener HB = ∑ k = 240 319 U HB 1 k 2 + ε where s = 0.1 and with k the sample index and U HB1 (k) the spectrum of the extended decoded low-band signal, a spectrum of 320 samples covering the band 0-8000Hz; and - the room level control factor is defined by: Γ = β ener HB − ener tonal ener HB − βener tonal where ener tonal is the energy of the dominant tonal components, ener HB is the total energy of the extended decoded low-band signal, and β is a factor. - extension (E401a) on at least a second frequency band higher than the first frequency band of the decoded low band signal to form an extended decoded low band signal U HB1; - extraction of tonal components and an ambient signal from the signal from the extended low band signal; - combination of the tonal components and the ambient signal by adaptive mixing using energy level control factors to obtain a combined signal; and - synthesis (E404b) of an audio signal to bring back into the time domain a signal from the combined signal, and according to which the energy level control factors include an ambient level control factor, Γ, the method being characterized in that: - the extraction step of tonal components and an ambient signal is carried out in the 6000-8000Hz band; - the energy level control factors further include an energy level control factor, fac, calculated as a function of the total energy of the extended decoded low-band signal and the tonal components, the total energy of the signal extended decoded low band ener HB being calculated according to: ener HB = ∑ k = 240 319 U HB 1 k 2 + ε where s = 0.1 and with k the sample index and U HB1 (k) the spectrum of the extended decoded low-band signal, a spectrum of 320 samples covering the band 0-8000Hz; and - the room level control factor is defined by: Γ = β ener HB − ener tonal ener HB − βener tonal where ener tonal is the energy of the dominant tonal components, ener HB is the total energy of the extended decoded low-band signal, and β is a factor. - the extraction step of tonal components and an ambient signal is carried out in the 6000-8000Hz band; - the energy level control factors further include an energy level control factor, fac, calculated as a function of the total energy of the extended decoded low-band signal and the tonal components, the total energy of the signal extended decoded low band ener HB being calculated according to: ener HB = ∑ k = 240 319 U HB 1 k 2 + ε where s = 0.1 and with k the sample index and U HB1 (k) the spectrum of the extended decoded low-band signal, a spectrum of 320 samples covering the band 0-8000Hz; and - the room level control factor is defined by: Γ = β ener HB − ener tonal ener HB − βener tonal where ener tonal is the energy of the dominant tonal components, ener HB is the total energy of the extended decoded low-band signal, and β is a factor.