Method of evaluating intelligibility of a degraded speech signal received from an audio transmission system, by conveying through said audio transmission system a reference speech signal such as to provide said degraded speech signal, wherein the method comprises: - sampling said reference speech signal into a plurality of reference signal frames, sampling said degraded speech signal into a plurality of degraded signal frames, and forming frame pairs by associating said reference signal frames and said degraded signal frames with each other; - for each frame pair pre-processing said reference signal frames and said degraded signal frames for enabling a comparison between said frames of each frame pair; the method being further characterised by: - providing for each frame pair one or more difference functions representing a difference between said degraded signal frame and said associated reference signal frame; - selecting at least one of said difference functions for compensating said at least one of said difference functions for one or more disturbance types, such as to provide for each frame pair one or more disturbance density functions adapted to a human auditory perception model, wherein said selecting is performed by comparing a disturbance level of said degraded signal with a threshold disturbance level; and - deriving from said disturbance density functions of a plurality of frame pairs an overall quality parameter, said quality parameter being at least indicative of said intelligibility of said degraded speech signal; wherein said method comprises a step of determining at least one switching parameter indicative of an audio power level of said degraded signal, and using said at least one switching parameter for determining or adapting said threshold disturbance level that is used in performing said selecting of said at least one of said difference functions for optimizing said method for audio power level conditions of said degraded signal for assessment of said intelligibility of said degraded speech signal for said evaluation. - sampling said reference speech signal into a plurality of reference signal frames, sampling said degraded speech signal into a plurality of degraded signal frames, and forming frame pairs by associating said reference signal frames and said degraded signal frames with each other; - for each frame pair pre-processing said reference signal frames and said degraded signal frames for enabling a comparison between said frames of each frame pair; - providing for each frame pair one or more difference functions representing a difference between said degraded signal frame and said associated reference signal frame; - selecting at least one of said difference functions for compensating said at least one of said difference functions for one or more disturbance types, such as to provide for each frame pair one or more disturbance density functions adapted to a human auditory perception model, wherein said selecting is performed by comparing a disturbance level of said degraded signal with a threshold disturbance level; and - deriving from said disturbance density functions of a plurality of frame pairs an overall quality parameter, said quality parameter being at least indicative of said intelligibility of said degraded speech signal;