1. Method of testing the sufficiency of an audio transmission system for conveying speech signals, by evaluating intelligibility of a degraded speech signal received from an audio transmission system, wherein a reference speech signal is conveyed through said audio transmission system to provide said degraded speech signal, wherein the method comprises: sampling said reference speech signal into a plurality of reference signal frames and determining for each frame a reference signal representation; sampling said degraded speech signal into a plurality of degraded signal frames and determining for each frame a degraded signal representation; forming frame pairs by associating said reference signal frames and said degraded signal frames with each other, and providing for each frame pair a difference function representing a difference between said degraded signal frame and said associated reference signal frame; compensating said difference function for one or more disturbance types, such as to provide for each frame pair a disturbance density function which is adapted to a human auditory perception model; 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, and providing an output signal indicative of the derived overall quality parameter; wherein said method further comprises the steps of: determining a loudness value for each of said reference signal frames; and determining a weighting value dependent on said loudness value of said reference signal frame; wherein said step of compensating of said difference function comprises a step of weighting said difference function using said loudness dependent weighting value, for incorporating an impact of disturbance on said intelligibility of said degraded speech signal into said evaluation; said method further comprising applying said derived overall quality parameter to test the sufficiency of the audio transmission system for conveying speech signals.