The invention provides a softscaled frequency compensation function that allows the evaluation of a first quality measure indicating a global impact of all distortions in an audio transmission system, including linear frequency response distortions and second quality measure that only lakes into account the impact of linear frequency response distortions. The softscaled frequency compensation function is derived from a softscaled ratio between a time integrated output and a time integrated input power density functions. The first quality measure is derived from the difference loudness density function as function of time and frequency, using the frequency compensated input loudness density function and the gain compensated output loudness density function both as a function of time and frequency, in the same manner as carried out in ITU standard P.862. The second measure is established from a difference loudness density function as function of frequency only that is derived from a time integration of a second frequency compensated input loudness density function and the gain compensated output loudness density function both as a function of time and frequency.