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It is a small-signal transfer function from the converter input voltage to the converter output voltage. It is a measure of how noise at the input voltage line
is attenuated by the converter by the time it reaches the output. The word audio in audiosusceptability comes from the fact that range of noise frequencies that were considered were in the audio range 20Hz-20kHz. But I think this does not mean that the frequency range has to be in the audio range -- I think the name is a legacy term.
By the way, when the audiosusceptability transfer function is computed all the other disturbance inputs (like duty ratio, etc. are assumed to be zero).