Digital watermarking is often modelled as the transmission of a message over a noisy channel denoted as "watermark channel". Distortions introduced by the watermark channel result mainly from attacks but, depending on the attack, may include interference from the original signal. One of the main-differences with classical transmission situations comes from the fact that only perceptual distortions have to be taken into account. However, measuring the perceptual impact an attack has on a watermarked signal is currently an unsolved problem. Possible means of circumventing this problem would be (i) to define the distortion in a so-called "perceptual domain" and defining an "ad hoc" equivalence between objective and perceptual distortion, or (ii) to define an "equivalent distortion", by removing from the attack noise the part that is correlated to the host signal. We concentrate on the second approach, and first show that the resulting "equivalent" attacks is a particular case of a thoroughly studied channel: attacks by filtering plus additive noise. However, our approach emphasizes the fact that the additive noise has to be decorrelated with the signal. Finally, the method is applied to desynchronization attacks on audio signals, provides the corresponding capacities, and outlines further work.