Noise cancellation is effected for audio devices. In connection with various embodiments, a forward leakage transfer function is determined using a signal corresponding to detected portions of the generated sound that has leaked out of the user's ear canal with the proximate end of the earphone inserted in (or otherwise fitted to) the ear. This forward leakage transfer function can be used to characterize ambient noise entering a user's ear canal. A signal corresponding to ambient noise is processed with the forward leakage transfer function to generate and output a noise-cancelling signal for canceling at least a portion of the detected ambient noise.