1. A method of enhancing intelligibility of speech contained in an audio signal perceived by a subject via a communications path, where the communications path includes a intelligibility enhancing device having an adjustable gain, comprising generating a candidate frequency-wise gain which, if applied to the intelligibility enhancing device, would maximize an intelligibility metric of the communications path, where the intelligibility metric is a function of the relation: 
 AI=v×e×f×h where, AI is the intelligibility metric, V is a measure of audibility of the speech contained in the audio signal and is associated with a speech-to-noise ratio in the audio signal, E is a loudness limit associated the speech contained in the audio signal, F is a measure of spectral balance of the speech contained in the audio signal, H is a measure of any of (i) intermodulation distortion introduced by an ear of the subject, (ii) reverberation in the medium, (iii) frequency-compression in the communications path, (iv) frequency-shifting in the communications path and (v) peak-clipping in the communications path, (vi) amplitude compression in the communications path, (vii) any other noise or distortion in the communications path not otherwise associated with V, E and F.