Sound isolating (SI) earphones are increasingly used by the general public with portable media players in noisy urban and transport environments. The dangers of these SI earphones are becoming increasingly apparent and legislators are recommending an urgent review of their usage. The problem is that the user is removed from their local ambient scene: a reduction in their "situation awareness" that often leads to accidents involving unheard oncoming vehicles. This paper introduces a new automatic gain control system to automatically mix the ambient sound field with reproduced audio material to maintain a constant SNR and therefore enhance situation awareness. A discussion of the audio system architecture is given and an analysis of 20 different warning sounds is used to suggest suitable parameters.
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