Studying sound externalization in the context of hearing aids

Sound externalization is a complex perceptual phenomenon that is difficult to study experimentally. In their fascinating paper on this topic, Hartmann and Wittenberg [J. Acoust. Soc. Am. 99, 3678–3688 (1996)] used a headphone synthesis technique to create virtual sound images that were indistinguishable from real sound images. This allowed them to systematically vary the signals delivered to the ear canals and examine the impact of these variations on the “convincingness” of the virtual sound images. Their results suggested that externalization depends on interaural time and level differences, on a plausible relationship between the two, and on monaural spectral cues. Hearing aids disrupt the natural sound path to the ear canals, and it is easy to appreciate how they may distort the cues that support externalization. This talk will describe an ongoing series of experiments aimed at characterizing in detail the effects of hearing aids on externalization. Results suggest that breakdowns of externalization with hearing aids are (1) related to microphone position and openness of the ear canal; (2) highly dependent on the source stimulus; and (3) partly resolved by head movements. The talk will also consider the consequences of poor externalization for speech understanding in multitalker mixtures.