Effects of dynamic-range compression on the spatial attributes of sounds in normal-hearing listeners.

OBJECTIVES: Dynamic-range compression is routinely used in bilaterally fitted hearing devices. The objective of this study was to investigate how compression applied independently at each ear affects spatial perception in normal-hearing listeners and to relate the effects to changes in binaural cues caused by the compression for different types of sound.
DESIGN: A semantic-differential method was used to measure the spatial attributes of sounds. Eleven normal-hearing participants responded to questions addressing certainty of location, diffuseness, movement, image splits, and externalization of sounds. Responses were given on seven-point scales between pairs of opposing terms. Stimuli included speech and a range of synthetic sounds with varying characteristics. Head-related transfer functions were used to simulate a source at an azimuth of -60° or +60°. Three processing conditions were compared: (1) an unprocessed reference condition; (2) fast-acting, wide-dynamic-range compression operating independently at each ear; and (3) imposition of a static bias in interaural level difference (ILD) equivalent to that generated by the compression under steady state conditions. All processing was applied in a high-frequency channel above 2 kHz. The three processing conditions were compared separately in two bandwidth conditions: a high-pass condition in which the high-frequency channel was presented to listeners in isolation and a full-bandwidth condition in which the high-frequency channel was recombined with the unprocessed low-frequency channel.
RESULTS: Hierarchical cluster analysis was used to group related questions based on similarity of participants' responses. This led to the calculation of composite scores for four spatial attributes: "diffuseness," "movement," "image split," and "externalization." Compared with the unprocessed condition, fast-acting compression significantly increased diffuseness, movement, and image-split scores and significantly reduced externalization scores. The effects of compression were greater when listeners heard the high-frequency channel in isolation than when it was recombined with the unprocessed low-frequency channel. The effects were apparent only for sounds containing gradual onsets and offsets, including speech. Dynamic compression had a much more pronounced effect on the spatial attributes of sounds than imposition of a static bias in ILD.
CONCLUSIONS: Fast-acting compression at high frequencies operating independently at each ear can adversely affect the spatial attributes of sounds in normal-hearing listeners by increasing diffuseness, increasing or giving rise to a sense of movement, causing images to split, and affecting the externalization of sounds. The effects are reduced, but not eliminated, when listeners have access to undisturbed low-frequency cues. Sounds containing gradual onsets and offsets, including speech, are most affected. The effects arise primarily as a result of relatively slow changes in ILD that are generated as the sound level at one or both ears crosses the compression threshold. The results may have implications for the use of compression in bilaterally fitted hearing devices, specifically in relation to spatial perception in dynamic situations.