Michael J. Nilsson Sriram Jayaraman Sigfrid D. Soli
House Ear Inst., 2100 West Third St., Los Angeles, CA 90057
A replication of Bronkhorst and Plomp [J. Acoust. Soc. Am. 83, 1508--1516 (1988)] was undertaken to study the separate contributions of interaural level differences (ITDs); produced by head shadow, and interaural time differences (ITDs), which produce binaural interactions, to directional hearing in noise. Digital filtering of speech and noise signals was performed using head-related transfer functions (HRTFs) measured on a KEMAR mannequin to simulate sound field conditions under headphones. Speech materials from the hearing in noise test (HINT) were presented at a simulated 0-deg azimuth, while a noise signal matched to the spectral shape of the speech was filtered to include the ILDs and ITDs from either a 0-, 90-, or 270-deg presentation, including mixed combinations, e.g., 0-deg ILDs with 90-deg ITDs. Results from 19 normal hearing subjects showed the ILDs alone benefited the listener slightly more [3.97-dB improvement in sentence speech reception threshold (sSRT) versus a 0-deg condition] than ITDs alone (3.22-dB improvement in sSRT), as compared to 6.21-dB improvement in sSRT when both ILDs and ITDs are present. If the sSRT is calculated relative to the noise level in the shadowed ear, the ITDs alone produced the same sSRT as ITDs and ILDs together. Data from hearing impaired listeners will also be presented.