The effect of head-induced interaural time and level differences on speech intelligibility in noise

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Abstract
A study was made of the effect of interaural time delay (ITD) and acoustic headshadow on binaural speech intelligibility in noise. A free-field condition was simulated by presenting recordings, made with a KEMAR manikin in an anechoic room, through earphones. Recordings were made of speech, reproduced in front of the manikin, and of noise, emanating from seven angles in the azimuthal plane, ranging from 0.degree. (frontal) to 180.degree. in steps of 30.degree.. From this noise, two signals were derived, one containing only ITD, the other containing only interaural level differences (ILD) due to headshadow. Using this material, speech reception thresholds (SRT) for sentences in noise were determined for a group of normal-hearing subjects. Results show that (1) for noise azimuths between 30.degree. and 150.degree., the gain due to ITD lies between 3.9 and 5.1 dB, while the gain due to ILD ranges from 3.5 to 7.8 dB, and (2) ILD decreases the effectiveness of binaural unmasking due to ITD (on the average, the threshold shift drops from 4.6 to 2.6 dB). In a second experiment, also conducted with normal-hearing subjects, similar stimuli wre used, but now presented monaurally or with an overall 20-dB attenuation in one channel, in order to simulate hearing loss. In addition, SRTs were determined for noise with fixed ITDs, for comparison with the results obtained with head-induced (frequency dependent) ITDs. Results show that (1) for noise with only ILD, the gain relies on the ear presented with the most favorable signal-to-noise ratio, but decreases when the noise presented to the other ear becomes relatively loud; (2) the effect of ITD is fairly insensitive to a 20-dB attenuation of either channel, also when ITD is introduced in noise with ILD; and (3) the unmasking caused by a head-induced ITD can be predicted from the delay for low frequencies (250-500 Hz).

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