Binaural speech intelligibility through personal and non-personal HRTF via headphones, with added artificial noise and reverberation.

Subjective intelligibility tests were carried out by processing speech through personal and non-personal Head-Related Transfer Functions (HRTF) for azimuth angle θ=+30∘ (sound source to the right), presented through headphones, under simulated adverse listening conditions. Tests with noise disturbance were also conducted at azimuth angles of θ=0∘, 15° and 45°. Phonetically balanced bi-syllable words in Spanish, uttered by a mexican female speaker, were used as speech material. Stimuli were convolved with personal or non-personal HRTF, and artificially contaminated with noise or reverberation, interaurally correlated or uncorrelated at the left and right ears. Results at θ=30∘ show that binaural speech intelligibility scores reduce slightly to moderately with non-personal HRTF, compared to personal HRTF, for these types of acoustic disturbance. Average intelligibility score reductions of ΔI=−7.6%, and −12.1%, were found statistically significant, p(Δ I ≠ 0) > 0.99, respectively for interaurally correlated and uncorrelated noise. Reductions of intelligibility with reverberation were found smaller: −1.9%, p=0.63, and −4.1%, p > 0.99, respectively for interaurally correlated and uncorrelated reverberation, the reduction being smaller, and statistically less significant for interaurally correlated reverberation. A tentative simple model of speech intelligibility is also presented, based on the modulation index theory, under the better-ear binaural assumption, and the spectral distortion metric to quantify HRTF differences, in an attempt to incorporate the effects of non-personal HRTF in speech intelligibility. The model explains correctly the qualitative trend of the results, but it overpredicts the observed reductions of the intelligibility scores, showing that spectral distortion of HRTF is probably too simplified, and insufficient to provide an accurate or suitable explanation in this context. Results with noise disturbance at other angles are as follows. At 0°, intelligibility scores decrease for non-personal HRTF with interaurally correlated noise (L=R), but increase with uncorrelated noise (L ≠ R). At 15°, intelligibility scores decrease in both conditions. At 45°, no change is observed.