The Relation between Localization and Intelligibility

The binaural masked threshold for speech depends upon the relation between the interaural phase angles of the speech and those of the noise. When these phase angles are the same, the threshold is high, and both speech and noise appear to be in the same place. When the interaural phase angle of the speech is reversed relative to that of the noise, the threshold is low and the speech and noise appear to be in different places. These relations have been clearly demonstrated with earphones, and they suggest that in free-field listening the threshold of intelligibility might be affected by the relative locations of the sources of speech and of masking noise. It was found that when the azimuths of the sources of speech and of noise are changed relative to each other, the threshold of intelligibility changes by small but consistent amounts. When the sources are close together, the threshold is high; when the sources are far apart, the threshold is reduced. Although this relation is partially confounded by the effect of azimuth on the sound pressure levels at the ears, the factor of localization appears to play a significant role, especially when two ears are used and when the head is allowed to move. In order that the hard-of-hearing may take advantage of these effects, they must have a hearing aid with two separate microphones mounted near the ears and connected each to a separate earphone.