Effect of loudspeaker position on differences between earphone and free-field thresholds (MAP and MAF).

Abstract

Free-field and earphone measurements were obtained from eight practiced listeners under monaural and binaural conditions to assess the hypothesis that a major source of the disparity between minimum audible pressure (MAP) and minimum audible field (MAF) speech thresholds was the position of the loudspeaker relative to the listener’s head. Free-field measurements were made at seven different loudspeaker positions (0, 30, 60, 90, 270, 300, and 330 degrees). Stimuli were spondaic words and pure tones at five octave intervals from 250 to 4000 Hz. The smallest monaural MAP/MAF difference for spondees occurred at 0° azimuth (2.7 dB) and the largest appeared at the 60° near-ear position (7.1 dB). Similar results emerged for spondaic words under binaural conditions, although the magnitude of the changes due to variations in loudspeaker position was reduced considerably from comparable monaural conditions. These results indicated that the disparities in MAP/MAF differences of previous investigations were due principally to the location of the loudspeaker. The differences between MAP and MAF thresholds were compared to other published results on ear-canal pressure measured in free field and under earphone. Secondary analysis of the data suggests that the MAP/MAF differences observed in this study may be related partially to the differences in calibration techniques used to specify the level of the signal in free field and under earphone.