This experiment sought to establish the difference between earphone and sound-field measures of spondee threshold SPL (sound-pressure level). Recorded spondee words were delivered to both a conventional and an insert-type earphone as well as to a single loudspeaker that generated the signal in the sound field. Monaural spondee threshold SPL's were established for both normal hearing and hypoacousic subjects under the three listening conditions. The intensity level of the spondee words under the three conditions was expressed as the SPL of an “equivalent” speech spectrum noise. For both groups of subjects, the mean sound-field threshold SPL was about 7.5 dB lower than that measured under the conventional earphone and approximately 12.5 dB lower than that established under the insert-type earphone. These results tend to confirm the reality of the “missing 6 dB.” Further, they indicate that the difference between minimum audible pressure (MAP) and minimum audible field (MAF) is a product of both diffraction and “closed-ear” effects. The latter effect appears to vary as a function of the volume of air enclosed by the pressure transducer.
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Tillman et al. (1966) studied this question.