Effects of Vocal Effort on Intelligibility
J. M. Pickett · The Journal of the Acoustical Society of America · 1955
Changes in speech intelligibility which may occur when the talker is forced to lower or raise his voice are of considerable practical and theoretical interest. We consider here the intelligibility of speech produced by eight different amounts of vocal effort ranging from the weakest voiced whisper to the loudest possible shout (a range of speech intensities from 36 to 90 db one meter from the lips). PB-word intelligibility of five male talkers was measured at three signal-to-noise ratios for each level of vocal effort. The noise was random and had an over-all intensity of 70 db. The results show less than 5% deterioration from normal intelligibility over the range from a “low voice” to a “very loud voice” (48 to 78 db at one meter). Beyond these points intelligibility deteriorates abruptly and in a linear relation to the decibel change in vocal intensity. For whispering the rate is about 5% per db and for shouting it is 1.5 to 3%. The deterioration of intelligibility with louder shouting is such that the noise level tolerable for constant intelligibility remains constant above a vocal level of about 78 db at one meter. The extremes of vocal effort are shown to produce changes in vowel confusions and in the distribution of errors over the parts of the syllable.