Effects of Frequency Distortion upon the Intelligibility of Speech
James P. Egan, Francis M. Wiener · The Journal of the Acoustical Society of America · 1946
The effects of frequency distortion upon the intelligibility of speech were investigated by means of controlled articulation tests. A large number of systems having band widths ranging from about one-half octave to a system covering the entire range of speech frequencies was investigated. The responses of these systems were linear and approximately uniform over the pass band, with sharp cut-offs at either end. The acoustic gain of the systems was expressed relative to the transmission of speech through one meter of air between talker and listener. Two spectra of masking noise were used, and each system was tested over a wide range of speech-to-noise ratios. In one experiment the speech was filtered before mixing with noise and in the other experiment both the speech and the noise were passed through the same filter. The results show that as the width of the band of speech frequencies is decreased the slopes of the ‘gain functions’ relating articulation to level of received speech also decrease. For constant levels of received speech, the articulation obtained with wide bands of speech is always higher than that obtained with narrow bands. When the masking noise has a uniform spectrum level, the articulation scores obtained by passing both the noise and the speech through the same band-pass filter are only slightly higher than those yielded by filtering the speech alone. However, if the spectrum level of the noise falls rapidly as a function of frequency, a considerable improvement in articulation results from filtering the noise as well as the speech.