Experiments on the intelligibility of low-frequency speech codes

The intelligibility of. unprocessed and of low-pass filtered speech was compared to that of speech processed using three versions of an all-harmonic code consisting of many harmonic sinusoids, a largest harmonic code consisting of four harmonic sinusoids closest o the formants, and a formant code consisting of three sinusoids scaled from the formant frequencies. Fundamental frequency and formant frequencies were scaled down in frequency by different amounts in the various codes. Normal-hearing subjects were tested on three different types of tests. The Diagnostic Rhyme Test (DRT) was used on the two speech varieties and on codes that were not frequency lowered, a Diagnostic Discrimination Test (DDT) was used on frequency-lowered speech codes, and a prosodic test was run on all versions of the speech and speech codes. Results of each test are presented and compared for the various talker, speech, and speech-code combination; they show that the low-passfiltered speech was always more intelligible than any low-frequency speech code tested.