Phoneme-group specific octave-band weights in predicting speech intelligibility

In an earlier study we derived robust frequency-weighting functions for prediction of the intelligibility of short nonsense words. These frequency-weighting functions are applied for prediction of intelligibility such as with the speech transmission index (STI). Six independent experiments revealed essentially similar frequency-weighting functions for the prediction of the nonsense word scores with respect to signal-to-noise ratio and gender [Speech Communication 28 (1999) 109]. Although the frequency weightings do not vary significantly for signal-to-noise ratio or gender, other studies have shown that using different types of speech material (i.e., nonsense words, phonetically balanced words and connected discourse) resulted in quite different frequency-weighting functions. This may be related to the distribution of specific phonemes in the test material. In order to obtain a more generic description of the frequency weighting, four relevant groups of phonemes were identified. In situations with reduced intelligibility, a small confusion rate of the phonemes between the groups and a high confusion rate of the phonemes within each group was observed. For each group a specific frequency-weighting function and a good prediction of the phoneme group scores could be obtained. It was shown that from these (weighted) phoneme group scores, word scores could be predicted with a prediction accuracy of ca. 4% (this corresponds to a signal-to-noise ratio of about 1 dB). Hence, this method provides a more generic way to predict intelligibility scores for different types of speech material.