The monaural speech-reception threshold of sentences in noise, here defined as the 50% correct-syllables threshold, was measured for a female speaker with and without speechreading via a video monitor. The additional visual information resulted in a 4.6-dB lower threshold for a group of 12 young subjects and in a 4.0-dB lower threshold for a group of 18 elderly subjects compared to auditory presentation alone.
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Middelweerd et al. (1987) studied this question.