THE PERCEPTION OF SPEECH IN FLUCTUATING NOISE

Various phenomena associated with the perception of speech in fluctuating noise were investigated using a small set of VCV (Vowel-Consonant-Vowel) stimuli. In experiments 1 and 2, the performance intensity function and the speech reception threshold were measured for the speech in noise backgrounds which fluctuated on various time-scales. It was found that very rapid fluctuations in the masker had no effect on speech perception, whilst a masker containing fluctuations of medium duration (80 to 100's of milliseconds) provided a release from masking but did not affect the relative intelligibility of each consonant, nor the pattern of confusions. A noise environment containing longer interruptions (of duration 1 second) produced a flattening of the performance intensity function and a randomisation of the types of phonetic confusion occurring. In experiments 3 and 4, the effect on the speech reception threshold of varying the duration and magnitude of fluctuations was studied. It was found that the ability of subjects to ''glimpse'' speech information, which may be used to explain the release of masking, depends strongly upon both the duration of the quiet sections of the noise and also upon their magnitude. A method for quantifying the fluctuation of a speech masker is proposed and examples of the analysis are given for cafeteria noise, and white noise shaped to possess similar frequency content. The analysis shows no differences in the fluctuation of the two noises likely to influence speech intelligibility, a result confirmed by experiment.