Developing a speech intelligibility test based on measuring speech reception thresholds in noise for English and Finnish.

A subjective test was developed suitable for evaluating the effect of mobile communications devices on sentence intelligibility in background noise. Originally a total of 25 lists, each list including 16 sentences, were developed in British English and Finnish to serve as the test stimuli representative of adult language today. The sentences, produced by two male and two female speakers, were normalized for naturalness, length, and intelligibility in each language. The sentence sets were balanced with regard to the expected lexical and phonetic distributions in the given language. The sentence lists are intended for adaptive measurement of speech reception thresholds (SRTs) in noise. In the verification of the test stimuli, SRTs were measured for ten subjects in Finnish and nine subjects in English. Mean SRTs were -2.47 dB in Finnish and -1.12 dB in English, with standard deviations of 1.61 and 2.36 dB, respectively. The mean thresholds did not vary significantly between the lists or the talkers after two lists were removed from the Finnish set and one from the English set. Thus the numbers of lists were reduced from 25 to 23 and 24, respectively. The statistical power of the test increased when thresholds were averaged over several sentence lists. With three lists per condition, the test is able to detect a 1.5-dB difference in SRTs with the probability of about 90%.