# Characterization of the Intelligibility of
## 4 Discussion  
The results from the analysis of the VCV intelligibility using the STOI measure and validation with subjective listening experiments demonstrate that there are inherent differences in the intelligibility of spoken VCVs across the five different sets of recordings--English, French, German, Italian, and Portuguese. (Figure 2, Table 4). The estimated intelligibility of spoken VCVs recorded from the English speaker was, on average higher than those in other languages, especially at low SNRs (on average, from about 9% to about 10% higher at negative SNRs compared to the average of the other languages). This relatively higher intelligibility, especially at low SNRs, may be useful when testing non-native listeners as processing speech in a non-native language requires more effort, especially in adverse conditions, and the higher intelligibility of English VCVs at low SNRs may, at least in part, compensate for the increased demand. In general, the STOI estimates of intelligibility are highly correlated with subjective intelligibility. However, it should be noted that the quantitative relationship between changes in STOI values and changes in subjective perception is not unique and may depend on the speech material used [46; 47]. In other words, the minimum amount of increase (or decrease) in STOI that corresponds to a noticeable variation in speech perception is not known. However, in previous studies, changes in STOI values as small as 1% were considered meaningful [48]. As such, the differences in mean STOI observed suggest that there may be differences in VCV intelligibility across subsets, especially between VCVs in English versus those in French, German, and Portuguese. The lower intelligibility observed for stimuli in Portuguese may be due to particular language characteristics, as already pointed out in previous studies regarding, for example, nasal consonants [22; 49; 50]. Regarding the variance of STOI estimates, our results show that higher mean STOI is associated with lower variance throughout the SNR range (on average, from about 28% lower (at \(-12\) dB SNR) to about 73% lower (at \(+6\) dB SNR) for English compared to the average variance of the other languages). Low variance, i.e., limited variability of intelligibility with varying noise (as estimated in this study by the STOI variance), can be useful in a screening test to ensure better repeatability of results, especially at low SNR. Future studies will need to measure the actual repeatability of SRT estimates for spoken VCVs recorded from speakers of various languages in native and non-native listeners. Our STOI analysis, together with the estimated SRTs and slopes of the psychometric curves, overall points at English as a good candidate language for the implementation of VCV-based speech-in-noise tests for screening to be delivered in multilingual settings. Indeed, psychometric functions of English VCVs show, on average, high intelligibility, low variability of intelligibility, and larger dynamics. Of note, the STOI was considered in this study because it has been shown to result in better correlation with speech intelligibility in listening tests compared to the above-mentioned models, as they rely on the global statistics over longer segments than the 386 ms segments used by the STOI [33]. In addition, the use of STOI was recommended with respect to other intrusive measures for intelligibility prediction for users with assistive listening devices, and it was tested on three subjectively rated speech data sets covering reverberation-alone, noise-alone, and reverberation-plus-noise degradation conditions, as well as degradations resultant from nonlinear frequency compression and different speech enhancement strategies [43]. It will be important in future studies to address recently proposed intrusive measures, particularly the extended STOI [51] that can be applied to a wider range of input signals compared to STOI, as it is based on fewer assumptions.