The scores of speech intelligibility, obtained using objective and subjective methods for three university lecture rooms of the small, medium, and large sizes with different degrees of filling, were presented. The problem of achieving high speech intelligibility is relevant for both students and university administration, and for architects designing or reconstructing lecture rooms. Speech intelligibility was assessed using binaural room impulse responses which applied an artificial head and non-professional quality audio equipment for measuring. The Speech Transmission Index was an objective measure of speech intelligibility, while the subjective evaluation of speech intelligibility was carried out using the articulation method. Comparative analysis of the effectiveness of parameters of impulse response as a measure of speech intelligibility showed that Early Decay Time exceeded the score of the T30 reverberation time but was ineffective in a small lecture room. The C50 clarity index for all the considered lecture rooms was the most informative. Several patterns determined by the influence of early sound reflections on speech intelligibility were detected. Specifically, it was shown that an increase in the ratio of the energy of early reflections to the energy of direct sound leads to a decrease in speech intelligibility. The exceptions are small, up to 30‒40 cm, distances from the back wall of the room, where speech intelligibility is usually slightly higher than in the middle of the room. At a distance of 0.7–1.7 m from the side walls of the room, speech intelligibility is usually worse for the ear, which is closer to the wall. The usefulness of the obtained results lies in refining the quantitative characteristics of the influence of early reflections of sound on speech intelligibility at different points of lecture rooms.

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In this paper, the results of automated subjective assessment of Ukrainian speech intelligibility are presented. Speech monosyllables of the consonant-vowel-consonant (CVC) type were listened in two modes: through headphones and through acoustic monitors. The assessment was carried out with the help of specially developed software that allowed automating of articulation tests. Speech listening was done for four situations: pure language; speech distorted by noise; speech distorted by reverberation; speech distorted by the combined effect of noise and reverberation. In the first case, speech monosyllables of 3 articulation tables were listened, each of which contained 50 monosyllables. In the second case, speech distorted by the additive noise with the signal-to-noise ratios (SNR) varied in the range-15…+10 dB was listened. In this case, models of white, pink and brown noises were used, the masking properties of which are rather well-studied. In the third case, the reverberant speech for reverberation times in the range 0.3…2.7 s was modeled by convolution of pure speech signals with room impulse responces (RIRs) of various rooms, and in the fourth case the joint action of pink noise and reverberation was considered. It turned out that the masking ability of white noise exceeds one for brown noise for SNR less than minus 5 dB, which is not entirely consistent with preliminary predictive estimates. In addition, it turned out that listening to speech distorted by noise through acoustic monitors could lead to a significant increase in the speech intelligibility, compared to the case of listening through headphones. The analysis of possible causes of abnormal increase in speech intelligibility has been carried out. Early reflections, presence of two loudspeakers, binaural listening, psychophysical features of listeners, as well as peculiarities of software and articulatory testing organization were considered as possible reasons of the phenomenon. After correction of the software and some features of articulation tests it turned out that the results of the speech intelligibility estimation almost coincide when listening to the signals through the headphones and through acoustic monitors, if the distance between the listener and acoustic monitors does not exceed 0.6-0.8 meters. At the same time, these corrections did not differ in the behavior of the dependencies of speech intelligibility on the SNR for small (less minus 5 dB) SNR values The general conclusion may be that listening to speech signals distorted by noise and reverberation interferences, performed with the application of the proposed automated system of articulation tests, indicates the performance and high quality of the developed system. Ref. 13, fig. 7.