Objective prediction of speech intelligibility at high ambient noise levels using the speech transmission index

In many cases the intelligibility of speech in noise may be assumed independent of the absolute sound level; the speech-to-noise ratio (SNR) primarily determines intelligibility. However, at high sound levels, speech intelligibility is found to decrease. Subjective Speech Reception Threshold (SRT) measurements were performed at various speech and noise levels, and with various noise spectra. Decreases in intelligibility between noise levels of 75 and 105 dB(A) were found that correspond to 1 to 3 dB difference in SNR, depending on the noise spectrum. This decrease is not predicted by the standardized Speech Transmission Index (STI), which may be calculated from speech and noise spectra. By introducing level-dependent auditory masking in the STI-calculations, a decrease in intelligibility is predicted that corresponds well to the SRT results.