Assessing Speech Comprehension in Noise: Acoustic Quality of Speech for Near Real Communication Conditions

Abstract

To assess the quality of speech communication, reference is usually made to speech intelligibility, but what does that mean for practical speech conditions? Even with high signal-to-noise ratios speech intelligibility alone falls short of what is required for a broader scope of differentiation. The pursued objective here is to improve the description of the communication situation which means paying more attention to the perception and comprehension processes with the main focus on the listener's efforts to cope with the situation or the stress and strain he experiences. In two experiments - on speech intelligibility and the assessment of the comprehension process -listeners were presented sentences and monosyllables at six different speech levels (40 to 75 dB) and varying traffic noises (55 dB). For each sentence both speech intelligibility and answers to assessment questions concerning coping, concentration, annoyance, and subjectively rated intelligibility (on a 5-point scale) have been evaluated. The answers to the assessment questions always refer to three different sentences with the same experimental conditions. As a result, assessment parameters have shown to be more sensitive, especially in cases where speech intelligibility is nearly constant (for levels of high speech intelligibility: SI > 80%, SNR > 5 dB). The data indicates that not until a signal- to-noise ratio of beyond SNR A > 15 to 20 dB high intelligibility with lowest possible mental effort can be achieved. The noise level has a particular influence on the level of annoyance (always at the same SNR) whereas a simultaneously applied task only affects one's concentration. Based on the assessment of coping and concentration five quality levels for the comprehension process have been developed.