On the detection of the intelligibility advantage of clear speech vs. casual speech

Studies show that even though intelligibility is increased by the decrease of speech rate both in clear and casual speech, clea r speech can also be produced without the decrease of rate afte r training the speakers [1]. This suggests that clear speech h as inherent acoustic properties independent of rate, that con tribute to improved intelligibility. Many speakers in their effort to elicit clear speech change their pitch both in level and range. However, it is not luminous if pitch modification is a feature tha t contributes to intelligibility. In this work, we examine if clear speech signals are still more comprehensive than casual speech signals after equali zing the prosody features on the two signals. To this purpose, a database of clear and casual speech signals is analyzed. Spe akers in this database read sentences both in clear and casual w ay [2]. Clear speech sentences are modified in duration and pitc h to match the corresponding attributes of casual speech sign al . After the equalization, pilot acoustical test analysis and objective measure tests are performed on the four equal set of sign al ; on the initial database of clear and casual signals and addit ionally on the time-scaled and time and pitch-scaled clear sign als. In the acoustical pilot experiments, speech shaped noise is added to the signals to create the test signals, with Signal t o Noise Ratio of0dB. Results show that on a set of pairs of clear and casual sentences, in 64% of the cases listeners found more intelligible the clear sentences. However, in time-sc aled and time-pitch-scaled modified clear sentences intelligib i ty scores were deteriorated. Objective measure tests were als o performed, using a modified version of the extended Speech Intel ligibility Index (SII) [3]. SII was evaluated in a separate dat abase giving high correlation scores with perceptual acoustical tests. According to the SSI measure, clear signals have higher inte lligibility scores than casual signals (Fig.1(a)) with higher probability (Fig.1(b)) of identifying a sentence for SNR levels a bove −5dB. On the other hand, casual signals, time-scaled and timepitch-scaled clear signals that have the same duration, giv e the same score of SII independent of the SNR level (Fig.1(a)). Pi lot acoustical experiments and objective measures suggest tha t duration indeed plays a significant role to intelligibility, w hereas pitch modifications do not seem to contribute to intelligibi lity. −10 −8 −6 −4 −2 0 2 4 6 8 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1