A previous investigation [G. R. Miller and J. C. R. Licklider, J. Acoust. Soc. Amer. 22, 167–173 (1950)] reported that intelligibility of interrupted monosyllabic words was essentially equal in the presence and absence of intervening noise when speech-time fraction was 0.5 and switching rates were approximately below five interruptions per second (ips). Two experiments investigated the effect upon intelligibility of temporally interrupted C. I. D. sentences [S. R. Silverman and I. J. Hirsh, Ann. Otol. Rhinol. Laryngol. 64, 1234–1245 (1955)] when noise was introduced during the speech-free interval. In study 1, listeners received the test material under two conditions at five interruption rates ranging from 0.77 to 3.85 ips. In one instance the speech-free interval was silent; in the other it was switched to white noise of the same average level as the speech. Speech-time fraction was 0.5. Study 2 examined the intelligibility of the interrupted sentences as a function of noise level present during the speech-free interval. The switching condition was held constant and signal-to-noise ratio varied from +9 to −15 dB. Results indicate that for the particular interruption conditions selected, speech switched to noise was significantly more intelligible than speech interrupted in the silence. In addition, listener performance depends upon the level of noise present during the speech-free interval.

Experiments were performed to examine changes in the intelligibility of temporally interrupted sentences when white noise was introduced during the speech-free interval. In experiment 1, listeners received the test material under two conditions at five interruption rates ranging from 0.77 to 3.85 interruptions per second (ips). In one instance the speech-free interval was silent, while in the other it was switched to noise of the same average level as the speech. Experiment 2 examined the intelligibility of the speech message as a function of signal-to-noise (S/N) ratio. The switching rate was held constant at 1.5 ips and the S/N ratio was varied from --30 to + 18 dB. Results indicated that for the particular interruption conditions selected, speech switched to noise is significantly more intelligible than speech interrupted by silence. In addition, intelligibility of the interrupted sentences in the presence of intervening noise varies with the S/N ratio.

The variations in intelligibility of a temporally interrupted connected discourse passage that result from changes in both switching conditions and message rate were studied. Six normal-hearing listeners estimated the intelligibility of ten, 15-sec speech samples presented monaurally at 36 interruption conditions (12 interruption rates × 3 speech-time fractions) for each of two speaking rates. Results indicated that for a given speech-time fraction, changes in intelligibility as a function of interruption rate depend on the precise temporal correspondence of switching and message parameters. Specifically: (1) for slow interruption rates, where on-time and off-time are greater than average word duration, temporal features of the message are relatively unimportant and intelligibility varies directly with speech-time fraction; (2) intelligibility functions will begin to recover from minimum at an interruption rate where off-time is equal to, or just shorter than, average word duration; (3) differences in performance along the recovery portions of the functions for the two message rates are negligible when intelligibility is expressed in terms of percent off-time relative to average word duration. It is apparent that a trading relation exists between speech-time fraction and interruption rate. Within certain limits, decrements resulting from lowering the speech-time fraction can be overcome by increasing the switching rate.

Previous experiments have demonstrated that intelligibility of interrupted speech is dependent upon several parameters including interruption rate, speech-time fraction (STF), ON time, OFF time, and message rate. Our experiments suggest a more parsimonious model that incorporates only ON time and OFF time expressed relative to average word duration. Messages spoken at rates of 186 and 246 wpm were interrupted at rates ranging from 0.42 to 20.0 ips for STFs of 0.67, 0.50, and 0.33. At slow interruption rates, intelligibility was dependent primarily on STF. Intelligibility functions then passed through a minimum located in the vicinity of 1.0–2.5 ips, followed by rapid recovery to nearly 100% intelligibility at 10 ips. The exact location of minimum, however, varied with both message rate and STF. The data can be viewed more simply by a model that incorporates two switching parameters, ON time and OFF time, and temporal characteristics of the message. Equal-intelligibility contours will be described that illustrate the nature of the trading relation between ON time and relative OFF time that is required to maintain a criterion intelligibility level.

The final step of the semisynthetic route to paclitaxel involves cleavage of the triethylsilyl (TES) protecting group from the C-7 hydroxyl group. Paclitaxel is an extremely complex molecule, and standard deprotection conditions led to formation of several impurities. Trifluoroacetic acid in aqueous acetic acid was found to be very effective in the cleavage of the TES group without compromising the quality of the product.