DESIGN OF HIGH FREQUENCY ANECHOIC TANKS 105 for use in the megacycle region can be simply constructed with a tan rho-c rubber liner. At a frequency of 3.35 megacycles a -}-inch thick liner provides results which are close to optimum, yielding a reflection coefficient of about--30 db up to an angle of incidence of about 60 ø . Above 60 ø the liner is highly ineffective. Theoretically, one would desire that the real part of the dilatational velocity in the rubber be slightly higher than that in water. In addition, there is an optimum dilatational attenuation coefficient which is frequency dependent. ACKNOWLEDGMENTSThe author wishes to thank Dr. W. C. Wineland for his many helpful suggestions, and Mr. R. H. Yee for his • -40 -50 Fro. 10. Experimental reflection coefficient for an aluminum backed rho-c rubber sheet of indicated thickness at a frequency of 3.35 megacycles/sec and beam width of 3 ø. assistance in carrying out the calculations and experiments.The consonant environments of vowels were varied by forming non meaningful stimulus syllables consisting of 72 combinations of six vowels and 12 consonants. The syllables were spoken by subjects, and the duration, fundamental frequency, and relative power of the vowels were measured. All three factors varied significantly in response to changes of the consonant environment. The variations were systematically related to the attributes of the consonants, the most powerful attribute being the presence or absence of vocal fold vibration, followed by manner of articulation and place of articulation, in that order. MONG the acoustical investigations of vowels,experiments employing wave analysis have naturally been most numerous, while studies of the secondary characteristics--duration, fundamental frequency, and intensity--have been relatively few. Especially has this been true of variation in these secondary characteristics which may be systematically related to the widely varying consonantal environments of vowels in words.Fairbanks, House, and Stevens, • reporting the results of an experiment on the relative intensities of vowels, concluded that, "When the same vowel is spoken in different isolated words, its intensity sometimes varies significantly from word to word, and it seems probable that such variations are, in part at least, effects of differing consonantal environments." In that investigation the words spoken by the subjects were monosyllables in which vowels were preceded and followed by consonant elements. All consonants were voiceless and varied unsystematically among stopplosives, fricatives, and affricates, produced in bilabial, labio-dental, lingua-dental, and velar positions. Since • Fairbanks, House, and Stevens, J. Acoust. Soc. Am. 22, 457 (1950).voicing was held constant and since only 10 words were used for each vowel, such variations were implicitly restricted. The finding that significant variation in intensity obtained even under these conditions was of unusual interest. A few previous studies have shown variation in duration also. Heffner an...

Two types of analyses have been performed on the measured durations of recordings produced by six talkers reading two scripts of approximately 300 words each. The texts, the combined visual–auditory marking technique, and preliminary results were reported earlier by Crystal and House [J. Acoust. Soc. Am. 72, 705–716 (1982)]. The average durations and standard deviations of various classes of speech sounds, as well as individual speech sounds, have been determined and segmental measurements are compared to earlier data and to various pertinent published reports. The histograms of the measured durations of various sounds and categories have been fitted with distributions which are, equivalently, the exit-probability sequence for a Markov chain or the impulse response of an IIR digital-filter network.

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Average durations of 12 vowels of American English measured in bisyllabic nonsense utterances are reported. The vowels occurred in 14 symmetrical consonantal environments and the utterances were produced by three male talkers. The consonant environments consisted of the voiced and voiceless versions of three stop, one affricate, and three fricative consonant articulations. Four determinants of the characteristic durations of stressed vowels are identified and discussed. The hypothesis is advanced that the primary lengthening of vowels in English—that found in tense vowels and in vowels before voiced constants—is a part of the phonology of the language and is learned by speakers of the language, and that the secondary lengthening of vowels in English—that found in open vowels and in vowels before fricative constants—is a function of the articulatory process itself.

The development and evaluation of a new speech-intelligibility test suitable for routine use by operational personnel in determining the performance level of speech-communication systems is described. The format used is similar to that described for a rhyme test but makes use of a closed-response set. An experiment was performed to determine the general reliability of the test materials when administered to U.S. Air Force enlisted personnel under a wide range of signal-to-noise ratios. Testing of 18 listeners over a period of 30 days showed that repeated exposure to the materials did not change the levels of average response in any appreciable way. Analysis of the responses to individual phonetic elements shows that the test can be useful for diagnostic study as well as for over-all evaluation of communication systems. Talker differences that appeared during the experiment and the statistical reliability and sensitivity of the materials are analyzed and discussed.