摘要:

Ultrasound measurements of a large population of wavefronts transmitted through female breasts at 3 and 4 MHz show that the wavefront amplitude distribution is close to Rayleigh. This finding is consistent with a fully developed scatter field, implying that the scatter energy removed from the coherent incident beam dominates the wavefront. The wavefront received from an inhomogeneous medium is the superposition of an incident wave plus a scattered wave. If the scattered field is weak, the received field is dominated by the incident field and the wavefront amplitude distribution is Rician. If the scattered field is strong, the received field is primarily the scattered field and the wavefront amplitude distribution is Rayleigh. If, in addition to scattering, refraction between bodies of different refractive indexes occurs, the total net effect on the wavefront amplitude distribution is the same as for strong scattering. This is what we have observed in the highly refractive female breast. This result has implications for the design of high lateral‐resolution echo scanners that will incorporate adaptive phase deaberration algorithms. The published algorithms were developed for weak scattering and therefore may not be powerful enough. Alternatives have to be found to deaberrate the severe wavefront distortion in the breast.

ISSN:0001-4966    

DOI:10.1121/1.410466

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A fixed, octave‐based musical scale cannot remain faithful to the consonant simple integer ratio intervals and simultaneously be modulated to all keys. It is possible to reconcile these competing criteria, however, if the notes of the scale are allowed to vary. This paper presents a method of adjusting the pitches of notes dynamically, anadaptivetuning, that maintains fidelity to a desired set of intervals and can be modulated to any key. The adaptive tuning algorithm changes the pitches of notes in a musical performance so as to maximize consonance, which is calculated based on recent perceptual work. The algorithm can operate in real time, is responsive to the notes played, and can be readily tailored to the timbre (or spectrum) of the sound. This can be viewed as a generalization of the methods of just intonation, but it can operate without specifically musical knowledge such as key and tonal center and is applicable to timbres with nonharmonic spectra as well as the more common harmonic timbres.

ISSN:0001-4966    

DOI:10.1121/1.410471

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Recent studies have shown that the acoustic relationship between the murmur and the vowel at the nasal‐vowel boundary is highly informative for the [m]–[n]distinction. In the present paper, the contribution of relational information is reassessed by classifying 1946 syllable‐initial and 2848 syllable‐final nasal consonants taken from continuous speech data. Relational information in the acoustic waveform is based ondifferencespectra, in which spectral information in the vowel is subtracted from spectral information in the murmur, and oncombinedspectrain which classifications are made from combinations of murmur and vowel spectra. These two kinds of relational spectra are compared withstaticspectra, in which single spectral slices are taken in either the murmur or the vowel. Contrary to recent theoretical predictions, difference spectra are shown to perform more poorly than some kinds of static spectra. However, since classification scores from combined spectra are better than from either static or difference spectra, cues to nasal place of articulation can nevertheless be defined as relational. In the best scoring combined spectra, classification scores on open tests are just under 94% correct for syllable‐initial nasals and just under 82% correct for syllable‐final nasals. The high classification scores show that there is considerable information in the acoustic waveform for identifying nasal place of articulation from continuous speech data.

ISSN:0001-4966    

DOI:10.1121/1.410465

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

This work was aimed at exploring articulatory‐acoustic relationships in the production of French fricatives. More precisely, an attempt was made to find out whether the spectral shapes of [s] and [■]can be regenerated from thexandycoordinates of three electromagnetic transducers affixed to the tongue in the midsagittal plane. The corpus was composed of the two fricatives /s/ and /■/ combined with the vowels /a/ and /i/ in sequences of the type /Vs■V/ and /V■sV/, and was read by one male native speaker of French. The spectrum regeneration was based on a statistical procedure which consisted of estimating the factors explaining the main part of the acoustic variance from the position of the transducers, by means of multiple linear regression. The articulatory‐acoustic correlations were high and allowed us to regenerate the fricative spectra with a good accuracy. The way in which the acoustic parameters varied as a function of the articulatory ones in the statistical model was in good agreement with data reported in previous works. The results support the idea that the tongue has relatively few degrees of freedom in the production of [s] and [■].

ISSN:0001-4966    

DOI:10.1121/1.411435

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Acoustic observations are reported for English front vowels embedded in a /w—l/ frame and carrying constant main stress. The vowels were produced by five speakers in clear and citation‐form styles at varying durations but at a constant speaking rate. The acoustic analyses revealed (i) that formant patterns were systematically displaced in the direction of the frequencies of the consonants of the adjacent pseudosymmetrical context; (ii) that those displacements depended in a lawful manner on vowel duration; (iii) that this context and duration dependence was more limited for clear than for citation‐form speech, and that the smaller formant shifts of clear speech tended to be achieved by increases in the rate of formant frequency change. The findings are compatible with a revised, and biomechanically motivated, version of the vowel undershoot model [Lindblom, J. Acoust. Soc. Am.35, 1773–1781 (1963)] that derives formant patterns from numerical information on three variables: The ‘‘locus‐target’’ distance, vowel duration, and rate of formant frequency change. The results further indicate that the ‘‘clear’’ samples were not merely louder, but involved a systematic, undershoot‐compensating reorganization of the acoustic patterns.

ISSN:0001-4966    

DOI:10.1121/1.410492

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Voice‐onset time (VOT) and syllable duration were measured for the English plosives in /C■d/ (C=consonant) context spoken by four postlingually deafened recipients of multichannel (Ineraid) cochlear implants. Recordings were made of their speech before, and at intervals following, activation of the speech processors of their implants. Three patients reduced mean syllable duration following activation. Using measures of VOT and syllable duration from speakers with normal hearing [Volaitis and Miller, J. Acoust. Soc. Am.92, 723–735 (1992)] and from the subjects of this study, VOT is shown to vary approximately linearly with syllable duration over the ranges produced here. Therefore, the VOT of each token was adjusted for the change in syllable duration of that token relative to the mean syllable duration in the first baseline session. This variable, labeled VOTc, was used to evaluate the effects on voicing of the speakers’ renewed access to the voicing contrast provided by their implants. Preimplant, all four speakers characteristically uttered voiced plosives with too‐short VOT, compared to the measures for hearing speakers. Voiceless plosive mean VOT was also abnormally short for two of the speakers, and close to normal for the remaining two. With some hearing restored, subjects made relatively few errors with respect to voicing when identifying plosives in listening tests, and three of the four speakers lengthened VOTc. The findings are interpreted as supporting the hypothesis that speakers use their hearing to calibrate mechanisms of speech production by monitoring the relations between their articulations and their acoustic output.

ISSN:0001-4966    

DOI:10.1121/1.410442

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A recognition system based on linguistic features was developed for the semivowels /w j r l/ in American English. The features of interest aresonorant,syllabic,consonantal,high,back,front, andretroflex. Acoustic correlates and events related to these features were used to detect and classify the semivowels. The recognizer was tested across semivowels occurring in a wide range of phonetic environments. The corpora included polysyllabic words and sentences spoken by males and females of several dialects. The results show that a feature‐based approach to recognition is a viable methodology. Fairly consistent overall recognition results were obtained. Across the test data, acoustic events were detected within 97% of the semivowels and classification rates were 62% for /w/, 74% for /l/ (/w/ and /l/ were often confused), 90% for /r/ and 84% for /j/.

ISSN:0001-4966    

DOI:10.1121/1.410375

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Segment durations in 20 minimal‐paired words in Japanese were examined to identify regular patterns of acoustic manifestations of mora timing in Japanese. The results indicate that segments consistently stretch or compress, providing temporal compensation within a mora and between moras, to attain targeted length at the word level. A segment is 11% longer in the pre‐geminate‐stop position than in the pre‐single‐stop position; in the word final position, a segment is 9% shorter following a geminate stop than a segment following a single stop; and [t] in the same phonotactic environment shows varied durations according to different moraic conditions. Fricatives followed by a devoiced vowel are considerably longer than any other inherently long, nonmoraic segments, and are comparable to moraic segments and the average mora duration. Syllable durations are not isochronous, varying by as much as 50%. The word duration is affected more by the number of component sounds and their inherent durations in two‐mora than in three‐or four‐mora words. When the word durations are converted into ratios, they are markedly similar to the mora values. The results suggest a timing control mechanism adjusting segments to obtain targeted word durations. These phenomena describe mora timing in Japanese.

ISSN:0001-4966    

DOI:10.1121/1.410376

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The object of this study was to investigate whether subjects are able to compare the prominence caused by different types of accent‐lending pitch movements and, if so, whether some pitch movements lend more prominence to a syllable than others. These experiments were carried out with the utterance /ma’mama/, with the second syllable accented by either a rise, a fall, or a rise–fall. Subjects adjusted the variable excursion size of a comparison stimulus to the fixed excursion size of a test stimulus in such a way that the accented syllable in test and comparison stimuli had equal prominence. The rise–fall was only presented in one ‘‘standard’’ position, while the fall and the rise were tested for five different temporal positions in the syllable. Subjects were found to be quite capable of equating the prominence of syllables accented by the following types of pitch movement: the rise–fall in standard position, the rise starting before the vowel onset, and the fall whatever its temporal position in the syllable. When lending equal prominence, the early starting rise and the rise–fall had equal excursion sizes. The fall, however, appeared to lend more prominence to a syllable than the rise or the rise–fall of equal excursion size, independent of its position in the syllable. This difference between the fall on the one hand and the rise and the rise–fall on the other increased with increasing declination of the pitch contour. A model is presented which can explain these phenomena quantitatively.

ISSN:0001-4966    

DOI:10.1121/1.410377

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Absolute thresholds were estimated for pure tones of 500, 1000, and 2000 Hz using both a maximum‐likelihood yes–no procedure and a traditional three‐down one‐up forced‐choice adaptive procedure. Threshold estimates obtained with the two procedures were highly correlated. In the maximum‐likelihood yes–no procedure, the false‐alarm probability was poorly estimated. A simple change in that procedure produced less‐biased estimates of the false‐alarm rate. In a second experiment, the influence of the false‐alarm rate on the threshold estimates was investigated. The listeners’ absolute thresholds were estimated when they adopted either a liberal or conservative criterion for detecting the signals. Data were collected using both a constant‐stimulus yes–no procedure and a maximum‐likelihood yes–no procedure. Threshold estimates are 5 to 6 dB higher for the low than for the high false‐alarm rates. Psychometric functions based on a stimulus‐power model provided the best fit to the data obtained with the constant‐stimulus method. This stimulus‐power model was used to produce the assumed psychometric function in the maximum‐likelihood yes–no procedure. Computer simulations were conducted to determine how threshold estimates were affected by the listener’s inattention or by mismatches between the slope parameter of the listener’s psychometric function and that assumed in the maximum‐likelihood procedure.

ISSN:0001-4966    

DOI:10.1121/1.410378

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP