摘要:

Traditional methods for measuring acoustic speed require knowledge of either the specimen thickness or the distances between the transducers and the specimen. In general, the accuracy in measuring these quantities determines the accuracy of the experimental technique for measuring speed. This problem is particularly acute in measuring sound speed in biological specimens. A new method for measuring acoustic speed of materials, which eliminates the need for determining these quantities, has been developed. The technique, which necessitates the use of only one transducer, requires measurement of four times of flight of a sound pulse and the knowledge of the speed of sound in a reference fluid medium in which the specimen is placed. Ultrasonic speed in stainless steel and Plexiglas was measured using this method to verify its validity. Results on measurements on porcine liver, myocardium, and soft fat are also reported.

ISSN:0001-4966    

DOI:10.1121/1.400242

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Speaking rate is thought to affect the spectral features of vowels. Target‐undershoot models of vowel production predict more spectral reduction and coarticulation of vowels in fast‐rate speech than in normal‐rate speech. To test this prediction, a meaningful Dutch text of about 850 words was read twice by an experienced newscaster, once at a normal speaking rate and once as fast as possible. All realizations of seven different vowels and some realizations of the schwa (/E/) were isolated. The first and second formant frequency values of all realizations were measured at five different points, each time by making cross sections at different points in the vowel realization. The different selections of these points are based on procedures used in literature, such as maximalF1 or mean formant value. No spectral vowel reduction was found that could be attributed to a faster speaking rate, neither was a change in coarticulation found. The only systematic effect was a higherF1 value in fast‐rate speech irrespective of vowel identity. This possibly suggests a generally more open articulation of vowels, speaking louder, or some other general change in speaking style by our speaker when he speaks fast.

ISSN:0001-4966    

DOI:10.1121/1.400243

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This paper examines some of the factors that can affect the magnitude of comodulation masking release (CMR). In experiment I, psychometric functions were measured for the detection of a 1‐kHz sinusoidal signal in a ‘‘multiplied’’ narrow‐band noise centered at 1 kHz (reference condition) and the same noise with two comodulated flanking bands added. The functions were slightly steeper for the comodulated than for the reference masker. Thus CMRs measured at a high percent correct point were slightly (0.4 dB) larger than CMRs measured at a low percent correct point. Large individual differences were found for the reference masker but not for the comodulated masker. Experiment II compared CMRs obtained with narrow‐band Gaussian noise and multiplied noise, using a single flanking band. For a flanking band remote from the signal frequency, the CMRs were smaller and more variable for the multiplied noise than for the Gaussian noise. This variability arose mainly from individual differences in the reference condition. Experiment III compared growth‐of‐masking functions for a signal centered in Gaussian noise and multiplied noise. Thresholds were lower for the multiplied than for the Gaussian noise, and the differences were greatest at high noise levels. The results are consistent with the idea that, for multiplied noise, some subjects can detect a change in the distribution of the envelope of the stimulus, when the signal is added to the masker. Such subjects have low thresholds in the reference condition, and give small CMRs. Other subjects are relatively insensitive to this cue. They have higher thresholds in the reference condition, and give larger CMRs. For Gaussian noise, thresholds for the reference condition are relatively stable across subjects and CMRs tend to be substantial, even for flanking‐band frequencies remote from the signal frequency.

ISSN:0001-4966    

DOI:10.1121/1.400244

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This paper is concerned with aspects of temporal integration and across‐frequency integration in signal detection. Previous experiments on the detection of brief broadband signals (clicks) in continuous broadband noise revealed efficient spectral integration. The extent to which this effect is restricted to a critical time window was investigated by manipulating the temporal relations among the signal components in different frequency regions. In a typical experiment, the signal consists of nine brief Gaussian‐shaped tone pulses, equally distributed at 1/3‐oct intervals, each with a spectral width of about 1/3 oct, and each equally detectable in white noise. In the synchronized condition (i.e., coinciding peaks of the nine Gaussian envelopes), the detection threshold is reached when the levels of the nine individual tone pulses are about 8 dB below their individual threshold levels (efficient spectral integration). When the signal is progressively desynchronized (i.e., noncoinciding peaks of the Gaussian envelopes), detection threshold is found to increase. This suggests that efficient spectral integration in signal detection is confined to a narrow time window, with a typical value of 30 ms. Similar experiments were performed with respect to the efficiency of temporal integration. For constant‐duration signals (100 ms), the detection threshold is found to increase when progressively widening signal bandwidth. The data indicate that the efficient temporal integration in signal detection is confined to a narrow frequency window, which, not surprisingly, corresponds to the critical bandwidth.

ISSN:0001-4966    

DOI:10.1121/1.400245

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The ability of a listener to detect a mistuned harmonic in an otherwise periodic tone is representative of the capacity to segregate auditory entities on the basis of steady‐state signal cues. By use of a task in which listeners matched the pitch of a mistuned harmonic, this ability has been studied, in order to find dependences on mistuned harmonic number, fundamental frequency, signal level, and signal duration. The results considerably augment the data previously obtained from discrimination experiments and from experiments in which listeners counted apparent sources. Although previous work has emphasized the role of spectral resolution in the segregation process, the present work suggests that neural synchrony is an important consideration; our data show that listeners lose the ability to segregate mistuned harmonics at high frequencies where synchronous neural firing vanishes. The functional form of this loss is insensitive to the spacing of the harmonics. The matching experiment also permits the measurement of the pitches of mistuned harmonics. The data exhibit shifts of a form that argues against models of pitch shifts that are based entirely upon partial masking.

ISSN:0001-4966    

DOI:10.1121/1.400246

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The speech‐reception threshold (SRT) for sentences presented in a fluctuating interfering background sound of 80 dBA SPL is measured for 20 normal‐hearing listeners and 20 listeners with sensorineural hearing impairment. The interfering sounds range from steady‐state noise, via modulated noise, to a single competing voice. Two voices are used, one male and one female, and the spectrum of the masker is shaped according to these voices. For both voices, the SRT is measured as well in noise spectrally shaped according to the target voice as shaped according to the other voice. The results show that, for normal‐hearing listeners, the SRT for sentences in modulated noise is 4–6 dB lower than for steady‐state noise; for sentences masked by a competing voice, this difference is 6–8 dB. For listeners with moderate sensorineural hearing loss, elevated thresholds are obtained without an appreciable effect of masker fluctuations. The implications of these results for estimating a hearing handicap in everyday conditions are discussed. By using the articulation index (AI), it is shown that hearing‐impaired individuals perform poorer than suggested by the loss of audibility for some parts of the speech signal. Finally, three mechanisms are discussed that contribute to the absence of unmasking by masker fluctuations in hearing‐impaired listeners. The low sensation level at which the impaired listeners receive the masker seems a major determinant. The second and third factors are: reduced temporal resolution and a reduction in comodulation masking release, respectively.

ISSN:0001-4966    

DOI:10.1121/1.400247

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

In this paper previous experiments on auditory filter shapes in binaural masking experiments [A. Kohlrausch, J. Acoust. Soc. Am.84, 573–583 (1988)] are extended to a wider range of masker and signal durations. The masker was a dichotic broadband noise with frequency‐dependent interaural parameters. The interaural phase difference of the masker was 0 below 500 Hz and π above 500 Hz. Signal frequency varied between 200 and 800 Hz, and the signal was presented either monaurally (Sm) or binaurally in antiphase (Sπ). In the first experiment, the masker duration was fixed at 500 ms and signals of 250 and 20 ms were used. In the second experiment, the signal duration was fixed at 20 ms, and the masker duration was reduced to 25 ms. The results from both experiments are consistent with studies using No or Nπ maskers: The binaural masking level difference (BMLD) increases slightly for shorter test signals and decreases strongly for short maskers. The BMLD patterns of the first experiment are well described by the auditory‐filter model derived for stationary test signals, if the additional influence of ‘‘off‐frequency listening’’ for the short test signal is taken into account. The BMLDs resulting from the second experiment (25‐ms masker), however, are much lower than predicted by this filter model. This outcome supports previous observations that binaural unmasking becomes less effective for very short masker durations and indicates that this effect is even stronger for maskers with a complex structure of interaural parameters.

ISSN:0001-4966    

DOI:10.1121/1.400248

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This study investigates whether binaural signal detection is improved by the listener’s previous knowledge about the interaural phase relations of masker and test signal. Binaural masked thresholds were measured for a 500‐ms dichotic noise masker that had an interaural phase difference of 0 below 500 Hz and of π above 500 Hz. The thresholds for two different 20‐ms test signals were determined within the same measurement using an interleaved adaptive 3‐interval forced‐choice (3IFC) procedure. In each 3IFC trial, both signals could occur with equal probability (uncertainty). The two signals differed in frequency and interaural phase in such a way that one signal always had a frequency above the masker edge frequency (500 Hz) and no interaural phase difference (So), whereas the other signal frequency was below 500 Hz and the interaural phase difference was π (Sπ). The frequencies of a signal pair remained fixed during the whole 3IFC track. These two signals thus lead to two different binaural conditions, i.e., NoSπ for the low‐frequency signal and NπSo for the high‐frequency signal. For comparison, binaural masked thresholds were measured with the same masker for fixed signal frequency and phase. The binaural masking level differences (BMLDs) resulting from the two experimental conditions show no significant difference. This indicates that the binaural system is able to apply different internal transformations or processing strategies simultaneously in different critical bands and even within the same critical band.

ISSN:0001-4966    

DOI:10.1121/1.400249

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A simple virtual sound display built around a microcomputer and analog hardware is described. The display implements most of the primary cues for sound localization in the ear‐level plane. Judging both from informal observations by users and from objective data obtained in an experiment on homing to virtual and real sounds, it is concluded that simple displays like the one described are effective in creating the impression of external sounds to which observers can locomote with ease; in particular, this means that simulation of the direction‐dependent spectral shaping effects of the pinnae is not a necessary requirement for extracranial sound localization.

ISSN:0001-4966    

DOI:10.1121/1.400250

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Gross variations of the speech amplitude envelope, such as the duration of different segments and the gaps between them, carry information about prosody and some segmental features of vowels and consonants. The amplitude envelope is one parameter encoded by the Tickle Talker, an electrotactile speech processor for the hearing impaired which stimulates the digital nerve bundles with a pulsatile electric current. Psychophysical experiments measuring the duration discrimination and identification, gap detection, and integration times for pulsatile electrical stimulation are described and compared with similar auditory measures for normal and impaired hearing and electrical stimulation via a cochlear implant. The tactile duration limen of 15% for a 300‐ms standard was similar to auditory measures. Tactile gap detection thresholds of 9 to 20 ms were larger than for normal‐hearing but shorter than for some hearing‐impaired listeners and cochlear implant users. The electrotactile integration time of about 250 ms was shorter than previously measured tactile values but longer than auditory integration times. The results indicate that the gross amplitude envelope variations should be conveyed well by the Tickle Talker. Short bursts of low amplitude are the features most likely to be poorly perceived.

ISSN:0001-4966    

DOI:10.1121/1.400197

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP