摘要:

Anechoic wedges are conventionally installed with the tapered point of the wedge pointing toward the test space and with the flat base of the wedge either against a rigid wall or separated from the wall by an air gap. A comparative investigation has been carried out in which the wedges were first conventionally oriented and then turned 180° so that the points of the wedges faced the wall. These comparative tests were performed in reverberation rooms both here and in Europe and also in two different impedance tubes. The results are extremely interesting. They tend to indicate that a ‘‘reversed’’ anechoic wedge can give a lower cutoff frequency than when conventionally oriented. Some possible explanations are offered for the measured performance.

ISSN:0001-4966    

DOI:10.1121/1.390595

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A sequential hypothesis testing, optimal stopping problem in underwater acoustic detection is formulated and solved using dynamic programming. The problem calls for deciding whether acoustic signals being received over long ranges in the ocean are due to a source or to ambient noise alone, so as to minimize the expected value of a specified cost function over a given time horizon. The cost function incorporates a constant cost per observation as well as terminal costs for false acceptance of either hypothesis. According to previous work by the authors, and without loss of generality, modeling the acoustic signals assumes a two‐state discrete‐time Markov process for each of the two hypotheses, the state of the process depending on whether the intensity of the signal at the receiver is above a specified threshold or not. The decision process presented is based on observations of the signal’s ‘‘interarrival times,’’ that is, the time intervals between two successive detection events. The algorithm is then extended into more than two alternative hypotheses (several ‘‘false’’ targets) and results using both simulated and experimental acoustic data for the two and three hypotheses cases are presented. Computational issues in implementing the algorithms as well as possible extensions of this work are finally discussed.

ISSN:0001-4966    

DOI:10.1121/1.390569

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Discharge patterns of auditory‐nerve fibers in anesthetized cats were recorded in response to a set of nine steady‐state, two‐formant vowels presented at 60 and 75 dB SPL. The largest components in the discrete Fourier transforms of period histograms were almost always harmonics of the vowel fundamental frequency that were close to one of the formant frequencies, the fundamental frequency or the fiber characteristic frequency (CF). For any fiber, the position of its CF relative to the formant frequencies (F1 andF2) appears to determine which of these components dominates the response. Specifically, the response characteristics of the tonotopically arranged array of fibers can be described in terms of five CF regions: (1) a low‐CF region belowF1 in which the largest response components are the harmonics of the fundamental frequency closest to CF; (2) a region centered around CF = F1 in which the first formant and its harmonics are the largest components; (3) an intermediate region betweenF1 andF2 with prominent components at both the fiber CF and the fundamental frequency; (4) a region centered around CF = F2 in which harmonics close to the second formant are the largest for frequencies above the fundamental; and (5) a high‐CF region in which response spectra tend to show broad, multiple peaks at the formant and fundamental frequencies. These CF regions are related to the phonetic descriptions of vowels. For example, the extent of the low‐CF region is largest for ‘‘open’’ vowels (which have a highF1), and the intermediate region is distinct only for ‘‘spread’’ vowels for whichF1 andF2 are more than 1.5–2 octaves apart. For all vowels, response activity for the majority of fibers is concentrated near the formant frequencies, in contrast to responses to broadband noise for which components near CF are dominant.

ISSN:0001-4966    

DOI:10.1121/1.390596

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Several processing schemes by which phonetically important information for vowels can be extracted from responses of auditory‐nerve fibers are analyzed. The schemes are based on power spectra of period histograms obtained in response to a set of nine two‐formant, steady‐state, vowel‐like stimuli presented at 60 and 75 dB SPL. One class of ‘‘local filtering’’ schemes, which was originally proposed by Young and Sachs [J. Acoust. Soc. Am.66, 1381–1403 (1979)], consists of analyzing response patterns by filters centered at the characteristic frequencies (CF) of the fibers, so that a tonotopically arranged measure of synchronized response can be obtained. Various schemes in this class differ in the characteristics of the filter. For a wide range of filter bandwidths, formant frequencies correspond approximately to the CFs for which the response measure is maximal. If in addition, the bandwidths of the analyzing filters are made compatible with psychophysical measures of frequency selectivity, low‐frequency harmonics of the stimulus fundamental are resolved in the output profile, so that fundamental frequency can also be estimated. In a second class of processing schemes, a dominant response component is defined for each fiber from a 1/6 octave spectral representation of the response pattern, and the formant frequencies are estimated from the most frequent values of the dominant component in the ensemble of auditory‐nerve fibers. The local filtering schemes and the dominant component schemes can be related to ‘‘place’’ and ‘‘periodicity’’ models of auditory processing, respectively.

ISSN:0001-4966    

DOI:10.1121/1.390597

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Responses of auditory‐nerve fibers in anesthetized cats were recorded for synthetic voiceless fricative consonants. The four stimuli (/x/, /š/, /s/, and /f/) were presented at two levels corresponding to speech in which the levels of the vowels would be approximately 60 and 75 dB SPL, respectively. Discharge patterns were characterized in terms of PST histograms and their power spectra. For both stimulus levels, frequency regions in which the stimuli had considerable energy corresponded well with characteristic‐frequency (CF) regions in which average discharge rates were the highest. At the higher level, the profiles of discharge rate against CF were more distinctive for the stimulus onset than for the central portion. Power spectra of PST histograms had large response components near fiber characteristic frequencies for CFs up to 3–4 kHz, as well as low‐frequency components for all fibers. The relative amplitudes of these components varied for the different stimuli. In general, the formant frequencies of the fricatives did not correspond with the largest response components, except for formants below about 3 kHz. Processing schemes based on fine time patterns of discharge that were effective for vowel stimuli generally failed to extract the formant frequencies of fricatives.

ISSN:0001-4966    

DOI:10.1121/1.390598

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Discharge patterns of auditory‐nerve fibers in anesthetized cats were obtained for two stimulus levels in response to synthetic stimuli with dynamic characteristics appropriate for selected consonants. A set of stimuli was constructed by preceding a signal that was identified as /da/ by another sound that was systematically manipulated so that the entire complex would sound like either /da/, /ada/, /na/, /σa/, /sa/, or others. Discharge rates of auditory‐nerve fibers in response to the common /da/‐like formant transitions depended on the preceding context. Average discharge rates during these transitions decreased most for fibers whose CFs were in frequency regions where the context had considerable energy. Some effect of the preceding context on fine time patterns of response to the transitions was also found, but the identity of the largest response components (which often corresponded to the formant frequencies) was in general unaffected. Thus the response patterns during the formant transitions contain cues about both the nature of the transitions and the preceding context. A second set of stimuli sounding like /σ/ and /C/ was obtained by varying the duration of the rise in amplitude at the onset of a filtered noise burst. At both 45 and 60 dB SPL, there were fibers which showed a more prominent peak in discharge rate at stimulus onset for /C/ than for /σ/, but the CF regions that reflected the clearest distinctions depended on stimulus level. The peaks in discharge rate that occur in response to rapid changes in amplitude or spectrum might be used by the central processor as pointers to portions of speech signals that are rich in phonetic information.

ISSN:0001-4966    

DOI:10.1121/1.390599

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Responses of auditory‐nerve fibers to steady‐state, two‐formant vowels in low‐pass background noise (S/N=10 dB) were obtained in anesthetized cats. For fibers over a wide range of characteristic frequencies (CFs), the peaks in discharge rate at the onset of the vowel stimuli were nearly eliminated in the presence of noise. In contrast, strong effects of noise on fine time patterns of discharge were limited to CF regions that are far from the formant frequencies. One effect is a reduction in the amplitude of the response component at the fundamental frequency in the high‐CF regions and for CFs betweenF1 andF2 when the formants are widely separated. A reduction in the amplitude of the response components at the formant frequencies, with concomittant increase in components near CF or low‐frequency components occurs in CF regions where the signal‐to‐noise ratio is particularly low. The processing schemes that were effective for estimating the formant frequencies and fundamental frequency of vowels in quiet generally remain adequate in moderate‐level background noise. Overall, the discharge patterns contain many cues for distinctions among the vowel stimuli, so that the central processor should be able to identify the different vowels, consistent with psychophysical performance at moderate signal‐to‐noise ratios.

ISSN:0001-4966    

DOI:10.1121/1.390537

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The frequency specificity of the auditory brainstem response (ABR) was examined by means of pure‐tone masking profiles using click, 4000‐Hz, and 1000‐Hz filtered‐click stimuli. Simultaneous pure‐tone maskers were presented at one‐half octave intervals around stimulus center frequency. Masking profiles at two intensities (60 and 40 dB SL) were obtained by measuring both latency and amplitude shifts in wave V as a result of the discrete‐frequency maskers. Both latency and amplitude analyses showed masking profiles at 40 dB SL that were narrow and centered around stimulus frequency, whereas profiles at 60 dB SL showed high‐frequency spread of the cochlear excitation area.

ISSN:0001-4966    

DOI:10.1121/1.390538

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

We compare psychophysical tuning curves obtained with sinusoidal and narrow‐band (50‐Hz wide) noise maskers in both simultaneous and forward masking. In one experiment, we examine the effects of different combinations of duration and intensity of the 1‐kHz sinusoidal signal. In a second experiment, we compare tuning curves obtained with a sinusoidal signal to those obtained with a noise signal. In both experiments, a narrow‐band noise is a more effective simultaneous masker than a sinusoid for masker frequencies near the signal frequency. We argue that this is probably due to the use of different detection cues in the presence of sinusoidal and noise maskers, and that the greater masking produced by the noise is not simply due to its greater variability. As observed in other studies, tuning curves are narrower in forward masking than in simultaneous masking.

ISSN:0001-4966    

DOI:10.1121/1.390539

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Frequency effects in auditory backward masking were examined by psychophysically determining the thresholds of 0.5‐, 1‐, 3‐, 5‐, and 7‐kHz sinusoidal probes followed by a broadband noise masker. For each probe frequency, backward masking decreased approximately exponentially as the temporal interval between the probe and masker was increased. The rate of this exponential decay increased with increasing probe frequency. This latter result is consistent with Duifhuis’ [J. Acoust. Soc. Am.54, 1471–1488 (1973)] theory which attributes backward masking to the temporal overlap of cochlear responses to the probe and masker.

ISSN:0001-4966    

DOI:10.1121/1.390540

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP