摘要:

This article concerns the generation of waveforms by a digital oscillator in which sampled data in a memory buffer are recycled. The buffer contains a fixed waveform and the output sample rate is also fixed. Despite these constraints, the oscillator is capable of arbitrarily high frequency resolution if the technique of fractional addressing is used. However, fractional addressing introduces distortion. This article gives a theory of fractional addressing, resembling the theory of diffraction in crystal lattices with a basis. The theory shows how the spectrum of the distortion components can be calculated and how the distortion can be minimized. Attention is called to numerous symmetries in the distortion spectrum. These symmetries are especially interesting if the purpose of the system is to make use of the distortion components to create inharmonic signals. Of particular importance is the γpsymmetry theorem, which makes it possible to derive simple formulas for the level of the largest distortion component and for the total distortion power.

ISSN:0001-4966    

DOI:10.1121/1.395683

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Carolliaperspicillata(Phyllostomidae) is a frugivorous bat that emits low‐intensity, broadband, frequency‐modulated echolocation pulses through nostrils surrounded by a noseleaf. The emission pattern of this bat is of interest because the ratio between the nostril spacing and the emitted wavelength varies during the pulse, causing complex interference patterns in the horizontal dimension. Sound pressures around the bat were measured using a movable microphone and were referenced to those at a stationary microphone positioned directly in front of the animal. Interference between the nostrils was confirmed by blocking one nostril, which eliminated sidelobes and minima in the emission pattern, and by comparison of real emission patterns with simple computer models. The positions of minima in the patterns indicate effective nostril spacings of over a half‐wavelength. Displacement of the dorsal lancet of the noseleaf demonstrated that this structure directs sound in the vertical dimension.

ISSN:0001-4966    

DOI:10.1121/1.395684

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The 20‐Hz signals of finback whales (Balaenopteraphysalus) were analyzed from more than 25 years of recordings at a variety of geographic locations on near‐surface hydrophones close to whales and on deep hydrophone systems. These signals were composed of 1‐s pulses of sinusoidal waveform with downward sweeping frequency from approximately 23 to 18 Hz at variable source levels up to 186 dB (re: 1 μPa at 1 m), usually with slightly lower levels for the pulses at the beginning and end of sequences. These ‘‘20‐Hz’’ pulses were produced in signal bouts (separated by more than 2 h) lasting as long as 32.5 h. Bouts were composed of regularly repeated pulses at intervals of 7–26 s (typically), either at one nominal pulse rate or at two alternating (doublet) pulse intervals. Signal bouts were interrupted by rests of 1–20 min at roughly 15‐min intervals and by irregular gaps lasting between 20 and 120 min. The distribution of these signals throughout the year and their temporal sequence were analyzed from the continuous drum records of the Bermuda SOFAR Station. Signal bouts occurred during winter, sometimes beginning in September and ending in May. The sound sequences were never exactly replicated. Direct association of the bouts with the reproductive season for this species points to the 20‐Hz signals as possible reproductive displays by finback whales.

ISSN:0001-4966    

DOI:10.1121/1.395685

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

It is usually said that a force applied to an extended vibratory system is incapable of exciting any normal mode that has a node at that point. Thus piano hammers positioned to strike the strings at certain fractions of their length should produce spectra with ‘‘missing modes.’’ However, this has been disputed by some observers, and it is sometimes suggested that more careful theoretical treatments would justify a restoration of those modes. Spectra of string motion have been measured as a function of hammer position by moving a piano action frame in small increments along the bed of the case. It is found that there are indeed sharp minima in mode amplitudes (on the order of 20 to 30 dB down) as the striking point passes within a few millimeters of the nodal positions. Possible limitations on the theoretical arguments are discussed, and it is concluded that the small residual strengths in the nearly missing modes can be well accounted for by finite soundboard admittance.

ISSN:0001-4966    

DOI:10.1121/1.395686

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Six studies purporting to demonstrate complementary covariation in lip and jaw activity during speech are reviewed. The statistical procedures used to assess interactions among the upper lip, lower lip, and jaw movements are discussed for four different experiments analyzing repetition‐to‐repetition movement variation. The findings from two studies analyzing repetition‐to‐repetition variation for interactions in electromyographic activity recorded from either the jaw musculature or the labial musculature also are evaluated. It is concluded that these studies do not provide convincing evidence of complementary covariation among the articulators or the muscles.

ISSN:0001-4966    

DOI:10.1121/1.395687

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The effects of forward masker duration on psychophysical measures of frequency selectivity were investigated in two experiments. In both experiments, masker duration was 50 or 400 ms, signal duration was 20 ms, and there was no delay between masker offset and signal onset. In the first experiment, growth‐of‐masking functions were measured for a masker whose frequency was below, at, or above the 1000‐Hz signal frequency. From those data, input filter patterns (IFPs) were plotted for masker levels from 40–90 dB SPL. In the second experiment, masking patterns (MPs) were measured for a 1000‐Hz masker presented at 50, 70, and 90 dB SPL. Both measures of frequency selectivity (IFPs and MPs) indicate that frequency selectivity is greater for the 400‐ms masker. These data suggest that there may be a sharpening of frequency selectivity with time at a stage prior to the adaptation observed in forward masking.

ISSN:0001-4966    

DOI:10.1121/1.395688

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Results of experiments on the detection of silent intervals, or gaps, in broadband noise are reported for normal‐hearing listeners. In some preliminary experiments, a gap threshold of about 2 ms was measured. This value was independent of the duration of the noise burst, variation of the noise level on each presentation, or the temporal position of the gap within the noise burst. In the main experiments, the thresholds for partial decrements in the noise waveform as well as brief increments were determined. As predicted by a model that assumes a single fixed peak‐to‐valley detection ratio, thresholds for increments are slightly higher than thresholds for decrements when the signal is measured as the change in rms noise level. A first‐order model describes the temporal properties of the auditory system as a low‐pass filter with a 7‐ to 8‐ms time constant. Temporal modulation transfer functions were determined for the same subjects, and the estimated temporal parameters agreed well with those estimated from the gap detection data. More detailed modeling was carried out by simulating Viemeister’s three‐stage temporal model. Simulations, using an initial stage bandwidth of 4000 Hz and a 3‐ms time constant for the low‐pass filter, generate data that are very similar to those obtained from human subjects in both modulation and gap detection.

ISSN:0001-4966    

DOI:10.1121/1.395689

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

In experiment I, thresholds for 400‐ms sinusoidal signals were measured in the presence of a continuous 25‐Hz‐wide noise centered at signal frequencies ( fs) ranging from 250 to 8000 Hz in 1‐oct steps. The masker was presented either alone or together with a second continuous 25‐Hz‐wide band of noise (the flanking band) whose envelope was either correlated with that of the on‐frequency band or was uncorrelated; its center frequency ranged from 0.5fsto 1.5fs. The flanking band was presented either in the same ear (monotic condition) as the signal plus masker or in the opposite ear (dichotic condition). The on‐frequency band and the flanking band each had an overall level of 67 dB SPL. The comodulation masking release, CMR(U−C), is defined as the difference between the thresholds for the uncorrelated and correlated conditions. The CMR(U−C) showed two components: a broadly tuned component, occurring at all signal frequencies and all flanking‐band frequencies, and occurring for both monotic and dichotic conditions; and a component restricted to the monotic condition and to flanking‐band frequencies close tofs. This sharply tuned component was small for low signal frequencies, increased markedly at 2000 and 4000 Hz, and decreased at 8000 Hz. Experiment II showed that the sharply tuned component of the CMR(U−C) was slightly reduced in magnitude when the level of the flanking band was 10 dB above that of the on‐frequency band and was markedly reduced when the level was 10 dB below, whereas the broadly tuned component and the dichotic CMR(U−C) were only slightly affected. Experiment III showed that the sharply tuned component of the CMR(U−C) was markedly reduced when the bandwidths of the on‐frequency and flanking bands were increased to 100 Hz, while the broadly tuned component and the dichotic CMR(U−C) decreased only slightly. The argument here is that the sharply tuned component of the monotic CMR(U−C) results from beating between the ‘‘carrier’’ frequencies of the two masker bands. This introduces periodic zeros in the masker envelope, which facilitate signal detection. The broadly tuned component, which is probably a ‘‘true’’ CMR, was only about 3 dB.

ISSN:0001-4966    

DOI:10.1121/1.395639

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Sound image position associated with the interaural onset or offset disparity of a signal was quantified by a scaling procedure in three experiments. Lateralization cues derived from the steady‐state portion of the broadband noise signal that would support a specific image position were minimized by the use of independent noise sources for each ear. Onset disparities produced lateralization toward the ear at which the sound was presented first, while offset disparity produced lateralization toward the ear at which the sound remained on longer. Disparity was systematically varied between 0 and 10 ms and for a given disparity, a greater shift in the sound image position was obtained when the disparity was at the onset rather than the offset. The duration of the shorter signal ranged from 2.5–100 ms and for either onset or offset disparity, the image of stimuli of long duration tended to remain near the center of the head, while those of shorter duration could be moved to more extreme positions. In an attempt to rule out dichotic loudness cues as a basis for the lateralization associated with offset disparity, stimuli were presented with equal energy at each ear. Image position for equal energy was virtually identical to that for equal sound pressure, suggesting that loudness differences are not mediating lateralization associated with offset disparity.

ISSN:0001-4966    

DOI:10.1121/1.395640

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Paired‐comparison judgments of intelligibility of speech in noise were obtained from eight hearing‐impaired subjects on a large number of hearing aids simulated by a digital master hearing aid. The hearing aids which comprised a 5×5 matrix differed systematically in the amount of low‐frequency and high‐frequency gain provided. A comparison of three adaptive strategies for determining optimum hearing aid frequency‐gain characteristics (an iterative round robin, a double elimination tournament, and a modified simplex procedure) revealed convergence on the same or similar hearing aids for most subjects. Analysis revealed that subjects for whom all three procedures converged on the same hearing aid showed a single pronounced peak in the response surface, while a broader peak was evident for the subjects for whom the three procedures identified similar hearing aids. The modified simplex procedure was found to be most efficient and the iterative round robin least efficient.

ISSN:0001-4966    

DOI:10.1121/1.395641

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP