The just audible tonality of short exponential and Gaussian pure tone bursts

This paper reports on listening tests performed to investigate the just audible tonality (JAT) of decaying pure tone bursts. Both exponential and Gaussian functions are used to shape the envelopes of the tone bursts and critical band center frequencies between 150 and 7000 Hz are studied. Loudness compensation is implemented to compensate for the reduced loudness of short tone bursts and attack functions are used for minimizing clicks. By using the method of limits, a sequence of tone bursts with increasing decay times and constant frequencies is presented to the listeners at 0.9 s intervals. The first burst in the sequence which is perceived as being tonal is indicated by the listeners. When that happens, the decay times of the tone bursts decrease and the listeners are asked to select the first tone burst with no audible tonality. The listeners are allowed to freely define tonality. No reference is given. For frequencies above 3.4 kHz, the results indicate that tonality is just audible for tone burst lengths of approximately 2.6-3.0 ms. For the lowest stimuli frequencies, the corresponding burst length is approximately 20-23 ms.     

The effect of broadband noise on the human brainstem auditory evoked response. II. Frequency specificity

A series of experiments evaluated the effects of broadband noise (ipsilateral) on wave V of the brainstem auditory evoked response (BAER) elicited by tone bursts or clicks in the presence of high-pass masking noise. Experiment 1 used 1000- and 4000-Hz, 60-dB nHL tone bursts in the presence of broadband noise. With increasing noise level, wave V latency shift was greater for the 1000-Hz tone bursts, while amplitude decrements were similar for both tone-burst frequencies. Experiment 2 varied high-pass masker cutoff frequency and the level of subtotal masking in the presence of 50-dB nHL clicks. The effects of subtotal masking on wave V (increase in latency and decrease in amplitude) increased with increasing derived-band frequency. Experiment 3 covaried high-pass masker cutoff frequency and subtotal masking level for 1000- and 4000-Hz tone-burst stimuli. The effect of subtotal masking on wave V latency was reduced for both tone-burst frequencies when the response-generating region of the cochlear partition was limited by high-pass maskers. The results of these three experiments suggest that most of the wave V latency shift associated with increasing levels of broadband noise is mediated by a place mechanism when the stimulus is a moderate intensity (60 dB nHL), low-frequency (1000 Hz) tone burst. However, the interpretation of the latency shifts produced by broadband noise for 4000-Hz tone-burst stimuli is made more complex by multiple technical factors discussed herein.     

Propeller tone bursts

Intense high frequency (25–38 kHz) tone bursts have been observed in acoustic tests of a scale model of a general aviation propeller. The amplitude of the tone burst is approximately equal to the amplitude of the propeller noise signature. The conditions necessary for the production of these tone bursts are described. The experiments indicate that the origin of these bursts is a periodic flow oscillation on the suction surface of the propeller blade tips which may be due to the interaction between an oscillating shock wave and a laminar boundary layer.     

Manifestation of peripherial coding in the effect of increasing loudness and enhanced discrimination of the intensity of tone bursts before and after tone burst noise

To find the possible reasons for the midlevel elevation of the Weber fraction in intensity discrimination of a tone burst, a comparison was performed for the complementary distributions of spike activity of an ensemble of space nerves, such as the distribution of time instants when spikes occur, the distribution of interspike intervals, and the autocorrelation function. The distribution properties were detected in a poststimulus histogram, an interspike interval histogram, and an autocorrelation histogram—all obtained from the reaction of an ensemble of model space nerves in response to an auditory noise burst–useful tone burst complex. Two configurations were used: in the first, the peak amplitude of the tone burst was varied and the noise amplitude was fixed; in the other, the tone burst amplitude was fixed and the noise amplitude was varied. Noise could precede or follow the tone burst. The noise and tone burst durations, as well as the interval between them, was 4 kHz and corresponded to the characteristic frequencies of the model space nerves. The profiles of all the mentioned histograms had two maxima. The values and the positions of the maxima in the poststimulus histogram corresponded to the amplitudes and mutual time position of the noise and the tone burst. The maximum that occurred in response to the tone burst action could be a basis for the formation of the loudness of the latter (explicit loudness). However, the positions of the maxima in the other two histograms did not depend on the positions of tone bursts and noise in the combinations. The first maximum fell in short intervals and united intervals corresponding to the noise and tone burst durations. The second maximum fell in intervals corresponding to a tone burst delay with respect to noise, and its value was proportional to the noise amplitude or tone burst amplitude that was smaller in the complex. An increase in tone burst or noise amplitudes was caused by nonlinear variations in the two maxima and the ratio between them. The size of the first maximum in the of interspike interval distribution could be the basis for the formation of the loudness of the masked tone burst (implicit loudness), and the size of the second maximum, for the formation of intensity in the periodicity pitch of the complex. The auditory effect of the midlevel enhancement of tone burst loudness could be the result of variations in the implicit tone burst loudness caused by variations in tone-burst or noise intensity. The reason for the enhancement of the Weber fraction could be competitive interaction between such subjective qualities as explicit and implicit tone-burst loudness and the intensity of the periodicity pitch of the complex.     

Effects of relative starting phase and frequency separation of two-tone stimuli on the brain-stem auditory-evoked response

Brain-stem auditory-evoked responses (BAERs) were obtained in six normal-hearing adults using single-tone and two-tone stimuli arithmetically centered around 4000 Hz. Two-tone stimuli varied in frequency separation from 200 to 3200 Hz, and started in-phase (homophasic) or 180 deg out-of-phase (antiphasic) with each other. Responses to each of the single-tone components of the two-tone stimuli were elicited and then summed for comparison with responses to the two-tone stimuli. Results indicated no significant difference in wave V latency between homophasic or antiphasic two-tone conditions, and summed single-tone conditions. Under the homophasic condition, the mean latency for the widest frequency separation of the tones was significantly longer than those for narrower separations. A significant difference in wave V amplitude between two-tone phase conditions was found for frequency separations of 200, 400, and 3200 Hz only. Summed single-tone BAERs demonstrated a significantly larger wave V amplitude than responses from either two-tone phase condition at all frequency separations.     

Discriminability of bursts of reproducible noise.

The ability of human listeners to discriminate pairs of bursts of reproducible noise was examined. A same-different psychophysical method was used. Bursts in a pair were identical on same trials. On different trials, bursts were identical except for tau ms of independent noise located at either the beginning, middle, or end of the pairs of bursts. As the temporal position of the tau ms of independent noise was moved from the beginning to the end of the bursts of noise, discriminability increased. For each temporal position of the independent noise, discriminability was a function of the ratio of the duration of the independent noise (tau) to the total burst duration.     

Binaural image position distributions for phase-shifted low frequency tone bursts

This experiment was designed to yield precise measures of the statistical properties of perceived sound images. Results are reported for listeners' judgments of intracranial sound image lateral positions in response to binaural tone burst stimuli (250 Hz, 50 ms) with varying interaural phase differences, conditional on the absence or presence of a (left or right) reference monaural tone burst (also 250 Hz, 50 ms) ending 500 ms prior to the test signal. The monaural-reference shifted the position distributions toward the opposite side of the head. The position distribution variance and skewness depended on the mean of the position distribution, not on the interaural phase difference of the stimulus. The standard deviation increased as the mean moved laterally from midline. Near the midline the position distributions were skewed ipsilaterally. Near either ear they were skewed toward the midline. The results suggest that the most important noise limiting performance originates central to brainstem coincidence detector networks.     

Spectral loudness summation for sequences of short noise bursts

Recent loudness data of single noise bursts indicate that spectral loudness summation depends on signal duration. To gain insight into the mechanisms underlying this duration effect, loudness was measured as a function of signal bandwidth centered around 2 kHz for sequences of 10-ms noise bursts at various repetition rates and, for comparison, for single noise bursts of either 10- or 1000-ms duration. The test-signal bandwidth was varied from 200 to 6400 Hz. For the repeated noise bursts, the reference signal had a bandwidth of 400 Hz. For the single noise bursts, data were obtained for two reference bandwidths: 400 and 3200 Hz. In agreement with previous results, the magnitude of spectral loudness summation was larger for the 10-ms than for the 1000-ms noise bursts. The reference bandwidth had no significant effect on the results for the single noise bursts. Up to repetition rates of 50 Hz, the magnitude of spectral loudness summation for the sequences of noise bursts was the same as for the single short noise burst. The data indicate that the mechanism underlying the duration effect in spectral loudness is considerably faster than the time constant of about 100 ms commonly associated with the temporal integration of loudness.     

Pitch matches between unresolved complex tones differing by a single interpulse interval

The experiment compared the pitches of complex tones consisting of unresolved harmonics. The fundamental frequency (F0) of the tones was 250 Hz and the harmonics were bandpass filtered between 5500 and 7500 Hz. Two 20-ms complex-tone bursts were presented, separated by a brief gap. The gap was an integer number of periods of the waveform: 0, 4, or 8 ms. The envelope phase of the second tone burst was shifted, such that the interpulse interval (IPI) across the gap was reduced or increased by 0.25 or 0.75 periods (1 or 3 ms). A "no shift" control was also included, where the IPI was held at an integer number of periods. Pitch matches were obtained by varying the F0 of a comparison tone with the same temporal parameters as the standard but without the shift. Relative to the no-shift control, the variations in IPI produced substantial pitch shifts when there was no gap between the bursts, but little effect was seen for gaps of 4 or 8 ms. However, for some conditions with the same IPI in the shifted interval, an increase in the IPI of the comparison interval from 4 to 8 ms (gap increased from 0 to 4 ms) changed the pitch match. The presence of a pitch shift suggests that the pitch mechanism is integrating information across the two tone bursts. It is argued that the results are consistent with a pitch mechanism employing a long integration time for continuous stimuli that is reset in response to temporal discontinuities. For a 250-Hz F0, an 8-ms IPI may be sufficient for resetting. Pitch models based on a spectral analysis of the simulated neural spike train, on an autocorrelation of the spike train, and on the mean rate of pitch pulses, all failed to account for the observed pitch matches.     

Brain-stem auditory-evoked responses elicited by maximum length sequences: effect of simultaneous masking noise

The effects of masking noise on wave V of the brain-stem auditory-evoked response (BAER) obtained to pseudorandom pulse sequences are evaluated in two experiments. In the first experiment, the level of broadband noise was covaried with minimum pulse interval (rate) using maximum length sequence analysis (MLSA). Both increasing noise level and decreasing minimum pulse interval decrease wave V amplitude and increase wave V latency. A nonadditivity of rate and noise level was observed such that, at the shortest interpulse intervals, simultaneous background noise produced virtually no latency change and minimal amplitude change, for the noise levels tested. In a second experiment, high-pass masking was performed to assess the feasibility of derived-band techniques using maximum length sequence analysis (MLSA) and to compare the frequency regions responsible for the BAER using MLSA versus conventional averaging. Results of experiment 2 showed that reliable responses across high-pass masker cutoff frequency could be obtained in normal-hearing listeners. The frequency specificity of the MLSA-based responses was nearly identical to that obtained by conventional averaging, although both amplitude and latency of wave V were affected by the high-pass masker cutoff and minimum pulse interval values. These studies suggest that the neuronal populations and frequency regions responsible for the BAER are virtually the same for MLSA and conventional averaging.     

Cosmic gamma-ray bursts

All aspects of cosmic gamma-ray bursts are reviewed. First, instrumentation and experimental technique are briefly covered. Then the observable burst properties are described, and empirical classification schemes are offered. Searches for coinciding bursts at other frequencies are enumerated. The observed spatial distribution of the burst sources is given, as well as various theoretical interpretations. A section is devoted to the unusual gamma-ray burst of March 5, 1979; its features are compared to more typical events and analyzed for insights into burst origins. Theoretical models for gamma-ray bursts are considered in general, and then examined in more detail under the categories of extragalactic models, accretion onto compact objects, thermonuclear explosions, flare models, and exotic models.     

The effects of real and illusory glides on pure-tone frequency discrimination

Experiment 1 measured pure-tone frequency difference limens (DLs) at 1 and 4 kHz. The stimuli had two steady-state portions, which differed in frequency for the target. These portions were separated by a middle section of varying length, which consisted of a silent gap, a frequency glide, or a noise burst (conditions: gap, glide, and noise, respectively). The noise burst created an illusion of the tone continuing through the gap. In the first condition, the stimuli had an overall duration of 500 ms. In the second condition, stimuli had a fixed 50-ms middle section, and the overall duration was varied. DLs were lower for the glide than for the gap condition, consistent with the idea that the auditory system contains a mechanism specific for the detection of dynamic changes. DLs were generally lower for the noise than for the gap condition, suggesting that this mechanism extracts information from an illusory glide. In a second experiment, pure-tone frequency direction-discrimination thresholds were measured using similar stimuli as for the first experiment. For this task, the type of the middle section hardly affected the thresholds, suggesting that the frequency-change detection mechanism does not facilitate the identification of the direction of frequency changes.     

Response magnitude and timing of auditory response initiation in the inferior colliculus of the awake chinchilla.

Recent single-unit studies in anesthetized cats have revealed that the latency and strength of transient responses to tone burst stimuli are determined largely by stimulus events in the first few ms of the signal. The present study sought to extend these findings by studying the inferior colliculus potential (ICP) in unanesthetized chinchillas. The ICP magnitude and latency were studied as a function of the plateau amplitude and rise time of noise burst stimuli. ICP amplitude increased with stimulus amplitude and decreased with stimulus rise time. ICP latency decreased with stimulus amplitude and increased with stimulus rise time. The absolute values of the ICP latencies confirmed that it is only the first few ms of the stimulus which determine the timing of response initiation, and therefore, that it is not the plateau level of the stimulus that directly determines the latent period. These data constitute a direct link between earlier single-unit studies in anesthetized animals and brainstem-evoked potential data in animals and man.     

Perceived tonal continuity through two noise bursts separated by silence

Three experiments measured the perceived continuity of two pure tones "flankers" through a masker containing a silence. Experiment 1 used a 2I-2AFC procedure; one interval contained two noise bursts separated by a silent gap, and the other contained two noise bursts separated by a tone of the same duration as the silence. Discrimination between masker conditions was very accurate when the flankers were absent but was impaired substantially when the flankers were present. This was taken as evidence that illusory flanker continuity during the silent gap was heard as similar to the physical presence of a tone in the gap. In experiment 2, performance remained poor when the flankers were frequency glides aligned along a common trajectory. Performance improved significantly when the flankers were misaligned in trajectory. In experiment 3, listeners rated directly perceived flanker continuity. Strong continuity was reported in the silent gap conditions for which poor performance had been observed in experiments 1 and 2. These findings show that continuity may be heard through a masker that cannot mask a physically continuous tone but can mask the flankers' offset and onset. The results are explained in terms of the perceptual grouping of onsets and offsets of the flankers.     

The perceptual tone/noise ratio of merged, iterated rippled noises with octave, harmonic, and nonharmonic delay ratios

The perceptual tone/noise ratio was measured for merged iterated ripple noise stimuli (IRNs) in which one of the individual IRNs always had a delay of 16 ms. The second IRN was chosen to create merged IRNs with single octave delay ratios (e.g., 16 ms:8 ms), double octave delay ratios (e.g., 16 ms:4 ms), harmonic delay ratios (e.g., 16 ms:12 ms), and nonharmonic delay ratios (e.g., 16 ms:3.9 ms). All stimuli were high-pass filtered at 800 Hz. The tone/noise ratio was significantly enhanced for the octave ratios, and there was a strong interaction between the single and double octave delay ratios and number of iterations. But, the perceptual tone/noise ratio for nonoctave ratios was determined solely by the number of iterations. The pattern of the results can be explained in terms of the height of the largest peak in the summary autocorrelogram [Meddis and Hewitt, J. Acoust. Soc. Am. 89, 2866-2882 (1991)] provided the model is modified to improve the simulation of the loss of phase locking.     

Age-related differences in discrimination of an interval separating onsets of successive tone bursts as a function of interval duration

The study measured listener sensitivity to increments in the inter-onset interval (IOI) separating pairs of successive 20-ms 4000-Hz tone pulses. A silent interval between the tone pulses was adjusted across conditions to create reference tonal IOI values of 25-600 ms. For each condition, a duration DL for increments of the tonal IOI was measured in listeners comprised of young normal-hearing adults and two groups of older adults with and without high-frequency hearing loss. Discrimination performance of all listeners was poorest for the shorter reference IOIs, and improved to stable levels for longer reference intervals exceeding about 200 ms. Temporal sensitivity of the young listeners was significantly better than that of the elderly listeners in each condition, with the largest age-related differences observed for the shortest reference interval. Age-related differences were also observed for duration DLs measured using single 4000-Hz tone bursts set to three reference durations in the range 50-200 ms. The tone DLs of all listeners were smaller than the corresponding tone-pair IOI DLs, particularly for the shorter reference stimulus durations. There were no significant performance differences observed between the older listeners with and without hearing loss for either discrimination task.     

Coding of amplitude-modulated signals in the cochlear nucleus of a grass frog

To study the mechanisms that govern the coding of temporal features of complex sound signals, responses of single neurons located in the dorsal nucleus of the medulla oblongata (the cochlear nucleus) of a curarized grass frog (Rana temporaria) to pure tone bursts and amplitude modulated tone bursts with a modulation frequency of 20 Hz and modulation depths of 10 and 80% were recorded. The carrier frequency was equal to the characteristic frequency of a neuron, the average signal level was 20–30 dB above the threshold, and the signal duration was equal to ten full modulation periods. Of the 133 neurons studied, 129 neurons responded to 80% modulated tone bursts by discharges that were phase-locked with the envelope waveform. At this modulation depth, the best phase locking was observed for neurons with the phasic type of response to tone bursts. For tonic neurons with low characteristic frequencies, along with the reproduction of the modulation, phase locking with the carrier frequency of the signal was observed. At 10% amplitude modulation, phasic neurons usually responded to only the onset of a tone burst. Almost all tonic units showed a tendency to reproduce the envelope, although the efficiency of the reproduction was low, and for half of these neurons, it was below the reliability limit. Some neurons exhibited a more efficient reproduction of the weak modulation. For almost half of the neurons, a reliable improvement was observed in the phase locking of the response during the tone burst presentation (from the first to the tenth modulation period). The cooperative histogram of a set of neurons responding to 10% modulated tone bursts within narrow ranges of frequencies and intensities retains the information on the dynamics of the envelope variation. The data are compared with the results obtained from the study of the responses to similar signals in the acoustic midbrain center of the same object and also with the psychophysical effect of a differential sensitivity increase in the process of adaptation.     

Multiple bursts, multiple looks, and stream coherence in the release from informational masking

In the simultaneous multitone masking paradigm introduced by Neff and Green [Percept. Psychophys. 41, 409-415 (1987)] the masker typically is a small number of tones having frequencies and levels that are randomly drawn on every presentation. Large amounts of masking for a pure-tone signal often occur that are thought to reflect central, rather than peripheral, limitations on processing. Previous work from this laboratory has indicated that playing a rapid succession of randomly drawn multitone maskers in each observation interval dramatically reduces the amount of masking that is observed relative to a single burst (SB). In this multiple-bursts-different (MBD) procedure, the signal tone is the only constant frequency component during the sequence of bursts and tends to perceptually segregate from the masker. In this study, the number of masker bursts and the interburst interval (IBI) were varied. The goals were to determine how the release from masking relative to the SB condition depends on the number of bursts and to examine whether increasing the IBI would cause each burst to be processed independently. If the latter were true, it might disrupt the perception of signal stream coherence, thereby diminishing the MBD advantage. However, multiple independent looks could also lead to an improvement in performance. For those subjects showing large amounts of informational masking in the SB condition, substantial reduction in masked thresholds occurred as the number of masker bursts increased, while masking increased as IBI lengthened. The results were not consistent with a simple version of a multiple-look model in which the information from each burst was combined optimally, but instead appear to be attributable to mechanisms involved in the perceptual organization of sounds.     

Stochastic incoherence in the response of rebound bursters

At an optimal value of the noise intensity, the maximum variability in rebound burst durations is observed and referred to as a response stochastic incoherence. A general mechanism underlying this phenomenon is given, being different from those reported so far in excitable systems. It is shown to be determined by (i) the monotonic reduction of the hysteresis responsible for bursting caused by noise and consequent transformation of responses from rebound bursts to single spikes, and (ii) a symmetry breaking in distributions of burst durations caused by the existence of the minimum response length. The phenomenon is studied numerically in a Morris-Lecar model for neurons and its mechanism is explained with the use of canonical models describing hard excitation states.