Masker Bursts Research Articles

Temporal Effects in Simultaneous Masking by Narrow-Band Bursts

The behavior of a masking transient which indicates that masking of a short signal pulse by a longer narrow-band noise burst is stronger at the beginning of the masker burst than later was investigated. The threshold of signal pulses masked by masker bursts as a function of different variables such as bandwidth, cutoff characteristics and rise time of the masker, and delay time between onset of masker and onset of signal was measured.The results were as follows:1) There were some overshoots for the tonal signals with a frequency at the upper and lower masking slopes (4 kHz and 1 kHz) of a single narrow-band masker (centered at 2 kHz and bandwidth 1.7-2.4 kHz) and the overshoots increases up to 11 dB.2) The present data showed that overshoot was increasing sharply as rise time of masker increased from 1 msec to 14 msec and then decreased gradually and exponentially as rise time of masker increased from 10 msec to 130 msec.3) The results, especially those obtained when signal pulses were located at the upper and lower masking slopes, lead to equal overshoot above threshold.4) When the frequency separation between main masker (centered at 2 kHz, f1) and the second masking tone (2.4 kHz, f2) was small, the amplitude of the combination band would be at its greatest, and overshoot at lower masking slope (1 kHz) would be masked by the combination band.

Spectral Effect and Overshoot in Simultaneous Masking by Broad-Band Noise Bursts

The behavior of a making transient which indicates that simultaneous masking of a short signal by a longer broad-band noise burst is larger at the onset of masker burst than later was investigated. The masked threshold of signal pulses as a function of different variables such as rise-fall time of signal and masker, cut-off frequency of masker, frequency of signal, delay time between onset of masker and onset of signal, and duration of signal and masker was measured.The results were as follows:1) The overshoot depended on the masker components about three critical band away. As a consequence, the spectral effect was not responsible for most of the overshoots.2) This overshoot increased up to 12dB when masker is white noise.3) The same overshoot occured at the slopes of high-passed and low-passed noise as well as on the plateau. This means that the overshoot could not be interpreted as the development of the critical band. But the overshoot which can be measured on the center frequency of bandsuppressed noise masker increased.4) The overshoot could not show any influence of signal duration on the shape of the threshold pattern for longer duration than 20msec.5) The masked threshold rose up several decibels when the duration of the masker decreased until approximately 50msec. The same high threshold appeared even for 20msec duration. This indicated that the maximal excitation of the masker appeared almost instantaneously.

Functional characteristics of cochlear nucleus in behaving cat examined by acoustic masking of electrical stimuli.

1. Cats were trained, using an operant procedure, to detect and respond to electrical stimulation delivered in the vicinity of the cochlear nucleus. The electrical stimuli were presented both in silence and in synchrony with repeated noise bursts to determine whether detection thresholds for the electrical stimuli were elevated by the acoustic masking noise. 2. For stimulation sites centered within auditory structures (cochlear nucleus or acoustic nerve root), the acoustic maskers caused a consistent elevation of the electrical detection thresholds. For stimulation sites that were in or bordered on nonacoustic neural structures (e.g., vestibular), the acoustic maskers caused little or no elevation of electrical detection thresholds. 3. The magnitude of the acoustic masking effect was monotonically related to the intensity of the acoustic masker across the range of intensities tested. 4. The magnitude of the masking effect was strongly dependent on the relative timing of the stimulus pulse and the masker noise burst. Maximum masking occurred when the pulse just followed the onset of the neural activity in cochlear nucleus evoked by the masker burst. Less masking occurred when the electrical pulse occurred at the middle or end of the masker burst, and still less when the pulse occurred just prior to the onset (backward masking) or just after the offset (forward masking) of the masker burst. 5. The magnitude of the masking effect also depended on the frequency of the acoustic masker. For tone bursts, masking was maximal for each electrode at a particular frequency and declined monotonically for masker frequencies above or below the optimal frequency. 6. It is concluded that the masking of an electrical stimulus by an acoustic stimulus depends on a direct interaction between the neural responses evoked by the two stimuli, and that similar central, neural interactions may contribute to acoustic masking of acoustic stimuli. It is also concluded that the technique of masking an electrical stimulus by an acoustical stimulus is a precise and useful tool for the study of sensory-neural organization in intact behaving animals.

Psychophysical evidence for lateral inhibition in hearing.

Although there are some indications of the possible role of lateral inhibition in hearing, there has been no clear demonstration of it in psychophysical experiments. Either the phenomenon plays only a minor role, or it has escaped psychophysical verification. Accepting for a moment the second possibility, is argued in this paper that the threshold of a test tone presented simultaneously with a masker does not reflect clear lateral-inhibition effects since the inhibition affects both the test tone and the masker. Two different methods, in which the test tone and the masker were presented succsesively, give clear psychophysical evidence of lateral inhibition in hearing. Firstly, the threshold curve of short test-tone bursts presented in the gaps between repeated masker bursts (noise with a steep negative or positive gradient at a particular frequency) shows marked edge effects. Secondly, the results of psychophysical measurements on two-tone suppression indicate that the nervous activity due to one frequency component may be suppressed by another component. The effect at the edges of the frequency spectrum are comparable with visual Mach bands, and the interaction of two tones is suggestive of the two-tone inhibition found in auditory-nerve fibers.

TEMPORAL EFFECTS IN SIMULTANEOUS MASKING AND LOUDNESS.

The intention of the research described was twofold. One purpose was to investigate the difference between threshold curves for tones masked by bands of noise and the corresponding displacement curves obtained from models of the basilar membrane. The second object of investigation was the behavior of a strange masking transient which indicates that masking of a short signal pulse by a longer masker burst is stronger at the beginning of the masker burst than later under certain circumstances, but that under other conditions masking remains constant during the total duration of the masker. The threshold of signal pulses masked by masker bursts was measured as a function of different variables such as bandwidth and center frequency of masker, delay between onset of masker and onset of signal, duration of signal, and spectrum shape of masker. The results, especially those obtained when white noise with a frequency gap was the masker, lead to the hypothesis that the masking excitation appears almost instantaneously. This hypothesis was tested and accepted on the basis of loudness judgments comparing equally long tone pulses and white-noise pulses of very short and long duration. The conclusion that excitation occurs almost instantaneously, taken together with the known relationship between the masking effect of white noise and that of narrow-band noise, is construed to mean that a complex spatial inhibition in the nervous system may not be responsible for the narrow sensitivity curve; in fact, a sharp mechanical filter could account for the findings.

Temporal Effects in Simultaneous Masking by White-Noise Bursts

The motivation of the research described was to investigate the behavior of a masking transient that indicates that masking of a short signal pulse by a longer white-noise burst is stronger at the beginning of the masker burst than later. The threshold of signal pulses masked by masker bursts was measured as a function of different variables such as bandwidth and center frequency of the signal, delay between onset of masker and onset of signal, duration of signal and masker, level of masker, and repetition rate. The results reveal very little “overshoot” of the threshold of short pulses as a function of the ON time of the masker if the signal and the masker have the same or similar broad spectra. The overshoot increases up to 13 dB as the bandwidth of the signal decreases down to that of a tone. The size of the “overshoot” and the prior excitation seem to be related to each other. Taking this in account, the thresholds under different conditions can be calculated on the basis of detection models. The measured and the calculated values are in good agreement.