Asymmetry of masking between complex tones and noise: partial loudness

In-channel cancellation: A model of early auditory processing

A model of early auditory processing is proposed in which each peripheral channel is processed by a delay-and-subtract cancellation filter, tuned independently for each channel with a criterion of minimum power. For a channel dominated by a pure tone or a resolved partial of a complex tone, the optimal delay is its period. For a channel responding to harmonically related partials, the optimal delay is their common fundamental period. Each peripheral channel is thus split into two subchannels-one that is cancellation-filtered and the other that is not. Perception can involve either or both, depending on the task. The model is illustrated by applying it to the masking asymmetry between pure tones and narrowband noise: a noise target masked by a tone is more easily detectable than a tone target masked by noise. The model is one of a wider class of models, monaural or binaural, that cancel irrelevant stimulus dimensions to attain invariance to competing sources. Similar to occlusion in the visual domain, cancellation yields sensory evidence that is incomplete, thus requiring Bayesian inference of an internal model of the world along the lines of Helmholtz's doctrine of unconscious inference.

Noise thresholds in harmonic series maskers

The presence of noise is a salient cue to the perception of breathiness and aspiration in speech sounds. The detection of noise within harmonic series (maskers) composed of unresolved components was found to depend on the fundamental frequency (f_o) and the overall level of the masker [Gockel, Moore, and Patterson (2002). J. Acoust. Soc. Am., 111 (6), 2759-2770]. In the present study, noise detection thresholds were measured as a function of the frequency range, the f_o, and the overall level of harmonic maskers. Frequency range was specified in equivalent rectangular bandwidth (ERB) units (3-13, 13-23, 23-33, or 3-33 ERBs). The results were consistent with the idea that listeners rely on spectral cues when maskers comprise only resolved components (3-13 ERBs), and on temporal (dip listening) cues when maskers contain only unresolved components (23-33 ERBs). Noise detection thresholds were generally lower when masker level was high (70 dBA) than when it was low (50 dBA). Masker f_o affected thresholds only when listeners relied on spectral cues for noise detection. With the wideband (3-33 ERBs) masker, listeners likely detected noise by focusing on the frequency band (23-33 ERBs) with the most advantageous noise-to-harmonic ratio.

Harmonic Cancellation-A Fundamental of Auditory Scene Analysis

This paper reviews the hypothesis of harmonic cancellation according to which an interfering sound is suppressed or canceled on the basis of its harmonicity (or periodicity in the time domain) for the purpose of Auditory Scene Analysis. It defines the concept, discusses theoretical arguments in its favor, and reviews experimental results that support it, or not. If correct, the hypothesis may draw on time-domain processing of temporally accurate neural representations within the brainstem, as required also by the classic equalization-cancellation model of binaural unmasking. The hypothesis predicts that a target sound corrupted by interference will be easier to hear if the interference is harmonic than inharmonic, all else being equal. This prediction is borne out in a number of behavioral studies, but not all. The paper reviews those results, with the aim to understand the inconsistencies and come up with a reliable conclusion for, or against, the hypothesis of harmonic cancellation within the auditory system.

Partial loudness at masker onset indicates temporal effects at supra-threshold levels

This study examines temporal effects both at threshold and at supra-threshold levels. The level needed to detect a short-duration 4.0-kHz signal was measured for signals presented with different onset delays relative to a 300-ms broadband noise masker: 100 ms and 5 ms before the onset of the masker and 5 ms and 100 ms after the onset of the masker. Loudness matches between the signal in quiet and the signal at the same four onset delays were obtained for five presentation levels of the short-duration signal and for three masker levels. The temporal effect was defined as the level difference between the signals near masker onset and the signals well before or well after masker onset, needed to reach threshold and/or achieve equal loudness. Both at threshold and at supra-threshold levels temporal effects were observed consistent with a decrease in gain at the masker frequency during the course of the masker. The temporal effect was not restricted to simultaneous masking, but was also found for backward masking. In both cases the temporal effects were stronger at supra-threshold levels than at threshold. This may be caused by a transient effect at masker onset. The almost simultaneous onset of the signal and the masker makes it difficult for subjects to separate signal from the masker, especially when the signal level is close to masked threshold.

Loudness perception of low tones undergoing partial masking by higher tones in orchestral music in concert halls

Objective acoustical parameters for halls are often measured in 1-octave bands with mid-frequencies from 125 to 4000 Hz. In reality, the frequency range of musical instruments is much wider than that, and the fundamentals of the lower notes of bass instruments are contained in 31.5 or 63 Hz bands. Overtones of fundamentals in these bands fall in 125 Hz band. This report presents subjective experiments designed to determine to what extent the overtones in 125 Hz band and higher bands influence the loudness sensation of the components in 63 Hz band. In the experiments, the 125 Hz and higher components of the musical tone are used to act as a masker against the lower component used as a maskee. The threshold of the difference between G(125 Hz) and G(lower band) that just enables one to hear the fundamental tones in the lower band is determined. Masked loudness of 63 Hz sinusoidal tone caused by partial masking noise with higher frequencies was determined based on a similar procedure to the masked loudness-matching function. The result indicates that the difference in loudness of low tone will not be noticeable even if G changed by ±2.5 to ±3 dB, at least when there are other accompanying instruments.

Perceptual interaction of the harmonic source and noise in voice

Although the amount of inharmonic energy (noise) present in a human voice is an important determinant of vocal quality, little is known about the perceptual interaction between harmonic and inharmonic aspects of the voice source. This paper reports three experiments investigating this issue. Results indicate that perception of the harmonic slope and of noise levels are both influenced by complex interactions between the spectral shape and relative levels of harmonic and noise energy in the voice source. Just-noticeable differences (JNDs) for the noise-to-harmonics ratio (NHR) varied significantly with the NHR and harmonic spectral slope, but NHR had no effect on JNDs for NHR when harmonic slopes were steepest, and harmonic slope had no effect when NHRs were highest. Perception of changes in the harmonic source slope depended on NHR and on the harmonic source slope: JNDs increased when spectra rolled off steeply, with this effect in turn depending on NHR. Finally, all effects were modulated by the shape of the noise spectrum. It thus appears that, beyond masking, understanding perception of individual parameters requires knowledge of the acoustic context in which they function, consistent with the view that voices are integral patterns that resist decomposition.

Representation of auditory-filter phase characteristics in the cortex of human listeners

Harmonic tone complexes with component phases, adjusted using a variant of a method proposed by Schroeder, can produce pure-tone masked thresholds differing by >20 dB. This phenomenon has been qualitatively explained by the phase characteristics of the auditory filters on the basilar membrane, which differently affect the flat envelopes of the Schroeder-phase maskers. We examined the influence of auditory-filter phase characteristics on the neural representation in the auditory cortex by investigating cortical auditory evoked fields (AEFs). We found that the P1m component exhibited larger amplitudes when a long-duration tone was presented in a repeating linearly downward sweeping (Schroeder positive, or m(+)) masker than in a repeating linearly upward sweeping (Schroeder negative, or m(-)) masker. We also examined the neural representation of short-duration tone pulses presented at different temporal positions within a single period of three maskers differing in their component phases (m(+), m(-), and sine phase m(0)). The P1m amplitude varied with the position of the tone pulse in the masker and depended strongly on the masker waveform. The neuromagnetic results in all cases were consistent with the perceptual data obtained with the same stimuli and with results from simulations of neural activity at the output of cochlear preprocessing. These findings demonstrate that phase effects in peripheral auditory processing are accurately reflected up to the level of the auditory cortex.

Contributions of individual components to the overall loudness of a multitone complex

The contributions of individual components to the overall loudness of a multitone complex were examined in a two-interval, loudness judgment task. Stimuli were five-tone complexes centered on 1000 Hz, with six different logarithmic frequency spacings, corresponding to bandwidths from 46 to 2119 Hz. Stimuli were presented for 300 ms (10 ms rise/fall). The overall level of the standard complex was fixed at 60 dB SPL (53 dB/component). Levels of the individual components of the comparison complex were selected at random from a rectangular distribution with a range of 20 dB. Perceptual weights were computed by calculating the point-biserial correlation between the difference in the level of each component across the two intervals and the subject's response. When all components fell within the same critical band, weights varied little across components. In contrast, the range of weights increased with increasing frequency separation, with more weight given to the lowest and highest frequency components. A relation between component weight and masked threshold was observed for all but the widest spacing condition. Perceptual weights and masked thresholds were largely in agreement with the loudness model [Moore et al., J. Audio. Eng. Soc. 45, 224-237 (1997)], except at the widest bandwidth.

Detection and F0 discrimination of harmonic complex tones in the presence of competing tones or noise

Normal-hearing listeners' ability to "hear out" the pitch of a target harmonic complex tone (HCT) was tested with simultaneous HCT or noise maskers, all bandpass-filtered into the same spectral region (1200-3600 Hz). Target-to-masker ratios (TMRs) necessary to discriminate fixed fundamental-frequency (F0) differences were measured for target F0s between 100 and 400 Hz. At high F0s (400 Hz), asynchronous gating of masker and signal, presenting the masker in a different F0 range, and reducing the F0 rove of the masker, all resulted in improved performance. At the low F0s (100 Hz), none of these manipulations improved performance significantly. The findings are generally consistent with the idea that the ability to segregate sounds based on cues such as F0 differences and onset/offset asynchronies can be strongly limited by peripheral harmonic resolvability. However, some cases were observed where perceptual segregation appeared possible, even when no peripherally resolved harmonics were present in the mixture of target and masker. A final experiment, comparing TMRs necessary for detection and F0 discrimination, showed that F0 discrimination of the target was possible with noise maskers at only a few decibels above detection threshold, whereas similar performance with HCT maskers was only possible 15-25 dB above detection threshold.

Perceptual interaction between carrier periodicity and amplitude modulation in broadband stimuli: a comparison of the autocorrelation and modulation-filterbank model

Recent temporal models of pitch and amplitude modulation perception converge on a relatively realistic implementation of cochlear processing followed by a temporal analysis of periodicity. However, for modulation perception, a modulation filterbank is applied whereas for pitch perception, autocorrelation is applied. Considering the large overlap in pitch and modulation perception, this is not parsimonious. Two experiments are presented to investigate the interaction between carrier periodicity, which produces strong pitch sensations, and envelope periodicity using broadband stimuli. Results show that in the presence of carrier periodicity, detection of amplitude modulation is impaired throughout the tested range (8-1000 Hz). On the contrary, detection of carrier periodicity in the presence of an additional amplitude modulation is impaired only for very low frequencies below the pitch range (<33 Hz). Predictions of a generic implementation of a modulation-filterbank model and an autocorrelation model are compared to the data. Both models were too insensitive to high-frequency envelope or carrier periodicity and to infra-pitch carrier periodicity. Additionally, both models simulated modulation detection quite well but underestimated the detrimental effect of carrier periodicity on modulation detection. It is suggested that a hybrid model consisting of bandpass envelope filters with a ripple in their passband may provide a functionally successful and physiologically plausible basis for a unified model of auditory periodicity extraction.

Effects of masker component phase on the forward masking produced by complex tones in normally hearing and hearing-impaired subjects

For normally hearing subjects, harmonic complex tones that give "peaky" waveforms on the basilar membrane (Schroeder-positive phase, sine phase or cosine phase) lead to less forward masking than complex tones that give less peaky waveforms (Schroeder-negative phase or random phase), but have the same power spectrum. This difference has been attributed mainly to the combined effects of peripheral compression and suppression, both of which depend on the operation of the active mechanism in the cochlea. If this explanation is correct, the phase effect should be reduced or absent for subjects with moderate cochlear hearing loss. We measured growth-of-masking functions for forward maskers containing the first 40 harmonics of a 100-Hz fundamental, with components added either in cosine phase or random phase, using both normally hearing subjects and subjects with moderate cochlear hearing loss. The signal frequency was 1 or 2 kHz. For the normally hearing subjects, the mean slopes of the growth-of-masking functions at 1 and 2 kHz, respectively, were 0.53 and 0.44 for the random-phase masker and 0.31 and 0.26 for the cosine-phase masker. For high masker levels, the former produced considerably more masking than the latter. The phase effect was smaller for the hearing-impaired than for the normally hearing subjects, which is consistent with the idea that it is partly caused by peripheral compression and suppression. However, three of the five hearing-impaired subjects showed a significant effect of masker phase for at least one signal frequency. In one case, this occurred when the hearing loss at the signal frequency was 65 dB. The slopes of the growth-of-masking functions were consistently less than one for the hearing-impaired subjects. Further testing suggested that the efferent system was not involved in producing the phase effect.