Sophie Savel

Additivity of nonsimultaneous masking for short Gaussian-shaped sinusoidsa)

The additivity of nonsimultaneous masking was studied using Gaussian-shaped tone pulses (referred to as Gaussians) as masker and target stimuli. Combinations of up to four temporally separated Gaussian maskers with an equivalent rectangular bandwidth of 600 Hz and an equivalent rectangular duration of 1.7 ms were tested. Each masker was level-adjusted to produce approximately 8 dB of masking. Excess masking (exceeding linear additivity) was generally stronger than reported in the literature for longer maskers and comparable target levels. A model incorporating a compressive input/output function, followed by a linear summation stage, underestimated excess masking when using an input/output function derived from literature data for longer maskers and comparable target levels. The data could be predicted with a more compressive input/output function. Stronger compression may be explained by assuming that the Gaussian stimuli were too short to evoke the medial olivocochlear reflex (MOCR), whereas for longer maskers tested previously the MOCR caused reduced compression. Overall, the interpretation of the data suggests strong basilar membrane compression for very short stimuli.

Temporal effects in simultaneous masking with on- and off-frequency noise maskers: Effects of signal frequency and masker level

Temporal effects in simultaneous masking were measured as a function of masker level for an on-frequency broadband masker and an off-frequency narrow-band masker for signal frequencies of 750, 1730, and 4000 Hz. The on-frequency masker was 10 equivalent rectangular bandwidths (ERBs) wide and centered at the signal frequency; the off-frequency masker was 500 Hz wide and its lower frequency edge was 1.038 ERBs higher in frequency than the signal. The primary goal of the study was to determine whether previously observed differences regarding the effects of signal frequency and masker level on the temporal effect for these two different types of masker might be due to considerably different signal levels at threshold. Despite similar masked thresholds, the effects of signal frequency and masker level in the present study were different for the two masker types. The temporal effect was significant for the two highest frequencies and absent for the lowest frequency in the presence of the broadband masker, but was more or less independent of frequency for the narrow-band masker. The temporal effect increased but then decreased as a function of level for the broadband masker (at the two higher signal frequencies, where there was a temporal effect), but increased and reached an asymptote for the narrow-band masker. Despite the different effects of signal frequency and masker level, the temporal effects for both types of masker can be understood in terms of a basilar-membrane input-output function that becomes more linear during the course of masker stimulation.

Human Auditory Localisation in a Distorted Environment: Water

This study is concerned with sound localisation abilities in water, where the cues for localisation are theoretically severely altered. The main goal of the study was to understand the discrepancy between the poor localisation abilities predicted by the degree of cues alteration and the relatively good performance empirically measured in past investigations. Nine experienced divers performed 8-alternative forced-choice identification tests in open water, with auditory targets presented both in the frontal and rear parts of the horizontal plane. Targets were trains of 300-ms bursts with 25-ms rise/decay times. Bursts were either pure tones (0.4, 1, 6 or 8 kHz) or white noise. With 0.4-kHz tones, the subjects exhibited very accurate left-right distinction and acceptable azimuth resolution. Four subjects made frequent front-back reversals. Also, a bias of the responses towards the median axis was observed in 24% of the data. Six subjects tested with head motion either restricted or free all showed a clear drop of front-back reversals due to head motion. Three subjects tested with all target frequencies exhibited correct localisation accuracy with the white noise, again associated with frequent front-back reversals for two of them. With the 1-, 6- and 8-kHz tones, however, results varied more between individuals. The same three subjects were tested with both the head-conduction and the ear-conduction pathways available, with the head-conduction path blocked off by the wear of a neoprene hood, and with the ear-conduction path blocked off by the wear of neoprene earplugs, in random order. They showed a strong degradation due to the covering of the ear-conduction pathway.

Simultaneous masking additivity for short Gaussian-shaped tones: Spectral effectsa)

Laback et al. [(2011). J. Acoust. Soc. Am. 129, 888-897] investigated the additivity of nonsimultaneous masking using short Gaussian-shaped tones as maskers and target. The present study involved Gaussian stimuli to measure the additivity of simultaneous masking for combinations of up to four spectrally separated maskers. According to most basilar membrane measurements, the maskers should be processed linearly at the characteristic frequency (CF) of the target. Assuming also compression of the target, all masker combinations should produce excess masking (exceeding linear additivity). The results for a pair of maskers flanking the target indeed showed excess masking. The amount of excess masking could be predicted by a model assuming summation of masker-evoked excitations in intensity units at the target CF and compression of the target, using compressive input/output functions derived from the nonsimultaneous masking study. However, the combinations of lower-frequency maskers showed much less excess masking than predicted by the model. This cannot easily be attributed to factors like off-frequency listening, combination tone perception, or between-masker suppression. It was better predicted, however, by assuming weighted intensity summation of masker excitations. The optimum weights for the lower-frequency maskers were smaller than one, consistent with partial masker compression as indicated by recent psychoacoustic data.

Auditory time-frequency masking: psychoacoustical data and application to audio representations

In this paper, the results of psychoacoustical experiments on auditory time-frequency (TF) masking using stimuli (masker and target) with maximal concentration in the TF plane are presented. The target was shifted either along the time axis, the frequency axis, or both relative to the masker. The results show that a simple superposition of spectral and temporal masking functions does not provide an accurate representation of the measured TF masking function. This confirms the inaccuracy of simple models of TF masking currently implemented in some perceptual audio codecs. In the context of audio signal processing, the present results constitute a crucial basis for the prediction of auditory masking in the TF representations of sounds. An algorithm that removes the inaudible components in the wavelet transform of a sound while causing no audible difference to the original sound after re-synthesis is proposed. Preliminary results are promising, although further development is required.

Additivity of auditory masking using Gaussian‐shaped tones

Both temporal and spectral masking have been studied extensively in the literature. Mostly, they have been regarded as separate phenomena. Very little is known about the interaction between these two effects, i.e. masking in the time‐frequency domain. Data on the time‐frequency spread of masking evoked by a single Gaussian‐shaped tone pulse are presented in an accompanying study at the same conference (Necciari et al.). The current study gathers data on the additivity of masking by up to four, approximately equally effective Gaussian maskers (ERB=600 Hz), separated either along the time or the frequency axis. For temporal separation, the amount of masking increases with the number of maskers, with excess masking (exceeding linear additivity) of up to 25 dB. For frequency separation (preliminary data) excess masking amounts up to 15 dB, and the higher‐frequency masker (relative to the target) contributes more to the additivity than the lower‐frequency maskers. Experiments with multiple maskers combining bo...

Effect of contralateral precursor type on the temporal effect in simultaneous masking with tone and noise maskers (L)

A sound (contralateral precursor) presented to the nontest ear prior to the onset of a masker and probe has been shown to reduce the temporal effect in simultaneous masking with noise maskers but not with tonal maskers. The present study examined this further. The probe was a 4.0-kHz tone. In experiment 1a, the masker was a 4.4-kHz tone and the precursor was a 4.4-kHz tone or an unmodulated (UM) or amplitude-modulated (AM) band of noise (4.4–8.0 kHz). In experiment 1b, the masker was a broadband noise and the precursor was a UM or an AM broadband noise. In both experiments the precursor consistently reduced the temporal effect for only one of the seven or eight subjects, regardless of precursor type. These largely negative results indicate that it may not be possible to use contralateral precursors to gain much insight into the mechanisms underlying temporal effects in simultaneous masking.

Auditory Azimuthal Localization Performance in Water as a Function of Prior Exposure

Objective: We report two psychoacoustical experiments that assessed the relationship between auditory azimuthal localization performance in water and duration of prior exposure to the milieu. Background: The adaptability of spatial hearing abilities has been demonstrated in air for both active and passive exposures to altered localization cues. Adaptability occurred faster and was more complete for elevation perception than for azimuth perception. In water, spatial hearing is believed to solely rely on smaller than normal cues-to-azimuth: interaural time differences. This should produce a medial bias in localization judgments toward the center of the horizontal plane, unless the listeners have adapted to the environment. Method: Azimuthal localization performance was measured in seawater for eight azimuthal directions of a low-frequency (<500 Hz) auditory target. Seventeen participants performed a forced-choice task in Experiment 1. Twenty-eight other participants performed a pointing task in Experiment 2. Results: In both experiments we observed poor front/back discrimination but accurate left/right discrimination, regardless of prior exposure. A medial bias was found in azimuth perception, whose size decreased as the exposure duration of the participant increased. Conclusion: The study resembles earlier results showing that passive exposure to altered azimuth cues elicits the adaptability of internal audio-spatial maps, that is, the behavioral plasticity of spatial hearing abilities. Application: Studies of the adaptability of the auditory system to altered spatial information may yield practical implications for scuba divers, hearing-impaired listeners with reduced sensitivity to spatial cues, and various normal-hearing users of virtual auditory displays.

Auditory masking using Gaussian‐windowed stimuli

This study investigates auditory masking with Gaussian‐windowed tones as target and masker stimuli. On the purpose of developing a time‐frequency masking model, such stimuli minimize the time‐frequency uncertainty. Also, as proposed by van Schijndel et al. (1999), they activate a single spectro‐temporal observation window of the auditory system. The study presented here measured auditory masking with Gaussian‐windowed stimuli with an ERB of 600 Hz and an effective duration of 9.6 ms. The masker was centered at 4 kHz. Its level was 60 dB SL. Four experiments were conducted. (1) Absolute thresholds for Gaussian‐windowed and 300‐ms‐sinusoidal targets were measured and compared for 11 frequencies. (2) Masking patterns were obtained with targets of various frequency separations from the masker. (3) Forward masking functions with 4‐kHz targets were measured at 5 temporal separations. (4) Forward masking was measured for different frequency separations between masker and target. These data are compared with thos...

Auditory Time-Frequency Masking for Spectrally and Temporally Maximally-Compact Stimuli.

Many audio applications perform perception-based time-frequency (TF) analysis by decomposing sounds into a set of functions with good TF localization (i.e. with a small essential support in the TF domain) using TF transforms and applying psychoacoustic models of auditory masking to the transform coefficients. To accurately predict masking interactions between coefficients, the TF properties of the model should match those of the transform. This involves having masking data for stimuli with good TF localization. However, little is known about TF masking for mathematically well-localized signals. Most existing masking studies used stimuli that are broad in time and/or frequency and few studies involved TF conditions. Consequently, the present study had two goals. The first was to collect TF masking data for well-localized stimuli in humans. Masker and target were 10-ms Gaussian-shaped sinusoids with a bandwidth of approximately one critical band. The overall pattern of results is qualitatively similar to existing data for long maskers. To facilitate implementation in audio processing algorithms, a dataset provides the measured TF masking function. The second goal was to assess the potential effect of auditory efferents on TF masking using a modeling approach. The temporal window model of masking was used to predict present and existing data in two configurations: (1) with standard model parameters (i.e. without efferents), (2) with cochlear gain reduction to simulate the activation of efferents. The ability of the model to predict the present data was quite good with the standard configuration but highly degraded with gain reduction. Conversely, the ability of the model to predict existing data for long maskers was better with than without gain reduction. Overall, the model predictions suggest that TF masking can be affected by efferent (or other) effects that reduce cochlear gain. Such effects were avoided in the experiment of this study by using maximally-compact stimuli.