Binaural masking release in symmetric listening conditions with spectro-temporally modulated maskers

Binaural detection thresholds and audio quality of speech and music signals in complex acoustic environments

Every-day acoustical environments are often complex, typically comprising one attended target sound in the presence of interfering sounds (e.g., disturbing conversations) and reverberation. Here we assessed binaural detection thresholds and (supra-threshold) binaural audio quality ratings of four distortions types: spectral ripples, non-linear saturation, intensity and spatial modifications applied to speech, guitar, and noise targets in such complex acoustic environments (CAEs). The target and (up to) two masker sounds were either co-located as if contained in a common audio stream, or were spatially separated as if originating from different sound sources. The amount of reverberation was systematically varied. Masker and reverberation had a significant effect on the distortion-detection thresholds of speech signals. Quality ratings were affected by reverberation, whereas the effect of maskers depended on the distortion. The results suggest that detection thresholds and quality ratings for distorted speech in anechoic conditions are also valid for rooms with mild reverberation, but not for moderate reverberation. Furthermore, for spectral ripples, a significant relationship between the listeners' individual detection thresholds and quality ratings was found. The current results provide baseline data for detection thresholds and audio quality ratings of different distortions of a target sound in CAEs, supporting the future development of binaural auditory models.

Effects of better-ear glimpsing, binaural unmasking, and spectral resolution on spatial release from masking in cochlear-implant users

Bilateral cochlear-implant (BICI) listeners obtain less spatial release from masking (SRM; speech-recognition improvement for spatially separated vs co-located conditions) than normal-hearing (NH) listeners, especially for symmetrically placed maskers that produce similar long-term target-to-masker ratios at the two ears. Two experiments examined possible causes of this deficit, including limited better-ear glimpsing (using speech information from the more advantageous ear in each time-frequency unit), limited binaural unmasking (using interaural differences to improve signal-in-noise detection), or limited spectral resolution. Listeners had NH (presented with unprocessed or vocoded stimuli) or BICIs. Experiment 1 compared natural symmetric maskers, idealized monaural better-ear masker (IMBM) stimuli that automatically performed better-ear glimpsing, and hybrid stimuli that added worse-ear information, potentially restoring binaural cues. BICI and NH-vocoded SRM was comparable to NH-unprocessed SRM for idealized stimuli but was 14%-22% lower for symmetric stimuli, suggesting limited better-ear glimpsing ability. Hybrid stimuli improved SRM for NH-unprocessed listeners but degraded SRM for BICI and NH-vocoded listeners, suggesting they experienced across-ear interference instead of binaural unmasking. In experiment 2, increasing the number of vocoder channels did not change NH-vocoded SRM. BICI SRM deficits likely reflect a combination of across-ear interference, limited better-ear glimpsing, and poorer binaural unmasking that stems from cochlear-implant-processing limitations other than reduced spectral resolution.

Further validation of a binaural model predicting speech intelligibility against envelope-modulated noises

Collin and Lavandier [J. Acoust. Soc. Am. 134, 1146-1159 (2013)] proposed a binaural model predicting speech intelligibility against envelope-modulated noises, evaluated in 24 acoustic conditions, involving similar masker types. The aim of the present study was to test the model robustness modeling 80 additional conditions, and evaluate the influence of its parameters using an approach inspired by a variance-based sensitivity analysis. First, the data from four experiments from the literature and one specifically designed for the present study were used to evaluate the prediction performance of the model, investigate potential interactions between its parameters, and define their values leading to the best predictions. A revision of the model allowed to account for binaural sluggishness. Finally, the optimized model was tested on an additional dataset not used to define its parameters. Overall, one hundred conditions split into six experiments were modeled. Correlation between data and predictions ranged from 0.85 to 0.96 across experiments, and mean absolute prediction errors were between 0.5 and 1.4 dB.

Prediction of individual speech recognition performance in complex listening conditions

This study examined how well individual speech recognition thresholds in complex listening scenarios could be predicted by a current binaural speech intelligibility model. Model predictions were compared with experimental data measured for seven normal-hearing and 23 hearing-impaired listeners who differed widely in their degree of hearing loss, age, as well as performance in clinical speech tests. The experimental conditions included two masker types (multi-talker or two-talker maskers), and two spatial conditions (maskers co-located with the frontal target or symmetrically separated from the target). The results showed that interindividual variability could not be well predicted by a model including only individual audiograms. Predictions improved when an additional individual "proficiency factor" was derived from one of the experimental conditions or a standard speech test. Overall, the current model can predict individual performance relatively well (except in conditions high in informational masking), but the inclusion of age-related factors may lead to even further improvements.

Binaural sensitivity and release from speech-on-speech masking in listeners with and without hearing loss

Listeners with sensorineural hearing loss routinely experience less spatial release from masking (SRM) in speech mixtures than listeners with normal hearing. Hearing-impaired listeners have also been shown to have degraded temporal fine structure (TFS) sensitivity, a consequence of which is degraded access to interaural time differences (ITDs) contained in the TFS. Since these "binaural TFS" cues are critical for spatial hearing, it has been hypothesized that degraded binaural TFS sensitivity accounts for the limited SRM experienced by hearing-impaired listeners. In this study, speech stimuli were noise-vocoded using carriers that were systematically decorrelated across the left and right ears, thus simulating degraded binaural TFS sensitivity. Both (1) ITD sensitivity in quiet and (2) SRM in speech mixtures spatialized using ITDs (or binaural release from masking; BRM) were measured as a function of TFS interaural decorrelation in young normal-hearing and hearing-impaired listeners. This allowed for the examination of the relationship between ITD sensitivity and BRM over a wide range of ITD thresholds. This paper found that, for a given ITD sensitivity, hearing-impaired listeners experienced less BRM than normal-hearing listeners, suggesting that binaural TFS sensitivity can account for only a modest portion of the BRM deficit in hearing-impaired listeners. However, substantial individual variability was observed.

The effect of room acoustical parameters on speech reception thresholds and spatial release from masking

In daily life, speech intelligibility is affected by masking caused by interferers and by reverberation. For a frontal target speaker and two interfering sources symmetrically placed to either side, spatial release from masking (SRM) is observed in comparison to frontal interferers. In this case, the auditory system can make use of temporally fluctuating interaural time/phase and level differences promoting binaural unmasking (BU) and better-ear glimpsing (BEG). Reverberation affects the waveforms of the target and maskers, and the interaural differences, depending on the spatial configuration and on the room acoustical properties. In this study, the effect of room acoustics, temporal structure of the interferers, and target-masker positions on speech reception thresholds and SRM was assessed. The results were compared to an optimal better-ear glimpsing strategy to help disentangle energetic masking including effects of BU and BEG as well as informational masking (IM). In anechoic and moderate reverberant conditions, BU and BEG contributed to SRM of fluctuating speech-like maskers, while BU did not contribute in highly reverberant conditions. In highly reverberant rooms a SRM of up to 3 dB was observed for speech maskers, including effects of release from IM based on binaural cues.

Contributions of lexical tone to Mandarin sentence recognition in hearing-impaired listeners under noisy conditions

Mandarin sentence recognition using natural-tone and flat-tone sentences was tested in 22 subjects with sensorineural hearing loss (SNHL) and 25 listeners with normal hearing (NH) in quiet, speech-shaped noise, and two-talker-babble conditions. While little effects of flat tones on sentence recognition were seen in the NH listeners when the signal-to-noise ratio (SNR) was ≥0 dB, the SNHL listeners showed decreases in flat-tone-sentence recognition in quiet and at +5-dB SNR. Such declined performance was correlated with their degrees of hearing loss. Lexical tone contributes greatly to sentence recognition in hearing-impaired listeners in both quiet and in noise listening conditions.

Better-ear glimpsing with symmetrically-placed interferers in bilateral cochlear implant users

For a frontal target in spatially symmetrically placed interferers, normal hearing (NH) listeners can use "better-ear glimpsing" to select time-frequency segments with favorable signal-to-noise ratio in either ear. With an ideal monaural better-ear mask (IMBM) processing, some studies showed that NH listeners can reach similar performance as in the natural binaural listening condition, although interaural phase differences at low frequencies can further improve performance. In principle, bilateral cochlear implant (BiCI) listeners could use the same better-ear glimpsing, albeit without exploiting interaural phase differences. Speech reception thresholds of NH and BiCI listeners were measured in three interferers (speech-shaped stationary noise, nonsense speech, or single talker) either co-located with the target, symmetrically placed at ±60°, or independently presented to each ear, with and without IMBM processing. Furthermore, a bilateral noise vocoder based on the BiCI electrodogram was used in the same NH listeners. Headphone presentation and direct stimulation with head-related transfer functions for spatialization were used in NH and BiCI listeners, respectively. Compared to NH listeners, both NH listeners with vocoder and BiCI listeners showed strongly reduced binaural benefit from spatial separation. However, both groups greatly benefited from IMBM processing as part of the stimulation strategy.

Effect of audibility on better-ear glimpsing as a function of frequency in normal-hearing and hearing-impaired listeners

Better-ear glimpsing (BEG) is an auditory phenomenon that helps understanding speech in noise by utilizing interaural level differences (ILDs). The benefit provided by BEG is limited in hearing-impaired (HI) listeners by reduced audibility at high frequencies. Rana and Buchholz [(2016). J. Acoust. Soc. Am. 140(2), 1192-1205] have shown that artificially enhancing ILDs at low and mid frequencies can help HI listeners understanding speech in noise, but the achieved benefit is smaller than in normal-hearing (NH) listeners. To understand how far this difference is explained by differences in audibility, audibility was carefully controlled here in ten NH and ten HI listeners and speech reception thresholds (SRTs) in noise were measured in a spatially separated and co-located condition as a function of frequency and sensation level. Maskers were realized by noise-vocoded speech and signals were spatialized using artificially generated broadband ILDs. The spatial benefit provided by BEG and SRTs improved consistently with increasing sensation level, but was limited in the HI listeners by loudness discomfort. Further, the HI listeners performed similar to NH listeners when differences in audibility were compensated. The results help to understand the hearing aid gain that is required to maximize the spatial benefit provided by ILDs as a function of frequency.

Better-ear rating based on glimpsing

The better ear of a listener is the ear that benefits most from head shadow effects in a setting with spatially separated sources. Traditionally, the better ear is considered to be the ear that receives a signal at the best signal-to-noise ratio. For a speech target in interfering speech, the concept of rating the better ear based on glimpses was explored. The laterality of the expected better ear was shown to be well represented by metrics based on glimpsing. When employing better-ear glimpsing as a microscopic predictor for speech intelligibility, a strong relation was found between the amount of glimpsed target speech received by the better ear and the performance on a consonant recognition task. This relation was investigated for two spatial processing methods that included or excluded the possibility to use better-ear listening. It was shown that the amount of glimpses at the better ear plus an effect of angular separation of speech sources could account for a substantial part of the performance, but that a small, additional role of the contralateral ear may need to be considered.