Speech intelligibility prediction in reverberation: Towards an integrated model of speech transmission, spatial unmasking, and binaural de-reverberation

Room acoustic indicators of intelligibility have focused on the effects of temporal smearing of speech by reverberation and masking by diffuse ambient noise. In the presence of a discrete noise source, these indicators neglect the binaural listener's ability to separate target speech from noise. Lavandier and Culling [(2010). J. Acoust. Soc. Am. 127, 387–399] proposed a model that incorporates this ability but neglects the temporal smearing of speech, so that predictions hold for near-field targets. An extended model based on useful-to-detrimental (U/D) ratios is presented here that accounts for temporal smearing, spatial unmasking, and binaural de-reverberation in reverberant environments. The influence of the model parameters was tested by comparing the model predictions with speech reception thresholds measured in three experiments from the literature. Accurate predictions were obtained by adjusting the parameters to each room. Room-independent parameters did not lead to similar performances, suggesting that a single U/D model cannot be generalized to any room. Despite this limitation, the model framework allows to propose a unified interpretation of spatial unmasking, temporal smearing, and binaural de-reverberation. I. INTROD

Psychoacoustic measurement of phase and level for cross-talk cancellation using bilateral bone transducers: Comparison of methods

Two bone-conduction hearing aids (BCHAs) could deliver improved stereo separation using cross-talk cancellation. Sound vibrations from each BCHA would be cancelled at the contralateral cochlea by an out-of-phase signal of the same level from the ipsilateral BCHA. A method to measure the level and phase required for these cancellation signals was developed and cross-validated with an established technique that combines air- and bone-conducted sound. Three participants with normal hearing wore bone transducers (BTs) on each mastoid and insert earphones. Both BTs produced a pure tone and the level and phase were adjusted in the right BT in order to cancel all perceived sound at that ear. To cross-validate, one BT was stimulated with a pure tone and participants cancelled the resultant signal at both cochleae via adjustment of the phase and level of signals from the earphones. Participants achieved cancellation using both methods between 1.5 and 8 kHz. Levels measured with each method differed by <1 dB between 3 and 5 kHz. The phase results also corresponded well for the cancelled ear (11° mean difference) but poorly for the contralateral ear (38.4° mean difference). The first method is transferable to patients with middle-ear dysfunction, but covers a limited frequency range

Reverberation limits the release from informational masking obtained in the harmonic and binaural domains

A difference in fundamental frequency (ΔF0) and a difference in spatial location (ΔSL) are two cues known to provide masking releases when multiple speakers talk at once in a room. Situations were examined in which reverberation should have no effect on the mechanisms underlying the release from energetic masking produced by these two cues. Speech reception thresholds using both unpredictable target sentences and the coordinate response measure followed a similar pattern. Both ΔF0s and ΔSLs provided masking releases in the presence of non-speech maskers (matched in excitation pattern and temporal envelope to speech maskers) which, as intended, were robust to reverberation. Larger masking releases were obtained for speech maskers, but critically, they were affected by reverberation. The results suggest that reverberation either limits the amount of informational masking there is to begin with, or affects its release by ΔF0s or ΔSLs

Speech intelligibility in virtual restaurants

Speech reception thresholds (SRTs) for a target voice on the same virtual table were measured in various restaurant simulations under conditions of masking by between 1 and 8 interferers at other tables. Results for different levels of reverberation and different simulation techniques were qualitatively similar. SRTs increased steeply with the number of interferers, reflecting progressive failure to perceptually unmask the target speech as the acoustic scene became more complex. For a single interferer, continuous noise was the most effective masker, and a single interfering voice of either gender was least effective. With two interferers, evidence of informational masking emerged as a difference in SRT between forward and reversed speech, but SRTs for all interferer types progressively converged at 4 and 8 interferers. In simulation based on a real room, this occurred at a signal-to-noise ratio of around -5 dB

Measurements of inter-cochlear level and phase differences of bone-conducted sound

Bone-anchored hearing aids are a widely used method of treating conductive hearing loss, but the benefit of bilateral implantation is limited due to interaural cross-talk. The present study measured the phase and level of pure tones reaching each cochlea from a single, mastoid placed bone transducer on normal hearing participants. In principle, the technique could be used to implement a cross-talk cancellation system in those with bilateral bone conductors. The phase and level of probe tones over two insert earphones was adjusted until they canceled sound from a bone transducer (i.e., resulting in perceived silence). Testing was performed in 50-Hz steps between 0.25 and 8 kHz. Probe phase and level results were used to calculate inter-cochlear level and phase differences. The inter-cochlear phase differences of the bone-conducted sound were similar for all three participants showing a relatively linear increase between 4 and 8 kHz. The attenuation characteristics were highly variable over the frequency range as well as between participants. This variability was thought to be related to differences in skull dynamics across the ears. Repeated measurements of cancellation phase and level of the same frequency produced good consistency across sessions from the same participant

The factor analysis of speech: limitations and opportunities for cochlear implants

Current spread is known to limit the number of independent spectral channels in cochlear implants. The outcome of an experiment employing cochlear implant simulations indicated that current spread is not the only limitation on the benefit of increasing the number of electrodes: for both sentences and digit triplets, improvements in speech reception threshold slowed markedly once more than seven electrodes/processing channels were simulated. Factor analysis of speech envelopes from the output of an auditory filterbank confirmed that speech contains 6-8 independent sources of information, causing finer spectral filtering to produce redundant information in adjacent channels. It is possible that factor analysis can be used to refine the frequency maps used in cochlear implants in order to minimise the effects of current spread

Unilateral crosstalk cancellation in normal hearing 1 participants using bilateral bone transducers

It is possible to psychophysically measure the phase and level of bone conducted sound at the cochleae using two bone transducers (BTs) [Mcleod and Culling (2019). J. Acoust Soc. Am. 146, 3295 − 3301]. The present work uses such measurements to improve masked thresholds by using the phase and level values to create a unilateral crosstalk cancellation system. To avoid changes in the coupling of the BT to the head, testing of tone and speech reception thresholds with and without crosstalk cancellation had to be performed immediately following the measurements without adjustment of the BT. To achieve this, a faster measurement method was created. Previously measured phase and level results were interpolated to predict likely results for new test frequencies. Testing time to collect the necessary phase and level values was reduced to approximately 15 min by exploiting listeners' previous measurements. The inter-cochlear phase difference and inter-cochlear level difference were consistent between experimental sittings in the same participant but different between participants. Addition of a crosstalk cancellation signal improved tone and speech reception thresholds for tones/speech presented with one BT and noise presented on the other by an average of 12.1 dB for tones and 13.67 dB for speech

Subcomponent cues in binaural unmasking

The addition of a signal in the N0Sπ binaural configuration gives rise to fluctuations in interaural phase and amplitude. Sensitivity to these individual cues was measured by applying sinusoidal amplitude modulation (AM) or quasi-frequency modulation (QFM) to a band of noise. Discrimination between interaurally in-phase and out-of-phase modulation was measured using an adaptive task for narrow bands of noise at center frequencies from 250 to 1500 Hz, for modulation rates of 2–40 Hz, and with or without flanking bands of diotic noise. Discrimination thresholds increased steeply for QFM with increasing center frequency, but increased only modestly for AM, and mainly for modulation rates below 10 Hz. Flanking bands of noise increased thresholds for AM, but had no consistent effect for QFM. The results suggest that two underlying mechanisms may support binaural unmasking: one most sensitive to interaural amplitude modulations that is susceptible to across-frequency interference, and a second, most sensitive to interaural phase modulations that is immune to such effects

Dichotic pitches as illusions of binaural unmasking. III. The existence region of the Fourcin pitch

Two experiments explored the existence region of the Fourcin pitch. In each experiment, detectability was assessed by measuring listeners’ ability to discriminate pitch changes. In the first experiment, the detectability of the pitch was measured as a function of the number of noises used to generate it. In the second experiment, the pitch was generated using two noises with equal and opposite interaural delays and detectability was measured as a function of the difference between these two delays, and thus of the perceived pitch height. In each case, the experimental results were compared with the predictions produced by a model of binaural unmasking, based on equalization cancellation, that had been designed to recover broadband sounds, such as speech, from interfering noise [Culling and Summerfield, J. Acoust. Soc. Am. 98, 785–797 (1995)]. The model accurately predicted the results from experiment 1, but failed to show an adequate decline in performance for small differences in interaural delay (corresponding to higher perceived pitches) in experiment 2. A revised model, based on similar principles, but using data on listeners’ sensitivity to interaural decorrelation, rather than an equalization-cancellation mechanism, was able to predict the results of both experiments successfully