Mathieu Lavandier

Prediction of binaural speech intelligibility against noise in rooms

In the presence of competing speech or noise, reverberation degrades speech intelligibility not only by its direct effect on the target but also by affecting the interferer. Two experiments were designed to validate a method for predicting the loss of intelligibility associated with this latter effect. Speech reception thresholds were measured under headphones, using spatially separated target sentences and speech-shaped noise interferers simulated in virtual rooms. To investigate the effect of reverberation on the interferer unambiguously, the target was always anechoic. The interferer was placed in rooms with different sizes and absorptions, and at different distances and azimuths from the listener. The interaural coherence of the interferer did not fully predict the effect of reverberation. The azimuth separation of the sources and the coloration introduced by the room also had to be taken into account. The binaural effects were modeled by computing the binaural masking level differences in the studied configurations, the monaural effects were predicted from the excitation pattern of the noises, and speech intelligibility index weightings were applied to both. These parameters were all calculated from the room impulse responses convolved with noise. A 0.95-0.97 correlation was obtained between the speech reception thresholds and their predicted value.

Revision and validation of a binaural model for speech intelligibility in noise

Lavandier and Culling [Lavandier, M. and Culling, J. F. 2010. Prediction of binaural speech intelligibility against noise in rooms. J. Acoust. Soc. Am. 127, 387-399] demonstrated a method of predicting human speech reception thresholds for speech in combined noise and reverberation. An updated version of the model is presented, which is substantially more computationally efficient. The updated model makes similar predictions for the SRT data considered by Lavandier and Culling, which tested the models ability to predict effects of binaural unmasking and room colouration. In addition, we show here that the model accurately predicts the effects of headshadow and reproduces a range of data sets from the literature, including situations with multiple interfering sounds in anechoic conditions.

Speech segregation in rooms: Effects of reverberation on both target and interferer

Speech reception thresholds were measured to investigate the influence of a room on speech segregation between a spatially separated target and interferer. The listening tests were realized under headphones. A room simulation allowed selected positioning of the interferer and target, as well as varying the absorption coefficient of the room internal surfaces. The measurements involved target sentences and speech-shaped noise or 2-voice interferers. Four experiments revealed that speech segregation in rooms was not only dependent on the azimuth separation of sound sources, but also on their direct-to-reverberant energy ratio at the listening position. This parameter was varied for interferer and target independently. Speech intelligibility decreased as the direct-to-reverberant ratio of sources was degraded by sound reflections in the room. The influence of the direct-to-reverberant ratio of the interferer was in agreement with binaural unmasking theories, through its effect on interaural coherence. The effect on the target occurred at higher levels of reverberation and was explained by the intrinsic degradation of speech intelligibility in reverberation.

Binaural speech intelligibility in rooms with variations in spatial location of sources and modulation depth of noise interferers

Four experiments investigated the effects on speech intelligibility of reverberation, sound source locations, and amplitude modulation of the interferers. Speech reception thresholds (SRTs) were measured using headphones and stimuli that simulated real-room listening, considering one or two interferers which were stationary or speech-modulated noises. In experiment 1, SRTs for modulated noises showed little variation with increasing interferer reverberation. Reverberation might have increased masking by filling in the modulated noise gaps, but simultaneously changed the noise spectra making them less effective maskers. In experiment 2, SRTs were lower when measured using a unique one-voice modulated interferer rather than a different interferer for each target sentence, suggesting that listeners could take advantage of the predictability of the interferer gaps. In experiment 3, increasing speech reverberation did not significantly affect the difference of SRTs measured with stationary and modulated noises, indicating that the ability to exploit noise modulations was still useful for temporally smeared speech. In experiment 4, spatial unmasking remained constant when applying modulations to the interferers, suggesting an independence of the abilities to exploit these modulations and the spatial separation of sources. Finally, a model predicting binaural intelligibility for modulated noises was developed and provided a good fit to the experimental data.

Comparative measurements of loudspeakers in a listening situation

Comparison of loudspeakers is a major concern during design or product selection. There are several standards for the measurement of loudspeaker characteristics, but none of them provides hints for a rigorous comparison between devices. In this study, different ways of evaluating acoustical dissimilarity between loudspeakers were compared. Several methods of signal analysis were used, and for each method a metric evaluating the dissimilarity between two signals was defined. The correlation between the different dissimilarity evaluations over a significant panel of loudspeakers led to identified classes of measurements. A specific aspect of this work is that measurements were performed in a standard listening environment, rather than in an anechoic or reverberant one. It allowed the use of the recorded signals for a simple listening test, providing a perceptual metric which was compared to the acoustical ones. It also allowed the introduction of auditory models in the computation of some acoustical metrics, so defining a new class of measurements which gave results close to the perceptual ones.

Sequential streaming, binaural cues and lateralization

Interaural time differences (ITDs) and interaural level differences (ILDs) associated with monaural spectral differences (coloration) enable the localization of sound sources. The influence of these spatial cues as well as their relative importance on obligatory stream segregation were assessed in experiment 1. A temporal discrimination task favored by integration was used to measure obligatory stream segregation for sequences of speech-shaped noises. Binaural and monaural differences associated with different spatial positions increased discrimination thresholds, indicating that spatial cues can induce stream segregation. The results also demonstrated that ITDs and coloration were relatively more important cues compared to ILDs. Experiment 2 questioned whether sound segregation takes place at the level of acoustic cue extraction (ITD per se) or at the level of object formation (perceived azimuth). A difference in ITDs between stimuli was introduced either consistently or inconsistently across frequencies, leading to clearly lateralized sounds or blurred lateralization, respectively. Conditions with ITDs and clearly perceived azimuths induced significantly more segregation than the condition with ITDs but reduced lateralization. The results suggested that segregation was mainly based on a difference in lateralization, although the extraction of ITDs might have also helped segregation up to a ceiling magnitude.

Identification of some perceptual dimensions underlying loudspeaker dissimilarities

This study investigated the dimensions underlying perceived differences between loudspeakers. Listeners compared the sound reproduction of 12 loudspeakers in a room, using three musical excerpts. For the loudspeakers to be compared one just after the other in exactly the same conditions, the sounds radiated by the loudspeakers were recorded in a listening room, and the recorded sounds were submitted to paired comparisons using headphones. The resulting perceptual dissimilarities were analyzed by using a multidimensional scaling technique, revealing two main perceptual dimensions used by listeners to discriminate the loudspeakers. These dimensions were identical for the three musical excerpts. As the signals heard by listeners were directly accessible, they were used to define acoustical attributes describing the perceptual dimensions. Instead of arbitrarily choosing one acoustical analysis to define these attributes, several analyses were compared. The temporal, spectral, and time-frequency domains were investigated, and different auditory models were tested. These auditory models allowed the best description of the differences perceived by listeners, and were used to define two acoustical attributes describing our perceptual dimensions: the bass/treble balance and the medium emergence.

Room and head coloration can induce obligatory stream segregation

Multiple sound reflections from room materials and a listeners head induce slight spectral modifications of sounds. This coloration depends on the listener and source positions, and on the room itself. This study investigated whether coloration could help segregate competing sources. Obligatory streaming was evaluated for diotic speech-shaped noises using a rhythmic discrimination task. Thresholds for detecting anisochrony were always significantly higher when stimuli differed in spectrum. The tested differences corresponded to three spatial configurations involving different levels of head and room coloration. These results suggest that, despite the generally deleterious effects of reverberation on speech intelligibility, coloration could favor source segregation.

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 listeners 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.

Predicting Binaural Speech Intelligibility in Architectural Acoustics

A binaural model of speech understanding in background noise is presented and applied to the problem of predicting intelligibility in noisy rooms. It is shown that the model can make accurate predictions from binaural room impulse responses that are short compared to the reverberation time of the room. The model indicates (1) that there can be wide variations in intelligibility even within a fairly uniform listening space when multiple noise sources are present, (2) reverberation time is a poor predictor of intelligibility, (3) intelligibility varies as a function of the listener’s’ head orientation. The effects of room occupancy, restaurant table orientation and hearing impairment are also discussed.