Speech Intelligibility In Rooms Research Articles

Relation between sound absorption and speech intelligibility in room, part 1: Theoretical investigation

In Japan, requirements and design guidelines on sound absorption of a variety of ordinary spaces are currently under consideration. In general, sound absorption works for reverberation suppression and noise reduction, both of which contributes to improve speech intelligibility performance. However, its combined effect is not fully clarified, especially on the relationship among of reverberation time, average absorption coefficient and speech intelligibility indices. In this paper, two theoretical case studies are performed based on the formulations of the clarity C50 and the useful-to-detrimental ratio U50 in the conditions without and with background noise. For rectangular rooms with an aspect ratio of 1:2:4, those indices are calculated at three source-to-receiver distances, changing the room volume widely under different noise levels. The results demonstrated that the absorption coefficient required for a target C50 increases with increasing the room volume, but it decreases beyond a certain volume. Moreover, it was shown that under high background noise, U50 is not effectively improved by increasing the absorption coefficient if the communication distance is much longer than the critical distance.

Relation between sound absorption and speech intelligibility in room, part 2: Case study based on geometric simulation

Sound absorption works for reverberation suppression and noise reduction, both of which contribute to improving speech transmission performance. In the previous paper, its combined effect was theoretically investigated to clarify the relationship among of reverberation time, average absorption coefficient and speech intelligibility indices. In this paper, a case study is first conducted to examine the applicability of the theoretical estimation of clarity C50 based on geometric simulation. In rectangular rooms with different volumes, aspect ratios and absorption distributions, impulse responses are simulated for various source-to-receiver positions, and then the distributions of the simulated C50 are compared with the theoretical values. Furthermore, a subjective experiment on speech intelligibility is conducted by using a 6-channel sound field reproduction system, where a speech signal is convolved with the simulated impulse responses under background noise conditions. The results show the correlation between the listening difficulty rating and the estimated useful-to-detrimental ratio U50.

Objective and subjective assessment of the speech intelligibility in rooms with an Active Acoustic Enhancement Systems

Speech Transmission Index (STI), apart from the Reverberation Time (RT), remains an important parameter used for the speech intelligibility assessment indoors. As the Active Acoustic Enhancement Systems (AAES) are getting more and more commonly used, it was verified how such systems affect speech perception. In accordance with the 3382-1 and 3382-2 regulations, objective parameters were measured in two enclosures with the AAES. The RT values varied from 0.91 to 1.61 [s] for the hall in Mińsk Mazowiecki and from 0.75 to 1.52 [s] in Puławy. The STI values varied from 0.57 to 0.64 in Mińsk Mazowiecki and 0.64 to 0.69 in Puławy. Despite the significant differences in the RT values, STI coefficients remained almost unchanged. The listening test was designed to verify if the subjectively assessed intelligibility would confirm the objective measurements' results. 50-element nonsense words (logatome) lists were used for in-situ experiments and, in the second part, binaural laboratory arrangement. Both parts involved over 120 listeners. Three scenes were selected: without AAES and two different AAES setups. In summary, subjective measurements' results were compared with the obtained STI, RT and Clarity (C50, C80) values to prove their importance in indoors speech intelligibility evaluation.

Comparing the effects of scattered and specular sound reflections on speech intelligibility in rooms

Speech intelligibility in rooms is assumed to depend on the amount of reverberation, and on the distribution of sound energy in the impulse response between early and later reflections, the former being considered beneficial, the latter detrimental. This assumption is based on the analysis of a single channel, which is monaural. When the binaural capacities of the auditory system are considered, other phenomena come into play, supporting speech recognition mainly by comparing signal levels and times of arrival at the ears. Starting from these basic mechanisms, the present work shows that, for given monaural conditions, binaural cues are influenced by the type of sound reflections in a room via the correlations they produce on the signals at the ears. A fully scattering scenario, and a totally flat boundary scenario were used to investigate listeners’ performance in a speech intelligibility task in a virtual room with spatialized noise conditions. The amount of correlation at the ears of the listener of the signals coming from both the source and the masker were found to affect speech intelligibility - the stronger either correlation, the greater the speech intelligibility - and their joint effect was larger than the two effects taken separately. A specular setting for the sound reflections was associated with both better reception thresholds and a better usage of spatial cues when deciphering speech. The implications for the acoustic design of rooms with a view to facilitating speech intelligibility are discussed.

Measurement and Prediction of Binaural-Temporal Integration of Speech Reflections.

For speech intelligibility in rooms, the temporal integration of speech reflections is typically modeled by separating the room impulse response (RIR) into an early (assumed beneficial for speech intelligibility) and a late part (assumed detrimental). This concept was challenged in this study by employing binaural RIRs with systematically varied interaural phase differences (IPDs) and amplitude of the direct sound and a variable number of reflections delayed by up to 200 ms. Speech recognition thresholds in stationary noise were measured in normal-hearing listeners for 86 conditions. The data showed that direct sound and one or several early speech reflections could be perfectly integrated when they had the same IPD. Early reflections with the same IPD as the noise (but not as the direct sound) could not be perfectly integrated with the direct sound. All conditions in which the dominant speech information was within the early RIR components could be well predicted by a binaural speech intelligibility model using classic early/late separation. In contrast, when amplitude or IPD favored late RIR components, listeners appeared to be capable of focusing on these components rather than on the precedent direct sound. This could not be modeled by an early/late separation window but required a temporal integration window that can be flexibly shifted along the RIR.

Speech intelligibility in rooms: Disrupting the effect of prior listening exposure.

It has been demonstrated that prior listening exposure to reverberant environments can improve speech understanding in that environment. Previous studies have shown that the buildup of this effect is brief (less than 1 s) and seems largely to be elicited by exposure to the temporal modulation characteristics of the room environment. Situations that might be expected to cause a disruption in this process have yet to be demonstrated. This study seeks to address this issue by showing what types of changes in the acoustic environment cause a breakdown of the room exposure phenomenon. Using speech carrier phrases featuring sudden changes in the acoustic environment, breakdown in the room exposure effect was observed when there was change in the late reverberation characteristics of the room that signaled a different room environment. Changes in patterns of early reflections within the same room environment did not elicit breakdown. Because the environmental situations that resulted in breakdown also resulted in substantial changes to the broadband temporal modulation characteristic of the signal reaching the ears, results from this study provide additional support for the hypothesis that the room exposure phenomenon is linked to the temporal modulation characteristics of the environment.

The effect of hearing loss on source-distance dependent speech intelligibility in rooms.

Westermann and Buchholz [(2015). J. Acoust. Soc. Am. 137(2), 757-767] found substantial improvements in speech reception thresholds (SRTs) for normal hearing listeners in a reverberant auditorium when the target talker was separated in distance from a two-talker masker. This study applied similar methodology, but tested listeners with a hearing impairment. On average, the participants received a 7 dB benefit in SRTs when the target was fixed at 0.5 m and the masker was moved from 0.5 to 10 m. But when the target was moved away, the SRTs increased by 5 dB. This indicates that hearing impaired listeners have difficulties suppressing nearby maskers while focusing attention on a far target.

Speech intelligibility in rooms: Effect of prior listening exposure interacts with room acoustics.

There is now converging evidence that a brief period of prior listening exposure to a reverberant room can influence speech understanding in that environment. Although the effect appears to depend critically on the amplitude modulation characteristic of the speech signal reaching the ear, the extent to which the effect may be influenced by room acoustics has not been thoroughly evaluated. This study seeks to fill this gap in knowledge by testing the effect of prior listening exposure or listening context on speech understanding in five different simulated sound fields, ranging from anechoic space to a room with broadband reverberation time (T60) of approximately 3 s. Although substantial individual variability in the effect was observed and quantified, the context effect was, on average, strongly room dependent. At threshold, the effect was minimal in anechoic space, increased to a maximum of 3 dB on average in moderate reverberation (T60 = 1 s), and returned to minimal levels again in high reverberation. This interaction suggests that the functional effects of prior listening exposure may be limited to sound fields with moderate reverberation (0.4 ≤ T60 ≤ 1 s).

Evaluating a distortion-weighted glimpsing metric for predicting binaural speech intelligibility in rooms

A distortion-weighted glimpse proportion metric (BiDWGP) for predicting binaural speech intelligibility were evaluated in simulated anechoic and reverberant conditions, with and without a noise masker. The predictive performance of BiDWGP was compared to four reference binaural intelligibility metrics, which were extended from the Speech Intelligibility Index (SII) and the Speech Transmission Index (STI). In the anechoic sound field, BiDWGP demonstrated high accuracy in predicting binaural intelligibility for individual maskers (ρ ≥ 0.95) and across maskers (ρ ≥ 0.94). The reference metrics however performed less well in across-masker prediction (0.54 ≤ ρ ≤ 0.86) despite reasonable accuracy for individual maskers. In reverberant rooms, BiDWGP was more stable in all test conditions (ρ ≥ 0.87) than the reference metrics, which showed different predictive patterns: the binaural STIs were more robust for the stationary than for the fluctuating noise masker, whilst the binaural SII displayed the opposite behaviour. The study shows that the new BiDWGP metric can provide similar or even more robust predictive power than the current standard metrics.

Speech Intelligibility in Rooms with and without an Induction Loop for Hearing Aid Users

Abstract The paper presents the results of sentence and logatome speech intelligibility measured in rooms with induction loop for hearing aid users. Two rooms with different acoustic parameters were chosen. Twenty two subjects with mild, moderate and severe hearing impairment using hearing aids took part in the experiment. The intelligibility tests composed of sentences or logatomes were presented to the subjects at fixed measurement points of an enclosure. It was shown that a sentence test is more useful tool for speech intelligibility measurements in a room than logatome test. It was also shown that induction loop is very efficient system at improving speech intelligibility. Additionally, the questionnaire data showed that induction loop, apart from improving speech intelligibility, increased a subject’s general satisfaction with speech perception

An improved speech transmission index for intelligibility prediction

The speech transmission index (STI) is a well known measure of intelligibility, most suited to the evaluation of speech intelligibility in rooms, with stimuli subjected to additive noise and reverberance. However, STI and its many variations do not effectively represent the intelligibility of stimuli containing non-linear distortions such as those resulting from processing by enhancement algorithms. In this paper, we revisit the STI approach and propose a variation which processes the modulation envelope in short-time segments, requiring only an assumption of quasi-stationarity (rather than the stationarity assumption of STI) of the modulation signal. Results presented in this work show that the proposed approach improves the measures correlation to subjective intelligibility scores compared to traditional STI for a range of noise types and subjected to different enhancement approaches. The approach is also shown to have higher correlation than other coherence, correlation and distance measures tested, but is unsuited to the evaluation of stimuli heavily distorted with (for example) masking based processing, where an alternative approach such as STOI is recommended.

Model of binaural speech intelligibility in rooms

Many models of speech intelligibility in rooms are based on monaural measures. However, the effect of binaural unmasking improves speech intelligibility substantially. The binaural speech intelligibility model (BSIM) uses multi-frequency-band equalization-cancellation (EC), which models human binaural noise reduction, and the Speech-Intelligibility-Index (SII), which calculates the resulting speech intelligibility. The model analyzes the signal-to-noise ratios at the left and the right ear (modeling better-ear-listening) and the interaural cross correlation of target speech and binaural interferer(s). The effect of the hearing threshold is modeled by assuming two uncorrelated threshold simulation noises for both ears. BSIM describes the (binaural) aspects of useful and detrimental room reflections, reverb, and background noise. Especially the interaction of delay time and direction of speech reflections with binaural unmasking in different acoustical situations was modeled successfully. BSIM can use eithe...

Speech intelligibility in rooms: An integrated model for temporal smearing, spatial unmasking, and binaural squelch

Speech intelligibility predictors based on room characteristics only consider the effects of temporal smearing of speech by room reflections and masking by diffuse ambient noise. In binaural listening conditions, a listener is able to separate target speech from interferering sounds. Lavandier and Culling (2010) proposed a model which incorporates this ability and its susceptibility to reverberation, but it neglects the temporal smearing of speech, so that prediction only holds for near-field targets. An extension of this model is presented here which accounts for both speech transmission and spatial unmasking, as well as binaural squelch in reverberant environments. The parameters of this integrated model were tested systematically by comparing the model predictions with speech reception thresholds measured in three experiments from the literature. The results showed a good correspondence between model predictions and experimental data for each experiment. The proposed model provides a unified interpretation of speech transmission, spatial unmasking, and binaural squelch.

Modeling the effects of a single reflection on binaural speech intelligibility

Recently the influence of delay and azimuth of a single speech reflection on speech reception thresholds (SRTs) was systematically investigated using frontal, diffuse, and lateral noise [Warzybok et al. (2013). J. Acoust. Soc. Am. 133, 269-282]. The experiments showed that the benefit of an early reflection was independent of its azimuth and mostly independent of noise type, but that the detrimental effect of a late reflection depended on its direction relative to the noise. This study tests if different extensions of a binaural speech intelligibility model can predict these data. The extensions differ in the order in which binaural processing and temporal integration of early reflections take place. Models employing a correction for the detrimental effects of reverberation on speech intelligibility after performing the binaural processing predict SRTs in symmetric masking conditions (frontal, diffuse), but cannot predict the measured interaction of temporal and spatial integration. In contrast, a model extension accounting for the distinction between useful and detrimental reflections before the binaural processing stage predicts the data with an overall R(2) of 0.95. This indicates that any model framework predicting speech intelligibility in rooms should incorporate an interaction between binaural and temporal integration of reflections at a comparatively early stage.

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.

A binaural model for predicting speech intelligibility in rooms using noise and reverberation suppression processes

Conventional metrics for predicting speech intelligibility are commonly described using the speech intelligibility index (SII) and speech transmission index (STI)--both of which are calculated using a monophonic signal. Under binaural conditions, these predictors often underestimate intelligibility due to the fact that beneficial binaural cues for unmasking detrimental effects are not accounted for. Beutelmann et al. [J. Acoust. Soc. Am. 127, 2479-2497 (2010)] proposed a binaural speech intelligibility model that determines the SII after suppressing spatially-separate noise using the equalization-cancellation (EC) process. This research expands on previous work by extending the model to analyze auditory scenes with more complex room acoustics, namely reverberation. In order to account for speech intelligibility degradation in the presence of room effects, the SII calculation is replaced by the STI. The current work also incorporates binaural release from reverberation by introducing an additional reverberation suppression mechanism into the model based on interaural coherence. The use of speech intelligibility metrics as an objective form of measurement may be used as a means to further understand binaural suppression processes within various room acoustic configurations through subjective listening tests.

Syllables intelligibility in relation to the autocorrelation and cepstrum model: the case of Chinese in Taiwan

The articulation of some special pronunciations of vowel inconsistently raises with the rapid speech transmission index (RASTI) tested using mono-syllables for Chinese phonics. In the researching of speech intelligibility in room, the factors being considered have to include not only the qualities of sound field but the pronunciation characteristics of syllables as well. Therefore, with regard to the defect of RASTI measurements, we utilized the autocorrelation and the cepstrum of mono-syllables recorded in rooms to compare the articulation with the physical phenomena of intelligibilities. Thus, we found that the minimum effective duration of autocorrelation function ({lower case tau}e) of all testing syllables signals recorded at same surveyed position in rooms, associated well with the articulation for each individual room. Specially, for individual syllable signals, they are further shown significant correlation between the cepstral energy of mono-syllables and the articulation collected in all rooms. The cepstrum acts as a spatially intelligible detector for syllables, and the autocorrelation is a good response of the pronunciation characteristics of syllables.

Relationship between Chinese speech intelligibility and speech transmission index in rooms based on auralization

Based on simulated monaural and binaural room impulse responses, the relationship between Chinese speech intelligibility scores and speech transmission index (STI) including the effect of noise is investigated using a phonetically balanced test in virtual rooms. The results show that Chinese speech intelligibility scores increase monotonically with STI values. The correlation coefficients are 0.95, 0.90 and the standard deviation is 5.6%, 6.7% under diotic and dichotic listening conditions, respectively. Compared with diotic listening based on monaural room impulse responses, dichotic listening based on binaural room impulse responses can improve by 2.7 dB signal-to-noise ratio for Chinese speech intelligibility. The STI method can better predict and evaluate Chinese speech intelligibility in rooms.

Speech intelligibility improvement in rooms using microphone arrays.

Microphones arrays reduce the effects of reverberation and background noise as compared to a single microphone. The resulting improvement in speech intelligibility in rooms is investigated here, following the approach discussed by Houtgast et al. [Acustica 46, 60–72 (1980)]. Room characteristics (volume, reverberation time), the background noise spectrum, and the source-receiver separation are used to compute a modulation transfer function. This function is input into the procedure described in IEC standard 60268-16(2003) to arrive at an objective measure of speech intelligibility, the speech transmission index (STI). Doing this for both omnidirectional and microphone array receivers, the improvement in intelligibility is determined. The signal source has the spectral shape of a human talker, as given in the IEC standard; the background noise spectrum is based on measurements made in a restaurant. The benefit of using microphone arrays is discussed for various conditions.

A binaural advantage in the subjective modulation transfer function with simple impulse responses

The speech transmission index (STI) has been a popular method for predicting speech intelligibility in rooms. It is based on the magnitude of the modulation transfer function, which can be derived from the impulse response of the room and the background noise levels. However, it does not take into account that humans listen with two ears. There can be large interaural phase differences in the modulation transfer functions, which can create detectable interaural level difference fluctuations. Measurements were made to determine whether these interaural modulation phase differences can be used to enhance the detectability of sinusoidal intensity modulations imposed on a broadband noise carrier and then convolved with simple, dichotic impulse responses. The results show that there can be a significant advantage to listening with two ears over listening with just one. Some further investigations were made to determine whether this advantage in signal detection can also be used as an advantage for speech intelligibility.