Louis D. Braida

Intensity Perception. I. Preliminary Theory of Intensity Resolution

An attempt is made to develop a quantitative theory of intensity resolution that is applicable to a wide variety of experiments on discrimination, identification, and scaling. The theory is composed of a Thurstonian decision model, which separates sensitivity from response bias, and an internal‐noise model, which separates sensory limitations from memory limitations. It is assumed that the subject has two memory operating modes, a sensory‐trace mode and a context‐coding mode, and that the use of these two modes is determined by the characteristics of the experiment. In one‐interval paradigms, it is assumed that the context‐coding mode is used, and the theory relates resolution to the total range of intensities in the stimulus set. In two‐interval paradigms, it is assumed that the two modes are combined, and the theory relates resolution to both the total intensity range and the duration between the two intervals. The theory provides, among other things, a new interpretation of the 7 ± 2 phenomenon.

Speaking Clearly for the Hard of Hearing I: Intelligibility Differences between Clear and Conversational Speech

The first paper of this series (Picheny, Durlach, & Braida, 1985) presented evidence that there are substantial intelligibility differences for hearing-impaired listeners between nonsense sentences...This paper is concerned with variations in the intelligibility of speech produced for hearing-impaired listeners under two conditions. Estimates were made of the magnitude of the intelligibility differences between attempts to speak clearly and attempts to speak conversationally. Five listeners with sensorineural hearing losses were tested on groups of nonsense sentences spoken clearly and conversationally by three male talkers as a function of level and frequency-gain characteristic. The average intelligibility difference between clear and conversational speech averaged across talker was found to be 17 percentage points. To a first approximation, this difference was independent of the listener, level, and frequency-gain characteristic. Analysis of segmental-level errors was only possible for two listeners and indicated that improvements in intelligibility occurred across all phoneme classes.

Acoustic properties of naturally produced clear speech at normal speaking rates

Sentences spoken “clearly” are significantly more intelligible than those spoken “conversationally” for hearing-impaired listeners in a variety of backgrounds [Picheny et al., J. Speech Hear. Res. 28, 96–103 (1985); Uchanski et al., ibid. 39, 494–509 (1996); Payton et al., J. Acoust. Soc. Am. 95, 1581–1592 (1994)]. While producing clear speech, however, talkers often reduce their speaking rate significantly [Picheny et al., J. Speech Hear. Res. 29, 434–446 (1986); Uchanski et al., ibid. 39, 494–509 (1996)]. Yet speaking slowly is not solely responsible for the intelligibility benefit of clear speech (over conversational speech), since a recent study [Krause and Braida, J. Acoust. Soc. Am. 112, 2165–2172 (2002)] showed that talkers can produce clear speech at normal rates with training. This finding suggests that clear speech has inherent acoustic properties, independent of rate, that contribute to improved intelligibility. Identifying these acoustic properties could lead to improved signal processing schemes for hearing aids. To gain insight into these acoustical properties, conversational and clear speech produced at normal speaking rates were analyzed at three levels of detail (global, phonological, and phonetic). Although results suggest that talkers may have employed different strategies to achieve clear speech at normal rates, two global-level properties were identified that appear likely to be linked to the improvements in intelligibility provided by clear/normal speech: increased energy in the 1000–3000-Hz range of long-term spectra and increased modulation depth of low frequency modulations of the intensity envelope. Other phonological and phonetic differences associated with clear/normal speech include changes in (1) frequency of stop burst releases, (2) VOT of word-initial voiceless stop consonants, and (3) short-term vowel spectra.

INTELLIGIBILITY OF CONVERSATIONAL AND CLEAR SPEECH IN NOISE AND REVERBERATION FOR LISTENERS WITH NORMAL AND IMPAIRED HEARING

The effect of articulating clearly on speech intelligibility is analyzed for ten normal-hearing and two hearing-impaired listeners in noisy, reverberant, and combined environments. Clear speech is more intelligible than conversational speech for each listener in every environment. The difference in intelligibility due to speaking style increases as noise and/or reverberation increase. The average difference in intelligibility is 20 percentage points for the normal-hearing listeners and 26 percentage points for the hearing-impaired listeners. Two predictors of intelligibility are used to quantify the environmental degradations: The articulation index (AI) and the speech transmission index (STI). Both are shown to predict, reliably, performance levels within a speaking style for normal-hearing listeners. The AI is unable to represent the reduction in intelligibility scores due to reverberation for the hearing-impaired listeners. Neither predictor can account for the difference in intelligibility due to speaking style.

Crossmodal integration in the identification of consonant segments

Although speechreading can be facilitated by auditory or tactile supplements, the process that integrates cues across modalities is not well understood. This paper describes two “optimal processing” models for the types of integration that can be used in speechreading consonant segments and compares their predictions with those of the Fuzzy Logical Model of Perception (FLMP, Massaro, 1987). In “pre-labelling” integration, continuous sensory data is combined across modalities before response labels are assigned. In “post-labelling” integration, the responses that would be made under unimodal conditions are combined, and a joint response is derived from the pair. To describe pre-labelling integration, confusion matrices are characterized by a multidimensional decision model that allows performance to be described by a subjects sensitivity and bias in using continuous-valued cues. The cue space is characterized by the locations of stimulus and response centres. The distance between a pair of stimulus centres determines how well two stimuli can be distinguished in a given experiment. In the multimodal case, the cue space is assumed to be the product space of the cue spaces corresponding to the stimulation modes. Measurements of multimodal accuracy in five modern studies of consonant identification are more consistent with the predictions of the pre-labelling integration model than the FLMP or the post-labelling model.

Resolution for speech sounds: Basic sensitivity and context memory on vowel and consonant continua

Discrimination and identification experiments were performed for a vowel continuum (/i/– /i/–/q/) and two consonant continua (/ba/–/pa/ and /ba/–/da/–/ga/). The results were interpreted in terms of a generalization of a theory of intensity resolution [N. I. Durlach and L. D. Braida, J. Acoust. Soc. Am. 46, 372–383 (1969)] that makes precise the distinction between basic sensitivity (sensory‐based resolution) and context coding (labeling processes). On the vowel continuum, basic sensitivity increased gradually across the range, whereas, for both consonant continua, sensitivity peaked between phonetic categories. All speech continua were found to have small ranges (measured in jnd’s); context memory was good, and better for consonants than for vowels. The stimuli that could be labeled most reliably were near the category boundaries on the vowel continuum, but near good phonetic exemplars for consonants. Introduction of a standard in identification primarily altered response bias, not sensitivity.

Intensity perception. XIII. Perceptual anchor model of context‐coding

In our preliminary theory of intensity resolution [e.g., see N. I. Durlach and L. D. Braida, J. Acoust. Soc. Am. 46, 372-383 (1969)], two modes of memory operation are postulated: the trace mode and the context-coding mode. In this paper, we present a revised model of the context-coding mode which describes explicitly a process by which sensations are coded relative to the context and which predicts a resolution edge effect [L. D. Braida and N. I. Durlach, J. Acoust. Soc. Am. 51, 483-502 (1972); J. E. Berliner, L. D. Braida, and N. I. Durlach, J. Acoust. Soc. Am. 61, 1256-1267 (1977)]. The sensation arising from a given stimulus presentation is coded by determining its distance from internal references or perceptual anchors. The noise in this process, combined with the sensation noise, constitutes the limitation on resolution in the model. In the revised model the probability density functions of the decision variable are not precisely Gaussian (and cannot be expressed analytically in closed form). This paper outlines the predictions of the model for one-interval paradigms and for fixed-level two-interval paradigms and derives estimates of the values of model parameters.

Study of multichannel amplitude compression and linear amplification for persons with sensorineural hearing loss

Experiments were conducted on five listeners with sensorineural hearing impairments using two 16-channel, computer-controlled, amplitude compression systems, and four linear systems. One of the compression systems was designed to restore normal equal loudness contours, the other employed reduced high-frequency emphasis and reduced compression ratios. The linear systems differed only in their frequency-gain characteristics (orthotelephonic plus three characteristics with high-frequency emphasis that were expected to produce better results than orthotelephonic). In the main experiment, all systems were compared for each listener using nonsense CVC monosyllables and sentence materials spoken by male and female talkers and presented in quiet/anechoic and noisy/reverberant environments at the most comfortable level for each listener. The linear systems with high-frequency emphasis performed substantially better than the orthotelephonic system. Performance with compression was generally slightly worse than with linear amplification. Compression was superior to linear amplification only when speech materials with significant item-to-item level variation were used in quiet with subjects with more severe losses and when reduced input speech levels were used. To the extent that these two conditions represent real-life communication conditions, these results suggest that compression is preferable to linear amplification in a wearable hearing aid.

Speaking clearly for the hard of hearing. III: An attempt to determine the contribution of speaking rate to differences in intelligibility between clear and conversational speech.

Previous studies (Picheny, Durlach, & Braida, 1985, 1986) have demonstrated that substantial intelligibility differences exist for hearing-impaired listeners for speech spoken clearly compared to speech spoken conversationally. This paper presents the results of a probe experiment intended to determine the contribution of speaking rate to the intelligibility differences. Clear sentences were processed to have the durational properties of conversational speech, and conversational sentences were processed to have the durational properties of clear speech. Intelligibility testing with hearing-impaired listeners revealed both sets of materials to be degraded after processing. However, the degradation could not be attributable to processing artifacts because reprocessing the materials to restore their original durations produced intelligibility scores close to those observed for the unprocessed materials. We conclude that the simple processing to alter the relative durations of the speech materials was not adequate to assess the contribution of speaking rate to the intelligibility differences; further studies are proposed to address this question.

Evaluating the articulation index for auditory–visual input

The current standard for calculating the Articulation Index (AI) includes a procedure to estimate the effective AI when hearing is combined with speechreading [ANSI S3.5‐1969 (R1978), “Methods for the Calculation of the Articulation Index” (American National Standards Institute, New York, 1969)]. This procedure assumes that the band‐importance function derived for auditory listening situations applies equally well to auditory‐visual situations. Recent studies have shown, however, that certain auditory signals that, by themselves, produce negligible speech reception scores (e.g., F0, speech‐modulated noise, etc.) can provide substantial benefits to speechreading. The existence of such signals suggests that the band‐importance function for auditory and auditory‐visual inputs may be different. In the present study, an attempt was made to validate the auditory‐visual correction procedure outlined in the ANSI‐1969 standard by evaluating auditory, visual, and auditory‐visual sentence identification performance of normal‐hearing subjects for both wideband speech degraded by additive noise and bandpass‐filtered speech presented in quiet. The results obtained for auditory listening conditions with an AI greater than 0.03 support the procedure outlined in the current ANSI standard. [Work supported by NIH.]