Across-formant integration and speech intelligibility: Effects of acoustic source properties in the presence and absence of a contralateral interferer.

Robert J Summers,Brian Roberts,Peter J Bailey

doi:10.1121/1.4960595

Robert J Summers, Brian Roberts + Show 1 more

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https://doi.org/10.1121/1.4960595

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Abstract

The role of source properties in across-formant integration was explored using three-formant (F1+F2+F3) analogues of natural sentences (targets). In experiment 1, F1+F3 were harmonic analogues (H1+H3) generated using a monotonous buzz source and second-order resonators; in experiment 2, F1+F3 were tonal analogues (T1+T3). F2 could take either form (H2 or T2). Target formants were always presented monaurally; the receiving ear was assigned randomly on each trial. In some conditions, only the target was present; in others, a competitor for F2 (F2C) was presented contralaterally. Buzz-excited or tonal competitors were created using the time-reversed frequency and amplitude contours of F2. Listeners must reject F2C to optimize keyword recognition. Whether or not a competitor was present, there was no effect of source mismatch between F1+F3 and F2. The impact of adding F2C was modest when it was tonal but large when it was harmonic, irrespective of whether F2C matched F1+F3. This pattern was maintained when harmonic and tonal counterparts were loudness-matched (experiment 3). Source type and competition, rather than acoustic similarity, governed the phonetic contribution of a formant. Contrary to earlier research using dichotic targets, requiring across-ear integration to optimize intelligibility, H2C was an equally effective informational masker for H2 as for T2.

Highlights

The frequencies of the first three formants and their patterns of change over time are a critical source of information for identifying the phonetic segments being articulated by a talker and for understanding speech
The experiments reported here have explored the effects of source properties per se, and of differences in acoustic form between formants, on the integration of phonetic information across formants when listening to single presentations of monaural targets with unpredictable lateralization
To explore how competition modulates the effects of differences in source properties, these effects were compared in the presence and absence of single-formant interferers in the contralateral ear

Summary

Introduction

The frequencies of the first three formants and their patterns of change over time are a critical source of information for identifying the phonetic segments being articulated by a talker and for understanding speech (see, e.g., Roberts et al, 2011). How the information carried by different formants is integrated across frequency into a phonetic percept is not fully understood, especially in contexts where more than one talker is speaking at once (see, e.g., Darwin, 2008). In such circumstances, successful communication depends on the extent to which the formant ensemble reaching the ears can be separated into a figure (target) and background (interferer). While it has long been known that acoustic cues such as differences in onset time and fundamental frequency (F0) can influence the ability to group and segregate formants, these influences can be complex and context-dependent. If the F0 of one formant is different from that of the others, that formant is usually heard as coming from a different source, but may still contribute to the perceived identity of the speech sounds (Cutting, 1976)

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