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:10.1121/1.4920891

Abstract

Three experiments used three-formant (F1 + F2 + F3) analogues of natural sentences to explore the role of acoustic source properties in across-formant integration. In experiment 1, F1 + F3 were generated using a monotonous periodic source (F0 = 140 Hz) and second-order resonators (H1 + H3); 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 target ear was assigned randomly on each trial. In some conditions, only the target was present; in others, a competitor for F2 (F2C) was added contralaterally. Listeners must reject F2C to optimize recognition. Competitors (H2C or T2C) were created using the time-reversed frequency and amplitude contours of F2. Without F2C, intelligibility was reasonably high and the effect of a source mismatch between F1 + F3 and F2 was negligible. For both shared- and hybrid-source targets, the impact of adding F2C was modest when it was tonal but large when it was harmonic, irrespective of whether F2C matched F1 + F3. Experiment 3 showed that this pattern was maintained when corresponding harmonic and tonal analogues were loudness-matched. These findings extend those from earlier research using dichotic targets. Source type and competition, rather than acoustic similarity, govern the phonetic contribution of a formant. [Work supported by ESRC.]

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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