Abstract
Masking experienced when target speech is accompanied by a single interfering voice is often primarily informational masking (IM). IM is generally greater when the interferer is intelligible than when it is not (e.g., speech from an unfamiliar language), but the relative contributions of acoustic-phonetic and linguistic interference are often difficult to assess owing to acoustic differences between interferers (e.g., different talkers). Three-formant analogues (F1+F2+F3) of natural sentences were used as targets and interferers. Targets were presented monaurally either alone or accompanied contralaterally by interferers from another sentence (F0 = 4 semitones higher); a target-to-masker ratio (TMR) between ears of 0, 6, or 12 dB was used. Interferers were either intelligible or rendered unintelligible by delaying F2 and advancing F3 by 150 ms relative to F1, a manipulation designed to minimize spectro-temporal differences between corresponding interferers. Target-sentence intelligibility (keywords correct) was 67% when presented alone, but fell considerably when an unintelligible interferer was present (49%) and significantly further when the interferer was intelligible (41%). Changes in TMR produced neither a significant main effect nor an interaction with interferer type. Interference with acoustic-phonetic processing of the target can explain much of the impact on intelligibility, but linguistic factors-particularly interferer intrusions-also make an important contribution to IM.
Highlights
Speech is a sparse signal in a frequency  time representation
These aspects of informational masking (IM) can have a considerable effect on the success of spoken communication even in circumstances where the properties of the target speech are well represented in the responses of the peripheral auditory system, and their effect is likely to be even greater when the peripheral representation of the target is degraded by energetic masking (EM) or by sensorineural hearing loss
We define acoustic-phonetic interference as those aspects of IM that hinder the extraction or integration of information about speech articulation carried by the time-varying formant-frequency contours, and linguistic interference as those aspects of IM that occur after lexical objects have been formed, such as the intrusion of words from an interfering sentence into the percept of the target sentence
Summary
Speech is a sparse signal in a frequency  time representation. when we listen to a talker in the presence of one or two interfering voices, there is often relatively little energetic masking (EM) of the target speech unless the level of the interfering speech is high. One way to tackle this problem is to retain a dichotic configuration but to degrade the monaural target speech before adding the contralateral masker—e.g., by adding a fixed-level ipsilateral noise masker to the target speech (Brungart and Simpson, 2002) This approach has been applied successfully in several studies of IM, but to our knowledge—with one exception, discussed below (Gallun et al, 2007)—whenever it has been used to compare the IM generated by intelligible and unintelligible interferers, their acoustic properties arguably have not been matched sufficiently closely. Three-formant interferers (time-reversed F1, F2, and F3 contours) have an even greater effect than a single-formant interferer derived from F1 (Roberts and Summers, 2018), and this difference cannot be attributed to the increase in total energy (typically
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