Non-native speech perception in adverse conditions: A review <s> Additive noise: informational masking <s> Shifts in masked spondee thresholds during several conditions of listening (monaural, homophasic, antiphasic, and with interaural time disparity) in the presence of one to four competing maskers were measured. The maskers used were white noise, white noise modulated four times per second by 10 dB with a 50% duty cycle, the same noise with 75% duty cycle, connected speech by one male talker, and connected speech by a second male talker. Results from three experiments that employed various permutations of the aforementioned conditions are reported. The findings, after equating conditions to equivalent masker levels, were four. First, the modulated noise with 50% duty cycle produced about 3.5 dB less masking than that produced by unmodulated white noise. Second, the modulated noise with 75% duty cycle allowed only about 1 dB less shift than did the unmodulated noise. Third, mixing one speech train with noise (either modulated or unmodulated) induced about 3.2 dB excess masking. This excess is here termed per... <s> BIB001 </s> Non-native speech perception in adverse conditions: A review <s> Additive noise: informational masking <s> Although many researchers have examined the role that binaural cues play in the perception of spatially separated speech signals, relatively little is known about the cues that listeners use to segregate competing speech messages in a monaural or diotic stimulus. This series of experiments examined how variations in the relative levels and voice characteristics of the target and masking talkers influence a listener's ability to extract information from a target phrase in a 3-talker or 4-talker diotic stimulus. Performance in this speech perception task decreased systematically when the level of the target talker was reduced relative to the masking talkers. Performance also generally decreased when the target and masking talkers had similar voice characteristics: the target phrase was most intelligible when the target and masking phrases were spoken by different-sex talkers, and least intelligible when the target and masking phrases were spoken by the same talker. However, when the target-to-masker ratio was less than 3 dB, overall performance was usually lower with one different-sex masker than with all same-sex maskers. In most of the conditions tested, the listeners performed better when they were exposed to the characteristics of the target voice prior to the presentation of the stimulus. The results of these experiments demonstrate how monaural factors may play an important role in the segregation of speech signals in multitalker environments. <s> BIB002 </s> Non-native speech perception in adverse conditions: A review <s> Additive noise: informational masking <s> Three experiments used the Coordinated Response Measure task to examine the roles that differences in F0 and differences in vocal-tract length have on the ability to attend to one of two simultaneous speech signals. The first experiment asked how increases in the natural F0 difference between two sentences (originally spoken by the same talker) affected listeners' ability to attend to one of the sentences. The second experiment used differences in vocal-tract length, and the third used both F0 and vocal-tract length differences. Differences in F0 greater than 2 semitones produced systematic improvements in performance. Differences in vocal-tract length produced systematic improvements in performance when the ratio of lengths was 1.08 or greater, particularly when the shorter vocal tract belonged to the target talker. Neither of these manipulations produced improvements in performance as great as those produced by a different-sex talker. Systematic changes in both F0 and vocal-tract length that simulated an incremental shift in gender produced substantially larger improvements in performance than did differences in F0 or vocal-tract length alone. In general, shifting one of two utterances spoken by a female voice towards a male voice produces a greater improvement in performance than shifting male towards female. The increase in performance varied with the intonation patterns of individual talkers, being smallest for those talkers who showed most variability in their intonation patterns between different utterances. <s> BIB003 </s> Non-native speech perception in adverse conditions: A review <s> Additive noise: informational masking <s> Three experiments investigated factors that influence the creation of and release from informational masking in speech recognition. The target stimuli were nonsense sentences spoken by a female talker. In experiment 1 the masker was a mixture of three, four, six, or ten female talkers, all reciting similar nonsense sentences. Listeners' recognition performance was measured with both target and masker presented from a front loudspeaker (F-F) or with a masker presented from two loudspeakers, with the right leading the front by 4 ms (F-RF). In the latter condition the target and masker appear to be from different locations. This aids recognition performance for one- and two-talker maskers, but not for noise. As the number of masking talkers increased to ten, the improvement in the F-RF condition diminished, but did not disappear. The second experiment investigated whether hearing a preview (prime) of the target sentence before it was presented in masking improved recognition for the last key word, which was not included in the prime. Marked improvements occurred only for the F-F condition with two-talker masking, not for continuous noise or F-RF two-talker masking. The third experiment found that the benefit of priming in the F-F condition was maintained if the prime sentence was spoken by a different talker or even if it was printed and read silently. These results suggest that informational masking can be overcome by factors that improve listeners' auditory attention toward the target. <s> BIB004
Energetic masking is not the only effect of signal degradation; a second important aspect is the role played by any additional energy contributed by the masker. While the focus is usually directed towards the effect of masking on a target signal such as speech, masking is a two-way process: the noise is itself masked by the target, and audible masker fragments can interfere with the perception of the target. The regions not depicted in red in Fig. 1 are dominated by audible contributions from the masker. This information has the capacity to interfere with decisions at higher levels of processing, resulting in what has been called informational masking (IM). IM was first studied for speech signals by BIB001 and more recently has been investigated by BIB002 and BIB004 . Unlike EM, informational masking covers more than one process whose relative contributions to a reduction in intelligibility remain poorly-understood. IM can arise due to supposedly low-level processes such as the grouping of time-frequency regions into larger units. Consider the spectral "holes" at the start of the word fear caused by the competing speech masker in Fig. 1 . These contain audible speech components from the masker that need to be excluded from the target speech. The success or failure of grouping will depend on factors such as the similarity of competing sources in F0 and vocal tract length BIB003 as well as how much the patches of energy make sense in the ongoing context formed during interpretation of the utterance. Although competing speech or multitalker babble for smaller numbers of talkers are usually regarded as the most effective form of informational masker, it is worth noting that all maskers have some occasional IM potential: audible time-frequency regions belonging to stationary maskers could, in principle, incorrectly cohere with the target signal in a form of informational masking based on target-masker misallocation.