Network
Latest external collaboration on country level. Dive into details by clicking on the dots.
Publication
Featured researches published by Uma Balakrishnan.
Journal of the Acoustical Society of America | 2001
Richard L. Freyman; Uma Balakrishnan; Karen S. Helfer
Three experiments were conducted to determine the extent to which perceived separation of speech and interference improves speech recognition in the free field. Target speech stimuli were 320 grammatically correct but nonmeaningful sentences spoken by a female talker. In the first experiment the interference was a recording of either one or two female talkers reciting a continuous stream of similar nonmeaningful sentences. The target talker was always presented from a loudspeaker directly in front (0 degrees). The interference was either presented from the front loudspeaker (the F-F condition) or from both a right loudspeaker (60 degrees) and the front loudspeaker, with the right leading the front by 4 ms (the F-RF condition). Due to the precedence effect, the interference in the F-RF condition was perceived to be well to the right, while the target talker was heard from the front. For both the single-talker and two-talker interference, there was a sizable improvement in speech recognition in the F-RF condition compared with the F-F condition. However, a second experiment showed that there was no F-RF advantage when the interference was noise modulated by the single- or multi-channel envelope of the two-talker masker. Results of the third experiment indicated that the advantage of perceived separation is not limited to conditions where the interfering speech is understandable.
Journal of the Acoustical Society of America | 1997
Richard L. Freyman; Patrick M. Zurek; Uma Balakrishnan; Yuan-Chuan Chiang
Saberi and Perrott [Acustica 81, 272-275 (1995)] found that the in-head lateralization of a relatively long-duration pulse train could be controlled by the interaural delay of the single pulse pair that occurs at onset. The present study examined this further, using an acoustic pointer measure of lateralization, with stimulus manipulations designed to determine conditions under which lateralization was consistent with the interaural onset delay. The present stimuli were wideband pulse trains, noise-burst trains, and inharmonic complexes, 250 ms in duration, chosen for the ease with which interaural delays and correlations of select temporal segments of the stimulus could be manipulated. The stimulus factors studied were the periodicity of the ongoing part of the signal as well as the multiplicity and ambiguity of interaural delays. The results, in general, showed that the interaural onset delay controlled lateralization when the steady state binaural cues were relatively weak, either because the spectral components were only sparsely distributed across frequency or because the interaural time delays were ambiguous. Onset dominance can be disrupted by sudden stimulus changes within the train, and several examples of such changes are described. Individual subjects showed strong left-right asymmetries in onset effectiveness. The results have implications for understanding how onset and ongoing interaural delay cues contribute to the location estimates formed by the binaural auditory system.
Journal of the Acoustical Society of America | 2004
Patrick M. Zurek; Richard L. Freyman; Uma Balakrishnan
Measurements and theoretical predictions of auditory target detection in simulated reverberant conditions are reported. The target signals were pulsed 1/3-octave bands of noise and the masker signal was a continuous wideband noise. Target and masker signals were passed through a software simulation of a reverberant room with a rigid sphere modeling a listeners head. The location of the target was fixed while the location of the masker was varied in the simulated room. Degree of reverberation was controlled by varying the uniform acoustic absorption of the simulated rooms surfaces. The resulting target and masker signals were presented to the listeners over headphones in monaural-left, monaural-right, or binaural listening modes. Changes in detection performance in the monaural listening modes were largely predictable from the changes in target-to-masker ratio in the target band, but with a few dB of extra masking in reverberation. Binaural detection performance was generally well predicted by applying Durlachs [in Foundations of Modern Auditory Theory (Academic, New York, 1972)] equalization-cancellation theory to the direct-plus-reverberant ear signals. Predictions in all cases were based on a statistical description of room acoustics and on acoustic diffraction by a sphere. The success of these detection models in the present well-controlled reverberant conditions suggests that they can be used to incorporate listening mode and source location as factors in speech-intelligibility predictions.
Journal of the Acoustical Society of America | 2002
Uma Balakrishnan; Richard L. Freyman
The effect of onset interaural time differences (ITDs) on lateralization and detection was investigated for broadband pulse trains 250 ms long with a binaural fundamental frequency of 250 Hz. Within each train, ITDs of successive binaural pulse pairs alternated between two of three values (0 micros, 500 micros left-leading, and 500 micros right-leading) or were invariant. For the alternating conditions, the experimental manipulation was the choice of which of two ITDs was presented first (i.e., at stimulus onset). Lateralization, which was estimated using a broadband noise pointer with a listener adjustable interaural delay, was determined largely by the onset ITD. However, detection thresholds for the signals in left-leading or diotic continuous broadband noise were not affected by where the signals were lateralized. A quantitative analysis suggested that binaural masked thresholds for the pulse trains were well accounted for by the level and phase of harmonic components at 500 and 750 Hz. Detection thresholds obtained for brief stimuli (two binaural pulse or noise burst pairs) were also independent of which of two ITDs was presented first. The control of lateralization by onset cues appears to be based on mechanisms not essential for binaural detection.
Journal of the Acoustical Society of America | 1989
Richard L. Freyman; Rachel K. Clifton; Ruth Y. Litovsky; Uma Balakrishnan
Dynamic variations in the strength of echo suppression were investigated through earphone simulation of the free‐field precedence effect. Each test stimulus consisted of two click pairs, with the interaural time parameters of the leading and lagging pairs reflecting stimuli originating from opposite sides of the head. Echo thresholds, the minimum lagging click delay required for subjects to report hearing an echo click, were obtained in two basic conditions: (a) test click presented in isolation, and (b) test click preceded by a train of clicks identical to the test click. The most striking finding was that the presence of the preceding click train usually elevated the echo threshold of the test stimulus relative to its threshold when presented in isolation, suggesting that echo suppression builds up during the click train. The magnitude of the effect was influenced by the number of clicks in the train and the period of silence between the end of the train and the test click. The effect is surprisingly pe...
Journal of the Acoustical Society of America | 2005
Richard L. Freyman; Karen S. Helfer; Uma Balakrishnan
The literature on speech recognition within a competing speech environment shows great variation with respect to the amount of masking produced, as well as variation in the improvement realized from spatial separation of the target and masking speech. Presumably, the sources of variation include differences in methodology and stimuli used by different researchers. In the current study the target speech stimuli were nonsense sentences spoken by a female talker. The competing speech was created from similar nonsense sentences recorded by 10 other female speakers. Five different maskers were created, each a combination of two of the 10 talkers who had similar fundamental frequencies. The results showed that the variation in masking produced by the different two‐talker combinations was substantial only when target and masker were presented from the same loudspeaker. As a consequence, the observed benefit from spatial separation varied widely among the different maskers, and was ordered according to fundamenta...
Journal of the Acoustical Society of America | 2001
Richard L. Freyman; Uma Balakrishnan; Patrick M. Zurek
This study explores the factors that influence the masking of narrow‐band signals in reverberant sound fields. Rectangular rooms were simulated using the image method with a rigid sphere modeling the head. The signals were third‐octave noise bands presented in a continuous broadband noise masker. For a given room simulation, the signals and masker were convolved with impulse responses obtained in the room at multiple azimuth angles and distances, for subsequent presentation via headphones. Adaptive forced‐choice procedures were used with the resulting signals to find monaural and binaural thresholds in listeners with normal hearing. The benefit of separating the masker and signal spatially in the simulation was reduced by reverberation, as expected, although the effects were complex. Among the predictions confirmed by these experiments is that reverberation actually improves thresholds when the masker is closer to the listener than the signal, apparently due to both an increase in monaural signal power a ...
Journal of the Acoustical Society of America | 1993
Uma Balakrishnan; Richard L. Freyman
Broadband click trains were used to investigate the effect of onset and ongoing interaural time delays (ITDs) on lateralization and binaural masking. Within a train, successive click pairs had ITDs that were either identical or alternated between two values. The interclick interval was 2 ms and the total duration of each train was 250 ms. Perceived intracranial position of the signals was estimated using an acoustic pointer. Detection thresholds for the same trains were obtained in the presence of a continuous white noise masker with a fixed interaural delay. It was found that lateralization generally followed the ITD of the very first click pair in the train. Masked thresholds were dependent upon the proximity of one or both signal ITDs to the masker ITD, but were independent of the perceived lateral position of the signal relative to that of the masker. These results suggest that while the onset ITD may dominate lateralization for click trains, it has no influence on masked binaural detection of the sam...
Journal of the Acoustical Society of America | 2004
Richard L. Freyman; Uma Balakrishnan; Karen S. Helfer
Journal of the Acoustical Society of America | 2007
Richard L. Freyman; Karen S. Helfer; Uma Balakrishnan