Generating partially correlated noise--a comparison of methods

Effects of interaural decoherence on sensitivity to interaural level differences across frequency

The interaural level difference (ILD) is a robust indicator of sound source azimuth, and human ILD sensitivity persists under conditions that degrade normally-dominant interaural time difference (ITD) cues. Nonetheless, ILD sensitivity varies somewhat with both stimulus frequency and interaural correlation (coherence). To further investigate the combined binaural perceptual influence of these variables, the present study assessed ILD sensitivity at frequencies 250-4000 Hz using stimuli of varied interaural correlation. In the first of two experiments, ILD discrimination thresholds were modestly elevated, and subjective lateralization slightly reduced, for both half-correlated and uncorrelated narrowband noise tokens relative to correlated tokens. Different from thresholds in the correlated condition, which were worst at 1000 Hz [Grantham, D.W. (1984). J. Acoust. Soc. Am. 75, 1191-1194], thresholds in the decorrelated conditions were independent of frequency. However, intrinsic envelope fluctuations in narrowband stimuli caused moment-to-moment variation of the nominal ILD, complicating interpretation of measured thresholds. Thus, a second experiment employed low-fluctuation noise tokens, revealing a clear effect of interaural decoherence per se that was strongly frequency-dependent, decreasing in magnitude from low to high frequencies. Measurements are consistent with known integration times in ILD-sensitive neurons and also suggest persistent influences of covert ITD cues in putative "ILD" tasks.

The "Missing 6 dB" Revisited: Influence of Room Acoustics and Binaural Parameters on the Loudness Mismatch Between Headphones and Loudspeakers

Generations of researchers observed a mismatch between headphone and loudspeaker presentation: the sound pressure level at the eardrum generated by a headphone has to be about 6 dB higher compared to the level created by a loudspeaker that elicits the same loudness. While it has been shown that this effect vanishes if the same waveforms are generated at the eardrum in a blind comparison, the origin of the mismatch is still unclear. We present new data on the issue that systematically characterize this mismatch under variation of the stimulus frequency, presentation room, and binaural parameters of the headphone presentation. Subjects adjusted the playback level of a headphone presentation to equal loudness as loudspeaker presentation, and the levels at the eardrum were determined through appropriate transfer function measurements. Identical experiments were conducted at Oldenburg and Aachen with 40 normal-hearing subjects including 14 that passed through both sites. Our data verify a mismatch between loudspeaker and binaural headphone presentation, especially at low frequencies. This mismatch depends on the room acoustics, and on the interaural coherence in both presentation modes. It vanishes for high frequencies and broadband signals if individual differences in the sound transfer to the eardrums are accounted for. Moreover, small acoustic and non-acoustic differences in an anechoic reference environment (Oldenburg vs. Aachen) exert a large effect on the recorded loudness mismatch, whereas not such a large effect of the respective room is observed across moderately reverberant rooms at both sites. Hence, the non-conclusive findings from the literature appear to be related to the experienced disparity between headphone and loudspeaker presentation, where even small differences in (anechoic) room acoustics significantly change the response behavior of the subjects. Moreover, individual factors like loudness summation appear to be only loosely connected to the observed mismatch, i.e., no direct prediction is possible from individual binaural loudness summation to the observed mismatch. These findings – even though not completely explainable by the yet limited amount of parameter variations performed in this study – have consequences for the comparability of experiments using loudspeakers with conditions employing headphones or other ear-level hearing devices.

Binaural sensitivity and release from speech-on-speech masking in listeners with and without hearing loss

Listeners with sensorineural hearing loss routinely experience less spatial release from masking (SRM) in speech mixtures than listeners with normal hearing. Hearing-impaired listeners have also been shown to have degraded temporal fine structure (TFS) sensitivity, a consequence of which is degraded access to interaural time differences (ITDs) contained in the TFS. Since these "binaural TFS" cues are critical for spatial hearing, it has been hypothesized that degraded binaural TFS sensitivity accounts for the limited SRM experienced by hearing-impaired listeners. In this study, speech stimuli were noise-vocoded using carriers that were systematically decorrelated across the left and right ears, thus simulating degraded binaural TFS sensitivity. Both (1) ITD sensitivity in quiet and (2) SRM in speech mixtures spatialized using ITDs (or binaural release from masking; BRM) were measured as a function of TFS interaural decorrelation in young normal-hearing and hearing-impaired listeners. This allowed for the examination of the relationship between ITD sensitivity and BRM over a wide range of ITD thresholds. This paper found that, for a given ITD sensitivity, hearing-impaired listeners experienced less BRM than normal-hearing listeners, suggesting that binaural TFS sensitivity can account for only a modest portion of the BRM deficit in hearing-impaired listeners. However, substantial individual variability was observed.

Localization of broadband sounds carrying interaural time differences: Effects of frequency, reference location, and interaural coherence

The auditory processes involved in the localization of sounds in rooms are still poorly understood. The present study investigated the auditory system's across-frequency processing of interaural time differences (ITDs) and the impact of the interaural coherence (IC) of the stimuli in ITD discrimination and localization. First, ITD discrimination thresholds were measured as a function of signal frequency, reference ITD, and IC using critical-band wide noises. The resulting data were fitted with a set of analytical functions and ITD weights were derived using concepts from signal detection theory. Inspired by the weighted-image model [Stern, Zeiberg, and Trahiotis. (1988). J. Acoust. Soc. Am. 84, 156-165], the derived ITD weights were then integrated in a simplified localization model using an optimal combination of ITD information across frequency. To verify this model, a series of localization experiments were conducted using broadband noise in which ITD and IC were varied across frequency. The model predictions were in good agreement with the experimental data, supporting the assumption that the auditory system performs a weighted integration of ITD information across frequency to localize a sound source. The results could be valuable for the design of new paradigms to measure localization in more complex acoustic conditions and may provide constraints for future localization models.

Auditory and Cognitive Factors Associated with Speech-in-Noise Complaints following Mild Traumatic Brain Injury

BACKGROUND

Auditory complaints following mild traumatic brain injury (MTBI) are common, but few studies have addressed the role of auditory temporal processing in speech recognition complaints.

PURPOSE

In this study, deficits understanding speech in a background of speech noise following MTBI were evaluated with the goal of comparing the relative contributions of auditory and nonauditory factors.

RESEARCH DESIGN

A matched-groups design was used in which a group of listeners with a history of MTBI were compared to a group matched in age and pure-tone thresholds, as well as a control group of young listeners with normal hearing (YNH).

STUDY SAMPLE

Of the 33 listeners who participated in the study, 13 were included in the MTBI group (mean age = 46.7 yr), 11 in the Matched group (mean age = 49 yr), and 9 in the YNH group (mean age = 20.8 yr).

DATA COLLECTION AND ANALYSIS

Speech-in-noise deficits were evaluated using subjective measures as well as monaural word (Words-in-Noise test) and sentence (Quick Speech-in-Noise test) tasks, and a binaural spatial release task. Performance on these measures was compared to psychophysical tasks that evaluate monaural and binaural temporal fine-structure tasks and spectral resolution. Cognitive measures of attention, processing speed, and working memory were evaluated as possible causes of differences between MTBI and Matched groups that might contribute to speech-in-noise perception deficits.

RESULTS

A high proportion of listeners in the MTBI group reported difficulty understanding speech in noise (84%) compared to the Matched group (9.1%), and listeners who reported difficulty were more likely to have abnormal results on objective measures of speech in noise. No significant group differences were found between the MTBI and Matched listeners on any of the measures reported, but the number of abnormal tests differed across groups. Regression analysis revealed that a combination of auditory and auditory processing factors contributed to monaural speech-in-noise scores, but the benefit of spatial separation was related to a combination of working memory and peripheral auditory factors across all listeners in the study.

CONCLUSIONS

The results of this study are consistent with previous findings that a subset of listeners with MTBI has objective auditory deficits. Speech-in-noise performance was related to a combination of auditory and nonauditory factors, confirming the important role of audiology in MTBI rehabilitation. Further research is needed to evaluate the prevalence and causal relationship of auditory deficits following MTBI.

Relating interaural difference sensitivities for several parameters measured in normal-hearing and hearing-impaired listeners

Just-noticeable differences (JNDs) in interaural time delay (ITD), interaural level difference (ILD), and interaural cross-correlation (ICC) were measured with low- and high-frequency noise bands over multiple sessions for 10 normal-hearing (NH) and 11 hearing-impaired (HI) listeners. Individual subject thresholds tended to improve with training then stabilize. Measured JNDs varied over these experienced listeners, for both subject groups and all tasks. Group JNDs were seldom predictable from hearing level. Individual listeners' JNDs were highly correlated across frequency for each task and group, except for ICC in the HI listeners. Further, ITD JNDs almost always significantly correlated with ILD JNDs within a group. Finally, although the ICC JNDs always significantly correlated with the ITD or ILD JNDs for the NH listeners, they often did not for the HI listeners. These findings suggest that little information about binaural sensitivity is added for NH listeners with multiple ITD, ILD, and ICC measures. For HI listeners, however, while ITD and ILD measures are well correlated, information is added with ICC measures. In general, the results suggest that less information is added with JND measures for NH listeners (15 significant correlations) than for HI listeners (six significant correlations).

Similar Impacts of the Interaural Delay and Interaural Correlation on Binaural Gap Detection

The subjective representation of the sounds delivered to the two ears of a human listener is closely associated with the interaural delay and correlation of these two-ear sounds. When the two-ear sounds, e.g., arbitrary noises, arrive simultaneously, the single auditory image of the binaurally identical noises becomes increasingly diffuse, and eventually separates into two auditory images as the interaural correlation decreases. When the interaural delay increases from zero to several milliseconds, the auditory image of the binaurally identical noises also changes from a single image to two distinct images. However, measuring the effect of these two factors on an identical group of participants has not been investigated. This study examined the impacts of interaural correlation and delay on detecting a binaurally uncorrelated fragment (interaural correlation = 0) embedded in the binaurally correlated noises (i.e., binaural gap or break in interaural correlation). We found that the minimum duration of the binaural gap for its detection (i.e., duration threshold) increased exponentially as the interaural delay between the binaurally identical noises increased linearly from 0 to 8 ms. When no interaural delay was introduced, the duration threshold also increased exponentially as the interaural correlation of the binaurally correlated noises decreased linearly from 1 to 0.4. A linear relationship between the effect of interaural delay and that of interaural correlation was described for listeners participating in this study: a 1 ms increase in interaural delay appeared to correspond to a 0.07 decrease in interaural correlation specific to raising the duration threshold. Our results imply that a tradeoff may exist between the impacts of interaural correlation and interaural delay on the subjective representation of sounds delivered to two human ears.

The perception of apparent auditory source width in hearing-impaired adults

In a previous study [Whitmer, Seeber and Akeroyd, J. Acoust. Soc. Am. 132, 369-379 (2012)], it was demonstrated that older hearing-impaired (HI) listeners produced visual sketches of headphone-presented noises that were insensitive to changes in interaural coherence. The current study further explores this insensitivity by comparing (a) binaural temporal fine-structure (TFS) resolution and (b) sound localization precision to (c) auditory source width judgments. Thirty-five participants aged 26-81 years with normal to moderately impaired hearing (a) discriminated interaurally phase-shifted tones from diotic tones presented over headphones, (b) located 500-ms speech-spectrum filtered click trains presented over loudspeakers between ±30° in quiet, and (c) sketched the perceived width of low-pass, high-pass, and speech-spectrum noise stimuli presented over loudspeakers from 0° and simultaneously from ±45° at attenuations of 0-20 dB to generate partially coherent stimuli. The results showed a decreasing sensitivity to width with age and impairment which was related to binaural TFS threshold: the worse one's threshold-which was correlated with age-the less the perceived width increased with decreasing interaural coherence. These results suggest that senescent changes to the auditory system do not necessarily lead to perceptions of broader, more diffuse sound images based on interaural coherence.

The effect of reverberation on personal audio devices

Personal audio refers to the creation of a listening zone within which a person, or a group of people, hears a given sound program, without being annoyed by other sound programs being reproduced in the same space. Generally, these different sound zones are created by arrays of loudspeakers. Although these devices have the capacity to achieve different sound zones in an anechoic environment, they are ultimately used in normal rooms, which are reverberant environments. At high frequencies, reflections from the room surfaces create a diffuse pressure component which is uniform throughout the room volume and thus decreases the directional characteristics of the device. This paper shows how the reverberant performance of an array can be modeled, knowing the anechoic performance of the radiator and the acoustic characteristics of the room. A formulation is presented whose results are compared to practical measurements in reverberant environments. Due to reflections from the room surfaces, pressure variations are introduced in the transfer responses of the array. This aspect is assessed by means of simulations where random noise is added to create uncertainties, and by performing measurements in a real environment. These results show how the robustness of an array is increased when it is designed for use in a reverberant environment.

Apparent auditory source width insensitivity in older hearing-impaired individuals

Previous studies have shown a loss in the precision of horizontal localization responses of older hearing-impaired (HI) individuals, along with potentially poorer neural representations of sound-source location. These deficits could be the result or corollary of greater difficulties in discriminating spatial images, and the insensitivity to punctate sound sources. This hypothesis was tested in three headphone-presentation experiments varying interaural coherence (IC), the cue most associated with apparent auditory source width. First, thresholds for differences in IC were measured for a broad sampling of participants. Older HI participants were significantly worse at discriminating IC across reference values than younger normal-hearing participants. These results are consistent with senescent increases in temporal jitter. Performance decreased with age, a finding corroborated in a second discrimination experiment using a separate group of participants matched for hearing loss. This group also completed a third, visual experiment, with both a cross-mapping task where they drew the size of the sound they heard and the identification task where they chose the image that best corresponded to what they heard. The results from the visual tasks indicate that older HI individuals do not hear punctate images and are relatively insensitive to changes in width based on IC.