Testing a computational model for aural detection of aircraft in ambient noise.

Computational models are used to predict the performance of human listeners for carefully specified signal and noise conditions. However, there may be substantial discrepancies between the conditions under which listeners are tested and those used for model predictions. Thus, models may predict better performance than exhibited by the listeners, or they may "fail" to capture the ability of the listener to respond to subtle stimulus conditions. This study tested a computational model devised to predict a listener's ability to detect an aircraft in various soundscapes. The model and listeners processed the same sound recordings under carefully specified testing conditions. Details of signal and masker calibration were carefully matched, and the model was tested using the same adaptive tracking paradigm. Perhaps most importantly, the behavioral results were not available to the modeler before the model predictions were presented. Recordings from three different aircraft were used as the target signals. Maskers were derived from recordings obtained at nine locations ranging from very quiet rural environments to suburban and urban settings. Overall, with a few exceptions, model predictions matched the performance of the listeners very well. Discussion focuses on those differences and possible reasons for their occurrence.

Enhanced multi-channel model for auditory spectrotemporal integration.

In psychoacoustics, a multi-channel model has traditionally been used to describe detection improvement for multicomponent signals. This model commonly postulates that energy or information within either the frequency or time domain is transformed into a probabilistic decision variable across the auditory channels, and that their weighted linear summation determines optimum detection performance when compared to a critical value such as a decision criterion. In this study, representative integration-based channel models, specifically focused on signal-processing properties of the auditory periphery are reviewed (e.g., Durlach's channel model). In addition, major limitations of the previous channel models are described when applied to spectral, temporal, and spectrotemporal integration performance by human listeners. Here, integration refers to detection threshold improvements as the number of brief tone bursts in a signal is increased. Previous versions of the multi-channel model underestimate listener performance in these experiments. Further, they are unable to apply a single processing unit to signals which vary simultaneously in time and frequency. Improvements to the previous channel models are proposed by considering more realistic conditions such as correlated signal responses in the auditory channels, nonlinear properties in system performance, and a peripheral processing unit operating in both time and frequency domains.

Noise-induced changes in cochlear compression in the rat as indexed by forward masking of the auditory brainstem response.

The current study was undertaken to investigate changes in forward masking patterns using on-frequency and off-frequency maskers of 7 and 10 kHz probes in the Sprague-Dawley rat. Off-frequency forward masking growth functions have been shown in humans to be non-linear, while on-frequency functions behave linearly. The non-linear nature of the off-frequency functions is attributable to active processing from the outer hair cells, and was therefore expected to be sensitive to noise-induced cochlear damage. For the study, nine Sprague-Dawley rats' auditory brainstem responses (ABRs) were recorded with and without forward maskers. Forward masker-induced changes in latency and amplitude of the initial positive peak of the rats' auditory brainstem responses were assessed with both off-frequency and on-frequency maskers. The rats were then exposed to a noise designed to induce 20-40 dB of permanent threshold shift. Twenty-one days after the noise exposure, the forward masking growth functions were measured to assess noise-induced changes in the off-frequency and on-frequency forward masking patterns. Pre-exposure results showed compressive non-linear masking effects of the off-frequency conditions on both latency and amplitude of the auditory brainstem response. The noise rendered the off-frequency forward masking patterns more linear, consistent with human behavioral findings. On- and off-frequency forward masking growth functions were calculated, and they displayed patterns consistent with human behavioral functions, both prior to noise and after the noise exposure.

Auditory acuity for aircraft in real-world ambient environments.

Although many psychoacoustic studies have been conducted to examine the detection of masked target sounds, the vast majority of these studies have been conducted in carefully controlled laboratory listening environments, and their results may not apply to the detection of real-world sounds in the presence of naturalistic ambient sound fields. Those studies that have examined the detection of realistic naturally-occurring sounds have been conducted in uncontrolled listening environments (i.e., outdoor listening tests) where the experimenters were unable to precisely control, or even measure, the specific characteristics of the target and masker at the time of the detection judgment. This study represents an attempt to bridge the gap between unrealistic laboratory listening studies and uncontrolled outdoor listening studies through the use of pseudorandomly-presented real world recordings of target and masking sounds. Subjects were asked to detect helicopter signals in the context of an ongoing ambient recording in a two interval detection task. The results show that the signal-to-noise ratio required to detect an aircraft sound varies across different types of ambient environments (i.e., rural, suburban, or urban).

Spectral and temporal integration of brief tones.

Both spectral and temporal integration of tones have been explored in detail, but integration of tones varying across both dimensions has received little attention. This study explores temporal integration of tone pulses that vary over a range of frequencies. Baseline thresholds were obtained for both spectral and temporal integration with the same signals and compared with prior research. The signals were then varied on both dimensions in several ways: with equivalent spectral and temporal step sizes, different spectral and temporal step sizes, and a random pattern of frequency presentation. The data were also analyzed by spectral step size, temporal step size, frequency range, direction and slope of frequency change, and predictability. The spectral and temporal integration conditions showed that the current procedures and signals yielded the same improvement in detection thresholds as prior studies. The spectrotemporal integration conditions showed the improvement for overall detection of the signals to be limited by spectral integration, with improvement related primarily to the number of tones, regardless of timing and frequency. Surprisingly, trial-by-trial random presentation of signal frequencies did not negatively influence detection. These results support the multiple looks hypothesis [Viemeister, N. F. and Wakefield, G. H. (1991). "Temporal integration and multiple looks," J. Acoust. Soc. Am. 90, 858-865] as applied to spectrotemporal integration.