Pierre Zakarauskas

A computational theory of spectral cue localization

This paper presents a computational theory of localization based on analysis of the cues contained in the spectrum received at the eardrum. It is well established that monaural localization, and binaural localization onto the cone of confusion, for pure tone stimuli are unrelated to the actual source position. On the other hand, broad spectrum stimuli are localized with some accuracy. This paper outlines the properties that the operations performed on a received spectrum must have in order for the source to be localized accurately. The use of the two simplest nontrivial such operations, the first and second finite differences of the spectrum, respectively, are explored in detail. This analysis shows that so long as the spectrum of the sound source has a locally constant slope, accurate localization using such operators is possible. A simulation of the localization process making use of measured human head‐related transfer functions (HRTF’s) and two recorded spectra demonstrates that the second‐order finit...

Meditation Experience Predicts Introspective Accuracy

The accuracy of subjective reports, especially those involving introspection of ones own internal processes, remains unclear, and research has demonstrated large individual differences in introspective accuracy. It has been hypothesized that introspective accuracy may be heightened in persons who engage in meditation practices, due to the highly introspective nature of such practices. We undertook a preliminary exploration of this hypothesis, examining introspective accuracy in a cross-section of meditation practitioners (1–15,000 hrs experience). Introspective accuracy was assessed by comparing subjective reports of tactile sensitivity for each of 20 body regions during a ‘body-scanning’ meditation with averaged, objective measures of tactile sensitivity (mean size of body representation area in primary somatosensory cortex; two-point discrimination threshold) as reported in prior research. Expert meditators showed significantly better introspective accuracy than novices; overall meditation experience also significantly predicted individual introspective accuracy. These results suggest that long-term meditators provide more accurate introspective reports than novices.

The auditory motionaftereffect: Its tuning and specificity in the spatial and frequency domains

In this paper, the auditory motion aftereffect (aMAE) was studied, using real moving sound as both the adapting and the test stimulus. The sound was generated by a loudspeaker mounted on a robot arm that was able to move quietly in three-dimensional space. A total of 7 subjects with normal hearing were tested in three experiments. The results from Experiment 1 showed a robust and reliable negative aMAE in all the subjects. After listening to a sound source moving repeatedly to the right, a stationary sound source was perceived to move to the left. The magnitude of the aMAE tended to increase with adapting velocity up to the highest velocity tested (20°/sec). The aftereffect was largest when the adapting and the test stimuli had similar spatial location and frequency content. Offsetting the locations of the adapting and the test stimuli by 20° reduced the size of the effect by about 50%. A similar decline occurred when the frequency of the adapting and the test stimuli differed by one octave. Our results suggest that the human auditory system possesses specialized mechanisms for detecting auditory motion in the spatial domain.

Underwater acoustic ambient noise levels on the eastern Canadian continental shelf

This paper reports the analysis of shallow‐water ambient noise levels collected by Defence Research Establishment Atlantic during 14 cruises over the period 1972 to 1985. A weighted average is formed to de‐bias the samples, with the aim of answering the question: ‘‘If one were to pick a site randomly on the eastern Canadian continental shelf at a random time, and perform an ambient noise measurement, what would be the expected noise level?’’ The samples are also grouped according to whether they were taken on the Scotian Shelf, the Grand Banks, or the Flemish Cap, and according to season. The frequency range covered is 30 to 900 Hz. The weighted mean and standard deviation of the noise levels are presented, as well as the correlation coefficient between the noise levels and wind speed. The results show that the eastern Canadian continental shelf as a whole presents levels that are characteristic of areas with high shipping density and good acoustic propagation, with the Scotian Shelf showing generally hig...

Method for estimating the distance of an acoustic signal

A system and method for detecting the range of an acoustic source within a reverberant space. The invention includes a method, apparatus, and computer program to determine whether a sound source is situated near or far from a pair of microphones situated within a reverberant space. A signal detector is applied to the data; only the data that passes the signal detector is processed any further. The signal at the two microphones is compared to obtain the angular distribution of acoustic power. The concentration of acoustic power in the direct path versus reverberant paths is determined and used to compute a direct-to-reverberant ratio. If this ratio is greater than a selected threshold, the sound source is determined to be near the microphones. Otherwise, the sound source is determined to be far from the microphones.

Aural intensity for a moving source

Considerable, highly specific information is available to the auditory system concerning the trajectory of a moving sound source. This paper delineates the set of stimuli that motion-sensitive systems might use. General expressions for the sound intensity, the interaural intensity difference, and their first time derivatives, are derived for a source moving along an arbitrary trajectory. The general expressions are then made explicit for three special cases of motion of an omnidirectional constant level source: a source moving directly away from or toward the observer, a source moving around the observers head, and a source moving in a straight line across the auditory field of the observer. The later special case combine characteristics of the two first ones. The functions are plotted and their characteristics compared. The combination of all four functions provides a unique signature for each source trajectory. The first time derivative of the monaural spectrum level function is found to be directly proportional to the velocity scaled by the distance of the source for omnidirectional sources of constant intensity. This makes the first time derivative of the spectrum level especially attractive as a component of a specialized source detection system in the brain.

Matched‐field inversion for source location and optimal equivalent bathymetry

Matched‐field processing (MFP) for source localization can be highly sensitive to mismatch between the true bathymetry and the bathymetry model used to compute the replica fields. This paper presents an efficient algorithm to invert for source location and an optimal representation of the (range‐dependent) bathymetry which is applicable whenever the adiabatic normal‐mode model applies. In the adiabatic mode formulation, the range dependence of the bathymetry between source and receiver is completely contained in range integrals of the wave numbers. Parametrizing these integrals by a single‐pole expansion maps all bathymetries which have an equivalent effect on the propagation into as little as a single parameter, resulting in a tremendous reduction in the dimensionality of the inverse problem. The resulting parameter space can then be searched for the bathymetry parameters which provide the best match with the measured acoustic fields. This search can be carried out very efficiently, and applied to optimi...

An artificial neural network for range and depth discrimination in matched field processing

Associative feedforward neural networks with no hidden layers were applied to the problem of localizing a source in range and depth using the acoustic signal arriving at a vertical array of sensors. A highly processed form of the signal (excitations of an orthogonal basis) was used as input in order to increase the robustness of the trained network. The output layer consisted of one unit for each possible range and one unit for each possible depth of the source. The networks were trained with a signal‐to‐noise ratio (S/N) at the hydrophone of 50 dB, and then their performance was evaluated with S/Ns of 50 and 0 dB. Network weights were found for narrow and broad target shapes that correspond to narrow and broad beamshapes. The narrow target produced the beamformer with the lowest sidelobes and highest gains with acceptable but somewhat higher sensitivity to noise. Performance in the region for which the network was trained compared favorably with minimum variance beamforming.

Matched-field localization for multiple sources in an uncertain environment, with application to Arctic ambient noise

This paper describes a general method to localize multiple broadband sources in a range- and azimuth-dependent environment when properties of the environment are not well known. Both the source locations and the unknown environmental parameters are included as variables in a simulated annealing inversion that searches for the best match between the measured and modeled fields. The example used to illustrate the method involves localizing an unknown number of ice-ridge building events in an Arctic environment where the bathymetry is poorly known. A number of aspects of this problem are examined in a synthetic study, including the effect of errors in the bathymetry on localization, the importance of simultaneous localization, and determining the number of sources present. The inversion accurately localizes multiple synthetic sources, particularly in cases where the sources are spatially separated and the spectral shape is known. The inversion is also applied to a set of ambient noise measurements recorded i...

Directionality of ice cracking events

This paper presents the measured vertical directivity of ice cracking events extracted from a 341 s sample of ambient noise recorded under the polar ice pack. The sample was taken with a vertical linear array of four hydrophones suspended over the Arctic continental shelf in 500 m of water. The hydrophone spacing was 100 m. The samples from each hydrophone were bandpass filtered between 40 and 1250 Hz, and searched for corresponding peaks in the acoustic pressure. The acoustic events are believed to be due predominantly to thermal ice cracking. The angular distribution of the 765 arrivals detected has its maximum at 15 deg below the horizontal, with most of the arrivals occurring between 0 and 25 deg below the horizontal. An explanation for this observation is given in terms of a simple ray‐based model of the propagation of ice cracking noise. The asymmetry in the angular distribution is shown to be due mostly to the source directivity favoring bottom reflected and refracted rays arriving at negative angl...