Elevation Of Sound Source Research Articles

Model for estimating elevation of sound source in the median plane from ear-input signals

Model for estimating elevation of sound source in the median plane from ear-input signals

Binaural model for artificial spatial sound localization based on interaural time delays and movements of the interaural axis

This paper presents a mathematical model for sound localization in space using two-microphone devices that possess at least two degrees of freedom. It proves a series of theorems and lemmas that are based on time difference of arrival measurements and movements of the interaural axis, forming a powerful instrument for practical robot applications. For instance, it shows that a single determined rotation of the interaural axis is sufficient to exactly yield the azimuth or the elevation of an immobile sound source in the far field, independently of microphone spacing and the speed of sound and hence of the surrounding medium. It proves that at any moment the knowledge of one value determines the magnitude of the other, with the restriction that the sign of the second value is undefined, which means that, depending on the rotation, either the back-front or the up-down ambiguity is kept unsolved. This paper also shows that parallax motion unlocks essential information about the distance and the Cartesian coordinates of the sound source. Shifting the microphone system sideways fixes the distance and the coordinate on the interaural axis. Combining rotation and translation movements completely solves the localization problem. In order to illustrate the efficacy of the model, this paper presents experiments with a low cost robot developer kit during which the azimuth, the elevation, and the distance of continuous sound sources are determined at a precision of 10 degrees and 0.5 m, respectively. Achieving this performance with low power material demonstrates how easily the model can be implemented into any robotic system.

A pair of spectral notches which plays a role as a spectral cue in the vertical localization, and it's application to estimation of sound source elevation from binaural signals

The author [Applied Acoustics, 68, 835‐850 (2007)] proposed a parametric HRTF model for vertical sound localization. The parametric HRTF is recomposed only of the spectral peaks and notches extracted from the measured HRTF. The results of the median plane localization tests, which were carried out using the parametric HRTFs with various combinations of spectral peaks and notches, show that the pair of first and second notches (N1 and N2) above 5 kHz can be regarded as spectral cues. In this research, estimation of the elevation of sound source in the upper median plane by extracting N1 and N2 frequencies from binaural input signal was carried out. The kinds of sound sources were female voice, male voice, music, white noise, and pink noise. The results show that the estimation is accurate for almost of all the elevation and of all the kind of sound sources.

Sound localisation during illusory self-rotation

Auditory elevation localisation was investigated under conditions of illusory self-rotation (i.e., vection) induced by movement of wide-field visual stimuli around participants' z-axes. Contrary to previous findings which suggest that auditory cues to sound-source elevation are discounted during vection, we found little evidence that vection affects judgements of source elevation. Our results indicate that the percept of auditory space during vection is generally consistent with the available head-centered auditory cues to source elevation. Auditory information about the head-centered location of a source appears to be integrated, without modification, with visual information about head motion to determine the perceived exocentric location of the source.

Spectral integration time of the auditory localisation system

For the elevation and front-versus-back hemifield of a sound source to be accurately determined, the sound must contain a broad range of frequencies. Experiment 1 of this study examined the spectral integration time of the auditory localisation system by measuring the accuracy with which frequency-modulated (FM) tones of modulation periods ranging from 0.5 to 200 ms can be localised. For each of the four participants, judgements of sound–source elevation and front–back hemifield were most accurate for a modulation period of 5 ms. Accuracy levels for the 5 ms modulation period approached those for a pink-noise stimulus. This suggests that the spectral integration time of the auditory localisation system is around 5 ms. Supporting evidence for this conclusion was sought in experiment 2, in which two participants localised noise stimuli that had magnitude spectra identical to those of 5 ms equivalent-rectangular-duration samples of the FM tones from experiment 1. For both participants, functions relating localisation error measures (i.e., elevation error and frequency of front–back confusion) to modulation period for spectrally matched noises were similar to those for FM tones.

Spatial orientation in the bushcricket Leptophyes punctatissima (Phaneropterinae; Orthoptera): III. Peripheral directionality and central nervous processing of spatial cues

We examined peripheral and central nervous cues underlying the ability of the bushcricket Leptophyes punctatissima to orient to elevated and depressed sound sources broadcasting the female acoustic reply. The peripheral spatial directionality of the ear was measured physiologically using monaural preparations of an auditory interneuron (T-fibre). In the azimuth, maximal interaural intensity differences of 18 dB occur between ipsi- and contralateral stimulation. With increasing elevation or depression of the sound sources, IIDs decrease systematically and reach zero with the source exactly above or below the preparation. Bilateral, simultaneous recordings of the activity of the pair of interneurons allowed determining the binaural discharge differences which occur in response to the extremely short (1 ms) female reply. These discharge differences are large (four action potentials/stimulus) and reliable in the azimuth with lateral stimulation, and decrease gradually with more frontal stimulation. With elevation and depression of sound sources these differences again decrease to one action potential/stimulus at 60 degrees or 75 degrees elevation, and lateral stimulus angles of about 60 degrees . We also calculated the reliability with which a receiver could correctly determine the location of the sound source. We discuss these quantitative measures in relation to the spatial phonotactic behaviour of male L. punctatissima.

The ability of the parasitoid fly Ormia ochracea to distinguish sounds in the vertical plane

The parasitic fly Ormia ochracea localizes its host, field crickets, by homing in on their calling song. Previous phonotactic studies indicate that their sound localization ability in azimuth is extraordinarily acute, but the fly's ability to localize the elevation of sound sources has not been tested to date. Here we show that in a freely-walking closed-loop Y-maze task elevational performance is well above chance, but slightly below the fly's performance in azimuth. Immobilizing the head or the halteres (sensory organs of balance) slightly lowered elevational discrimination, but performance was still well above chance. Because ormiine ears are thought to be symmetric and the pure-tone models of a cricket's call used in these experiments contained little to no spectral bandwidth, additional studies will be needed to elucidate the underlying cues. Nonetheless, it is clear that while walking, Ormia flies are capable of distinguishing sound sources that differ only in elevation.

Relearning Sound Localization with a New Ear

Human sound localization results primarily from the processing of binaural differences in sound level and arrival time for locations in the horizontal plane (azimuth) and of spectral shape cues generated by the head and pinnae for positions in the vertical plane (elevation). The latter mechanism incorporates two processing stages: a spectral-to-spatial mapping stage and a binaural weighting stage that determines the contribution of each ear to perceived elevation as function of sound azimuth. We demonstrated recently that binaural pinna molds virtually abolish the ability to localize sound-source elevation, but, after several weeks, subjects regained normal localization performance. It is not clear which processing stage underlies this remarkable plasticity, because the auditory system could have learned the new spectral cues separately for each ear (spatial-mapping adaptation) or for one ear only, while extending its contribution into the contralateral hemifield (binaural-weighting adaptation). To dissociate these possibilities, we applied a long-term monaural spectral perturbation in 13 subjects. Our results show that, in eight experiments, listeners learned to localize accurately with new spectral cues that differed substantially from those provided by their own ears. Interestingly, five subjects, whose spectral cues were not sufficiently perturbed, never yielded stable localization performance. Our findings indicate that the analysis of spectral cues may involve a correlation process between the sensory input and a stored spectral representation of the subject's ears and that learning acts predominantly at a spectral-to-spatial mapping level rather than at the level of binaural weighting.

Spatial sound resolution of an interpolated HRIR library

The paper evaluates the human directional resolution of virtual sound sources synthesised with the aid of a generalised head related impulse response (HRIR) library, i.e., an HRIR library measured using a dummy head and torso. The original HRIR set is first expanded using linear interpolation, and then directional resolution measurements are performed for playback through headphones. These results are compared to the results obtained using loudspeakers as sound sources in an anechoic chamber. Directional resolution is the ability of listeners to distinguish two closely-spaced sound sources alternately playing the same signal. Experiments show that two sound sources with insufficient spacing appear as a single source to the listener. Directional resolution for small azimuth changes is relatively high for both virtual and real sound sources. Most test subjects have no problem resolving two sound sources only 5° apart. Compared to real sound sources, detecting changes in elevation of virtual sound sources is much less accurate, which may be the main drawback of using a generalised HRIR library.

Contribution of head shadow and pinna cues to chronic monaural sound localization.

Monaurally deaf people lack the binaural acoustic difference cues in sound level and timing that are needed to encode sound location in the horizontal plane (azimuth). It has been proposed that these people therefore rely on spectral pinna cues of their normal ear to localize sounds. However, the acoustic head-shadow effect (HSE) might also serve as an azimuth cue, despite its ambiguity when absolute sound levels are unknown. Here, we assess the contribution of either cue in the monaural deaf to two-dimensional (2D) sound localization. In a localization test with randomly interleaved sound levels, we show that all monaurally deaf listeners relied heavily on the HSE, whereas binaural control listeners ignore this cue. However, some monaural listeners responded partly to actual sound-source azimuth, regardless of sound level. We show that these listeners extracted azimuth information from their pinna cues. The better monaural listeners were able to localize azimuth on the basis of spectral cues, the better their ability to also localize sound-source elevation. In a subsequent localization experiment with one fixed sound level, monaural listeners rapidly adopted a strategy on the basis of the HSE. We conclude that monaural spectral cues are not sufficient for adequate 2D sound localization under unfamiliar acoustic conditions. Thus, monaural listeners strongly rely on the ambiguous HSE, which may help them to cope with familiar acoustic environments.

Utility of Monaural Spectral Cues Is Enhanced in the Presence of Cues to Sound-Source Lateral Angle

The contention that normally binaural listeners can localize sound under monaural conditions has been challenged by Wightman and Kistler (J. Acoust. Soc. Am. 101:1050-1063, 1997), who found that listeners are almost completely unable to localize virtual sources of sound when sound is presented to only one ear. Wightman and Kistler's results raise the question of whether monaural spectral cues are used by listeners to localize sound under binaural conditions. We have examined the possibility that monaural spectral cues provide useful information regarding sound-source elevation and front-back hemifield when interaural time differences are available to specify sound-source lateral angle. The accuracy with which elevation and front-back hemifield could be determined was compared between a monaural condition and a binaural condition in which a wide-band signal was presented to the near ear and a version of the signal that had been lowpass-filtered at 2.5 kHz was presented to the far ear. It was found that accuracy was substantially greater in the latter condition, suggesting that information regarding sound-source lateral angle is required for monaural spectral cues to elevation and front-back hemifield to be correctly interpreted.

Electrophysiologic correlates of direction and elevation cues for sound localization in the human brainstem: Correlaciones electrofisiológicas en las claves de dirección y elevación para la localizatión del sonido en el tallo cerebral humano

Our objective was to study the effects of sound source direction and elevation on human brainstem electrical activity associated with sound localization. The subjects comprised 15 normal-hearing and symmetrically hearing adults. Auditory brainstem evoked potentials (ABEPs) were recorded from three channels, in response to alternating-polarity clicks, presented at a rate of 21.1/s, at nine virtual spatial locations with different direction and elevation attributes. Equivalent dipoles of the binaural interaction components (BICs) of ABEPs were derived by subtracting the response to binaural clicks at each spatial location from the algebraic sum of monaural responses to stimulation of each ear in turn. The BICs included two major components corresponding in latency to the vertex—neck-recorded components V and VI of ABEP. A significant decrease of the first BICs equivalent dipole magnitude was observed for clicks in the horizontal frontal position (no elevation) in the midsagittal plane, and for clicks in the left—horizontal (no elevation) and right diagonally above the head (medium elevation) positions in the coronal plane, compared to clicks positioned directly above the head. Significant effects on equivalent dipole latencies of this component were found for front-back positions in the midsagittal plane and left-right positions in the coronal plane, compared to clicks positioned directly above the head. The most remarkable finding was a significant change in equivalent dipole orientations across stimulus conditions. We conclude that the changes in BIC equivalent dipole latency, amplitude and orientation across stimulus conditions reflect differences in the distribution of binaural pontine activity evoked by sounds in different spatial locations.Nuestro objetivo fue estudiar los efectos de la dirección y la elevación de la fuente sonora sobre la actividad eléctrica del tallo cerebral humano asociados a la localización del sonido. Se estudiaron 15 adultos con audición normal y simétrica. Se registraron potenciales evocados auditivos del tallo cerebral (ABEPs) en tres canales, en respuestas a clicks de polaridad alternante, presentados a una tasa de 21.2/seg, en nueve localizaciones espaciales virtuales, con diferentes atributos de dirección y elevación. Se derivaron dipolos equivalentcs de los componentes de interacción binaural (BICs) de los ABEPs, restando la respuesta a los clicks binaurales en cada localización espacial, de la suma algebraica de las respuestas monoanrales a la estimulación de cada oido. Los BICs incluyeron dos componentes mayores, correspondientes en latencia a los componentes V y VI de los ABEPs registrados en la derivación vertex-cuello. Se observó una disminución significativa en la magnitud del primer dipolo equivalente de los BICs, cuando en el piano medio sagital se utilizaron clicks en la posición horizontal-frontal (sin elevación), y para clicks en el piano horizontal izquierdo (sin elevación) y en la posición derecha diagonal (elevación media), en el piano coronal, comparada con clicks colocados directamente por encima de la cabcza. Se encontraron efectos significativos sobre las latencies de los dipolos equivalentes dc este componente en las posiciones de-frentc-atrás en el piano medio-sagital y en las posiciones izquierda-derecha en el piano coronal, comparados con clicks presentados directamente por encima de la cabeza. El hallazgo más importante fue un cambio significativo en la orientación de los dipolos equivalentes en las diversas condiciones del estímulo. Concluimos que los cambios en latencia, amplitud y orientación en los dipolos equivalentes de los BICs, en las diferentes condiciones de estimulo, reflejan diferencias en la distribución de la actividad protuberancial binaural evocada por sonidos en diferentes localizaciones espaciales.

Auditory processing of spectral cues for sound localization in the inferior colliculus.

The head-related transfer function (HRTF) of the cat adds directionally dependent energy minima to the amplitude spectrum of complex sounds. These spectral notches are a principal cue for the localization of sound source elevation. Physiological evidence suggests that the dorsal cochlear nucleus (DCN) plays a critical role in the brainstem processing of this directional feature. Type O units in the central nucleus of the inferior colliculus (ICC) are a primary target of ascending DCN projections and, therefore, may represent midbrain specializations for the auditory processing of spectral cues for sound localization. Behavioral studies confirm a loss of sound orientation accuracy when DCN projections to the inferior colliculus are surgically lesioned. This study used simple analogs of HRTF notches to characterize single-unit response patterns in the ICC of decerebrate cats that may contribute to the directional sensitivity of the brain's spectral processing pathways. Manipulations of notch frequency and bandwidth demonstrated frequency-specific excitatory responses that have the capacity to encode HRTF-based cues for sound source location. These response patterns were limited to type O units in the ICC and have not been observed for the projection neurons of the DCN. The unique spectral integration properties of type O units suggest that DCN influences are transformed into a more selective representation of sound source location by a local convergence of wideband excitatory and frequency-tuned inhibitory inputs.

Transformation of external-ear spectral cues into perceived delays by the big brown bat, Eptesicus fuscus.

The external-ear transfer function for big brown bats (Eptesicus fuscus) contains two prominent notches that vary from 30 to 55 kHz and from 70 to 100 kHz, respectively, as sound-source elevation moves from -40 to +10 degrees. These notches resemble a higher-frequency version of external-ear cues for vertical localization in humans and other mammals. However, they also resemble interference notches created in echoes when reflected sounds overlap at short time separations of 30-50 micros. Psychophysical experiments have shown that bats actually perceive small time separations from interference notches, and here we used the same technique to test whether external-ear notches are recognized as a corresponding time separation, too. The bats' performance reveals the elevation dependence of a time-separation estimate at 25-45 micros in perceived delay. Convergence of target-shape and external-ear cues onto echo spectra creates ambiguity about whether a particular notch relates to the object or to its location, which the bat could resolve by ignoring the presence of notches at external-ear frequencies. Instead, the bat registers the frequencies of notches caused by the external ear along with notches caused by the target's structure and employs spectrogram correlation and transformation (SCAT) to convert them all into a family of delay estimates that includes elevation.

Auditory localization using direction-dependent spectral information

This work presents a biologically motivated neuronal model for detecting the elevation of unfamiliar natural sound sources using monoaural cues, based on head-related-transfer functions. This model can determine the elevation of an unfamiliar sound source to within less than 4° with no error using very small number of training samples. In addition, we suggest that the approximate logarithmic response of the cells in the cochlea is beneficial for localizing unfamiliar sound sources.

Cortical Mechanisms for Auditory Spatial Illusions

Frequency transformation by the external ears provides the spectral cues for localization of broadband sounds in the vertical plane. When human subjects listen to spectrally-impoverished narrowband sounds presented in a free field, the perceived locations vary with the centre frequency and are largely independent of the actual source locations. The present study explored the substrate of spatial illusion by examining the responses of cortical neurons to narrowband stimuli. Single-unit responses were recorded in area A2 of anaesthetized cats. Broadband noise bursts were presented at 14 locations in the vertical median plane, from 60° below the front horizon, up and over the head, to 20° below the rear horizon. Narrowband (1/6-oct) noise bursts were presented at +80° elevation. An artificial neural network was trained to recognize the spike patterns elicited by broadband noise and, thereby, to register the spike patterns with sound-source elevation. When the trained network was presented with neural responses elicited by narrowband noise, the elevation estimated by the neural network varied with the centre frequency of the narrowband stimuli. Consistent with psychophysical results in human, the locations associated with a given centre frequency could be predicted by comparing the stimulus spectrum with the directional transfer functions of the cat's external ear. The results support the hypothesis that full spike patterns (including spike counts and spike timing) of cortical neurons code information about sound location and that the auditory cortical neurons play a pivotal role in localization behaviour.

Role of acoustic striae in hearing: discrimination of sound-source elevation

After years of systematic experimentation, we finally uncovered one thing the dorsal system contributes to hearing which the ventral system may not – the mechanism for orienting to an elevated sound source [Sutherland, D.P., Masterton, R.B., Glendenning, K.K. (1998) Behav. Brain Res. in press]. This paper follows up this one positive result on a historical background of uniformly negative results. The focus of this report is on the fusiform cells of the dorsal cochlear nucleus whose axons course through the dorsal acoustic stria (DAS). Because electrophysiological studies have shown that the cues for sensing the elevation of a sound source would seem to be best analyzed by the dorsal cochlear nucleus, we tested, behaviorally, normal cats and cats deprived of their DAS or intermediate acoustic stria, bilaterally or ipsilaterally (with or without their contralateral ear deafened), for their ability to orient to elevated sources of broad-band noise. For behavioral testing, we made use of a conventional shock-avoidance procedure. The results lead to the conclusion that DCN and DAS may play no role in learned elevation discriminations. This result builds on that of another of our papers which suggests that a deficit in reflexive discrimination of elevation is strictly auditory in nature [Sutherland, D.P., Masterton, R.B., Glendenning, K.K. (1998) Behav. Brain Res. in press].

Auditory-nerve encoding of pinna-based spectral cues: rate representation of high-frequency stimuli.

The elevation of sound sources and their front-back position is encoded in spectral cues produced by direction-dependent filtering in the pinna. Auditory-nerve (AN) fiber population recordings were used to analyze the neural representation of the acoustic features which carry this information. The most prominent pinna-produced spectral features occur at frequencies greater than 5 kHz, so this information must be encoded in AN discharge rates and not in measures of phase locking. However, profiles of discharge rate versus fiber best frequency (BF) reveal a poor representation of the spectra of the stimuli, primarily because of fiber-to-fiber variation in rate. The variation is not controlled by rate normalization, but a clear representation of the ratio of the magnitude spectra of two stimuli is seen when responses are plotted as the difference between the rates in response to the two stimuli. This results suggests that precise information about stimulus spectrum is present in discharge rate, which could be revealed in rate profiles constructed with suitable normalization. When binaural stimuli are presented, a weak inhibitory effect, due to the olivocochlear bundle or the middle ear muscle reflex, is observed. The rate changes are small and are not correlated with the spectrum of the contralateral ear stimulus.

Bilateral inhibition generates neuronal responses tuned to interaural level differences in the auditory brainstem of the barn owl

I investigated the neural algorithms by which neurons gain selectivity for interaural level difference in the brainstem of the barn owl (Tyto alba). Differences in the timing and in the level of sounds at the ears are used by this owl to encode, respectively, azimuthal and vertical position of sound sources in space. These two cues are processed in two parallel neural pathways. Below the level of the inferior colliculus, all neurons in the pathway that processes level differences show responses to this cue that are monotonic, and thus not selective for a particular level difference. Only in the inferior colliculus, which contains a map of auditory space, are neurons sharply tuned to specific interaural level differences. How are these response properties generated from those of the nuclei that provide input to the inferior colliculus? I show that the posterior subdivision of the nucleus ventralis lemnisci lateralis (VLVp) projects bilaterally to the lateral shell of the central nucleus of the inferior colliculus, the input stage to the map of auditory space. Both these nuclei are part of the pathway that processes interaural level differences. Manipulations of the responses in VLVp affected the responses to level differences in the inferior colliculus; responses to time differences were unaffected. By systematically increasing or decreasing neural activity in VLVp, I show that the VLVp on each side provides inhibition to the colliculus at large level differences. This results in a peaked response that is tuned to level differences in the inferior colliculus. Some cells in the lateral shell of the inferior colliculus appear to receive direct GABAergic inhibition from VLVp. I suggest that this circuitry and the algorithms it supports are the neural substrates that allow the barn owl to exploit level differences for computation of sound source elevation.

Neurons in the inferior colliculus of cats sensitive to sound-source elevation

The sensitivity to variations in sound-source elevation was studied in 48 units, previously examined as to their azimuthal sensitivity, of the inferior colliculi of cats. Of these units, 36 were directionally-sensitive (firing rate varied by more than 50% across the range of positions studied) to both azimuthal and elevational changes. Elevation sensitivity was common to noise stimuli (19/25 units) and pure tones in excess of 6 kHz (17/17 units). Not one of the 8 azimuth-sensitive units with CFs below 6 kHz was directionally-sensitive to the elevation of CF stimuli. The 4 units omnidirectional to azimuthal variation were similarly insensitive to elevation. The shapes of functions relating sound-source elevation to spike count (elevation functions) varied across an apparent continuum, with some very sharply-peaked functions being observed. Peak spike counts almost invariably occurred at stimulus elevations above the horizontal plane. Comparisons of the widths of elevation and azimuth functions at the same sound pressure level were made for 36 units. The relative sharpness of elevation and azimuthal tuning varied across the population. The common association of sensitivity to both azimuth and elevation suggests that elevation sensitivity may be mediated partly by binaural comparisons.