Sound pressure transformations by the head and pinnae of the adult Chinchilla (Chinchilla lanigera)

There are three main cues to sound location: the interaural differences in time (ITD) and level (ILD) as well as the monaural spectral shape cues. These cues are generated by the spatial- and frequency-dependent filtering of propagating sound waves by the head and external ears. Although the chinchilla has been used for decades to study the anatomy, physiology, and psychophysics of audition, including binaural and spatial hearing, little is actually known about the sound pressure transformations by the head and pinnae and the resulting sound localization cues available to them. Here, we measured the directional transfer functions (DTFs), the directional components of the head-related transfer functions, for 9 adult chinchillas. The resulting localization cues were computed from the DTFs. In the frontal hemisphere, spectral notch cues were present for frequencies from ～6-18?kHz. In general, the frequency corresponding to the notch increased with increases in source elevation as well as in azimuth towards the ipsilateral ear. The ILDs demonstrated a strong correlation with source azimuth and frequency. The maximum ILDs were <10?dB for frequencies <5?kHz, and ranged from 10-30?dB for the frequencies >5?kHz. The maximum ITDs were dependent on frequency, yielding 236?μs at 4?kHz and 336?μs at 250?Hz. Removal of the pinnae eliminated the spectral notch cues, reduced the acoustic gain and the ILDs, altered the acoustic axis, and reduced the ITDs.     

Differential Effect of Near-Threshold Stimulus Intensities on Sound Localization Performance in Azimuth and Elevation in Normal Human Subjects

The ability of humans to localize sounds remains relatively constant across a range of intensities well above detection threshold, and increasing the spectral content of the stimulus results in an improvement in localization ability. For broadband stimuli, intensities near detection threshold result in fewer and weaker binaural cues used in azimuth localization because the stimulus energy at the high- and low-frequency ends of the audible spectrum fall below detection threshold. Thus, the ability to localize broadband sounds in azimuth is predicted to be degraded at audible but near threshold stimulus intensities. The spectral cues for elevation localization (spectral peaks and notches generated by the head-related transfer function) span a narrower frequency range than those for azimuth. As the stimulus intensity decreases, the ability to detect the stimulus frequencies corresponding to the spectral notches will be more strongly affected than the ability to detect frequencies outside the range where these spectral cues are useful. Consequently, decreasing the stimulus intensity should degrade localization in both azimuth and elevation and create a greater deficit in elevation localization due to the narrower band of audible frequencies containing elevation cues compared to azimuth cues. The present study measured the ability of 11 normal human subjects to localize broadband noise stimuli along the midsagittal plane and horizontal meridian at stimulus intensities of 14, 22, and 30 dB above the subject's detection threshold using a go/no-go behavioral paradigm. Localization ability decreased in both azimuth and elevation with decreasing stimulus intensity, and this effect was greater on localization in elevation than on localization in azimuth. The differential effects of stimulus intensity on sound localization in azimuth and elevation found in the present study may provide a valuable tool in investigating the neural correlates of sound location perception.     

Head-related transfer functions of the Rhesus monkey

Head-related transfer functions (HRTFs) are direction-specific acoustic filters formed by the head, the pinnae and the ear canals. They can be used to assess acoustical cues available for sound localization and to construct virtual auditory environments. We measured the HRTFs of three anesthetized Rhesus monkeys (Macaca mulatta) from 591 locations in the frontal hemisphere ranging from -90 degrees (left) to 90 degrees (right) in azimuth and -60 degrees (down) to 90 degrees (up) in elevation for frequencies between 0.5 and 15 kHz. Acoustic validation of the HRTFs shows good agreement between free field and virtual sound sources. Monaural spectra exhibit deep notches at frequencies above 9 kHz, providing putative cues for elevation discrimination. Interaural level differences (ILDs) and interaural time differences (ITDs) generally vary monotonically with azimuth between 0.5 and 8 kHz, suggesting that these two cues can be used to discriminate azimuthal position. Comparison with published subsets of HRTFs from squirrel monkeys (Saimiri sciureus) shows good agreement. Comparison with published human HRTFs from the frontal hemisphere demonstrates overall similarity in the patterns of ILD and ITD, suggesting that the Rhesus monkey is a good acoustic model for these two sound localization cues in humans. Finally, the measured ITDs in the horizontal plane agree well between -40 degrees and 40 degrees in azimuth with those calculated from a spherical head model with a radius of 52 mm, one-half the interaural distance of the monkey.     

Sound source localization

Sound source localization is paramount for comfort of life, determining the position of a sound source in 3 dimensions: azimuth, height and distance. It is based on 3 types of cue: 2 binaural (interaural time difference and interaural level difference) and 1 monaural spectral cue (head-related transfer function). These are complementary and vary according to the acoustic characteristics of the incident sound. The objective of this report is to update the current state of knowledge on the physical basis of spatial sound localization.     

Acoustic Cues for Sound Source Distance and Azimuth in Rabbits, a Racquetball and a Rigid Spherical Model

There are numerous studies measuring the transfer functions representing signal transformation between a source and each ear canal, i.e., the head-related transfer functions (HRTFs), for various species. However, only a handful of these address the effects of sound source distance on HRTFs. This is the first study of HRTFs in the rabbit where the emphasis is on the effects of sound source distance and azimuth on HRTFs. With the rabbit placed in an anechoic chamber, we made acoustic measurements with miniature microphones placed deep in each ear canal to a sound source at different positions (10–160 cm distance, ±150° azimuth). The sound was a logarithmically swept broadband chirp. For comparisons, we also obtained the HRTFs from a racquetball and a computational model for a rigid sphere. We found that (1) the spectral shape of the HRTF in each ear changed with sound source location; (2) interaural level difference (ILD) increased with decreasing distance and with increasing frequency. Furthermore, ILDs can be substantial even at low frequencies when distance is close; and (3) interaural time difference (ITD) decreased with decreasing distance and generally increased with decreasing frequency. The observations in the rabbit were reproduced, in general, by those in the racquetball, albeit greater in magnitude in the rabbit. In the sphere model, the results were partly similar and partly different than those in the racquetball and the rabbit. These findings refute the common notions that ILD is negligible at low frequencies and that ITD is constant across frequency. These misconceptions became evident when distance-dependent changes were examined.     

Directionality of sound pressure transformation at the pinna of echolocating bats

The directionality of sound pressure transformation at the pinna of three species of bats was studied by measuring the sound pressure level of a tone (25 45 65 and 85 kHz) at the tympanic membrane as a function of azimuth and elevation of the sound source under free-field conditions. The tympanic sound pressure level varied with location of the sound source. The directionality of sound pressure transformation pattern of the pinna of each bat was studied by plotting isopressure contours. The area within each isopressure contour decreased with increasing tonal frequency. For each tonal frequency, the point of maximal sound pressure was always located in the frontal ipsilateral sound field. This point shifted medially with increasing tonal frequency along the horizontal plane in all species tested, but it shifted in a species-specific manner along the vertical plane. Removal or distortion of the pinna and tragus resulted in either uncircumscribed or irregular isopressure contours for all tonal frequencies tested. Acoustic pressure gain of the external ear reached 16-23 dB for frequencies at 15-18 kHz. The importance of the external ear to the directionality of the bat's echolocation system is discussed.     

Effect of middle ear fluid on sound transmission and auditory brainstem response in guinea pigs

Combined measurements of middle ear transfer function and auditory brainstem response (ABR) in live guinea pigs with middle ear effusion (MEE) are reported in this paper. The MEE model was created by injecting saline into the middle ear cavity. Vibrations of the tympanic membrane (TM), the tip of the incus, and the round window membrane (RWM) were measured with a laser vibrometer at frequencies of 0.2-40?kHz when the middle ear fluid increased from 0 to 0.2?ml (i.e., full fill of the cavity). The click and pure tone ABRs were recorded as the middle ear fluid increased. Fluid introduction reduced mobility of the TM, incus and RWM mainly at high frequencies (f?>?1?kHz). The magnitude of this reduction was related to the volume of fluid. The displacement transmission ratio of the TM to incus varied with frequency and fluid level. The volume displacement ratio of the oval window to round window was approximately 1.0 over most frequencies. Elevation of ABR thresholds and prolongation of ABR latencies were observed as fluid level increased. Reduction of TM displacement correlated well with elevation of ABR threshold at 0.5-8?kHz. Alterations in the ratio of ossicular displacements before and after fluid induction are consistent with fluid-induced changes in complex ossicular motions.     

Masked threshold changes associated with angular separation of noise and signal sources

Threshold changes associated with separating a signal source and a masking white noise source from 0 degree to 90 degrees were determined for 0.5, 1 and 8 kHz pure tones and click trains. No changes occurred for the 0.5 and 1 kHz pure tones. Masked thresholds of 8 kHz pure tones and click trains decreased linearly by 9 and 13 dB respectively as angular separation was moved from 0 degree to 90 degrees. Changes in click train stimuli masked thresholds did not change significantly when the ear directed toward the masking source was occluded (11 dB drop at 90 degrees). The absence of changes at low frequencies and the similarity in magnitude of the changes in signals containing high frequency components with the responses to the monaural click trains, suggests that the threshold changes can be attributed to a head shadow effect. The casting of a sound shadow effectively lowers the noise level on the shielded side. These findings question the importance of cross-correlation techniques when detecting signals in noise.     

Directional properties of the auditory periphery in the guinea pig

The directional sensitivity of the outer ear of the guinea pig was determined by recording changes in the amplitude of the cochlear microphonic to frequencies between 1 and 20 kHz as the location of the sound source was changed throughout 360 degrees of horizontal auditory space. The directional responses to frequencies below 3 kHz were almost omnidirectional. The directional responses for frequencies between 3 and 12 kHz were progressively more directional toward the anterior midline. The responses for frequencies above 12 kHz were highly directional along the ipsilateral interaural axis. In contrast, the directional responses to all frequencies in animals whose pinnae had been removed were orientated along the ipsilateral interaural axis. The observations suggest that the orientation and strength of the directional response of the auditory periphery in the guinea pig are dependent on frequency and that this dependence is attributable, at least in part, to the acoustic properties of the pinna. The observations also indicate that there is a substantial change in the interaural intensity difference at various frequencies and in the spectral transfer function of the ear according to the location of the sound source in the ipsilateral hemifield. The observation that these changes are asymmetrical about the interaural axis for a substantial part of the auditory range of the animal is consistent with the hypothesis that the frequency dependent directionality of the auditory periphery provides a spectral cue for the localization of broad band sounds in the free field.     

Direction-dependent amplification of the human outer ear

The transfer function of the outer ear in humans was determined by using the impulse technique. Signals were delivered from 325 or 393 positions on an imaginary sphere surrounding the experimental subject. Changes of sound pressure level in the ear canal show that certain frequency bands are amplified maximally if they impinge onto the ear from certain directions. For some frequency bands there are two directions of sound incidence that cause best amplification in the ear canal. The directionality of the human pinna appears to increase at higher frequencies. The amount of amplification by the outer ear of frequencies between 2 and 15 kHz was also determined by measuring the free-field transfer function.     

Concurrent Development of the Head and Pinnae and the Acoustical Cues to Sound Location in a Precocious Species,the Chinchilla (<Emphasis Type="Italic">Chinchilla lanigera)</Emphasis>

Sounds are filtered in a spatial- and frequency-dependent manner by the head and pinna giving rise to the acoustical cues to sound source location. These spectral and temporal transformations are dependent on the physical dimensions of the head and pinna. Therefore, the magnitudes of binaural sound location cues—the interaural time (ITD) and level (ILD) differences—are hypothesized to systematically increase while the lower frequency limit of substantial ILD production is expected to decrease due to the increase in head and pinna size during development. The frequency ranges of the monaural spectral notch cues to source elevation are also expected to decrease. This hypothesis was tested here by measuring directional transfer functions (DTFs), the directional components of head-related transfer functions, and the linear dimensions of the head and pinnae for chinchillas from birth through adulthood. Dimensions of the head and pinna increased by factors of 1.8 and 2.42, respectively, reaching adult values by ~6 weeks. From the DTFs, the ITDs, ILDs, and spectral shape cues were computed. Maximum ITDs increased by a factor of 1.75, from ~160 μs at birth (P0-1, first postnatal day) to 280 μs in adults. ILDs depended on source location and frequency exhibiting a shift in the frequency range of substantial ILD (>10 dB) from higher to lower frequencies with increasing head and pinnae size. Similar trends were observed for the spectral notch frequencies which ranged from 14.7–33.4 kHz at P0-1 to 5.3–19.1 kHz in adults. The development of the spectral notch cues, the spatial- and frequency-dependent distributions of DTF amplitude gain, acoustic directionality, maximum gain, and the acoustic axis were systematically related to the dimensions of the head and pinnae. The dimension of the head and pinnae in the chinchilla as well as the acoustical properties associated with them are mature by ~6 weeks.     

Localization of Click Trains and Speech by Cats: the Negative Level Effect

Although localization of sound in elevation is believed to depend on spectral cues, it has been shown with human listeners that the temporal features of sound can also greatly affect localization performance. Of particular interest is a phenomenon known as the negative level effect, which describes the deterioration of localization ability in elevation with increasing sound level and is observed only with impulsive or short-duration sound. The present study uses the gaze positions of domestic cats as measures of perceived locations of sound targets varying in azimuth and elevation. The effects of sound level on localization in terms of accuracy, precision, and response latency were tested for sound with different temporal features, such as a click train, a single click, a continuous sound that had the same frequency spectrum of the click train, and speech segments. In agreement with previous human studies, negative level effects were only observed with click-like stimuli and only in elevation. In fact, localization of speech sounds in elevation benefited significantly when the sound level increased. Our findings indicate that the temporal continuity of a sound can affect the frequency analysis performed by the auditory system, and the variation in the frequency spectrum contained in speech sound does not interfere much with the spectral coding for its location in elevation.     

A combined sensitivity for frequency and interaural intensity difference in neurons in the auditory midbrain of the grassfrog

The relation between spectral tuning and sensitivity for interaural intensity difference (IID) was studied for single units in the auditory midbrain of the grassfrog. The stimuli consisted of sequences of pure tones of different frequency and interaural intensity differences presented by means of a closed sound system. At best excitatory frequency, three types of binaural interaction were observed: E0 (one ear excitatory 23%), EE (both ears excitatory 9%) and EI (one ear excitatory, the other inhibitory 67%). For a considerable number of units different types of binaural interaction were observed for different stimulus frequencies. More than 30% of the binaural units had multiple excitatory and inhibitory regions in their spectrotemporal selectivity. E0 and EI units had uniformly distributed best frequencies, EE units generally had best frequencies near 1.0 kHz. The E0 and EE categories had response latencies less than about 70 ms whereas EI units could have longer latencies. Most EE and all EI category units had sigmoidally shaped IID-rate curves. About 40% of the units had a combined sensitivity for sound spectrum and IID which was invariant to overall stimulus intensity. For nearly all EI units the inhibitory influence of the ipsilateral ear was confined to frequencies in the 0.4-1.6 kHz range and was not correlated with a unit's best frequency. By means of a simple additive model we demonstrated that determination of sound source laterality can be achieved by ensemble coding in the auditory midbrain.     

Localization of pure tones and click trains by untrained humans

Minimum audible angles (m.a.a.s) of untrained subjects were measured in a room using pure tone (0.5 to 8 kHz) and click train (noise) stimuli (two alternative, forced-choice, constant stimulus with feedback and head movements permitted, horizontal plane, 0 degree azimuth). The m.a.a.s and standard deviations (SD) were 3.0 degrees +/- 5.2 degrees for click trains and 10.9 degrees +/- 21.0 degrees for pure tones. The m.a.a.s did not vary significantly with frequency. The m.a.a.s and their SDs matched values reported from localization error studies. Narrowing the testing range from 32 degrees to 8 degrees resulted in random responses to the pure tones, though the click trains were readily localized. One subject presented with 2500 trials using an 8 kHz pure tone (with feedback, 16 degrees range) increased her responses from random to 88% correct during the testing. The click train m.a.a.s probably reflect the typical noise localizational abilities of the general population. For pure-tone m.a.a.s, experience/training may result in improved accuracy not applicable to the general public. The presence of a well defined time clue and a broad bandwidth sound results in significantly lower m.a.a.s than were obtained using pure tones which presumably present only interaural phase or intensity clues.     

Sound localization cues of binaural hearing

The ability to localize sound sources in space is of considerable importance to the human safety- and survival-system. Consequently the current scientific interest in improving the safety-standard i. e. in air-traffic control has provided a new momentum for investigating spatial hearing. This review deals with the nature and the relative salience of the localization cues. Localization refers to judgements of the direction and distance of a sound source but here we will deal with direction only. We begin with a short introduction into the so-called Duplex theory which dates back to John William Strutt (later Lord Rayleigh). The idea is that sound localization is based on interaural time differences (ITD) at low frequencies and interaural level differences (ILD) at high frequencies. If the head remains stationary neither a given ITD nor an ILD can sufficiently define the position of a sound source in space. On such a theoretical basis cones of confusion which open outward from each ear can be predicted ambiguously projecting any source on the surface of such a cone onto an interaural axis. Our restricted ability at localizing sound sources in the vertical median plane is another example of possible ambiguity. At the end of the 19th century scientists already realized that occlusion of the pinnae cavities decreases localization competence. As a result of later achievements in physics and signal-theory it became more obvious that the pinnae may provide an additional cue for spatial hearing and that the outer ear together with the head and the upper torso form a sophisticated direction-dependent filter. The action of such a filter is mathematically described by the so-called Anatomical Transfer Function (ATF). The spectral patterning of the sound produced by the pinnae and the head is most effective when the source has spectral energy over a wide range and contains frequencies above 6 kHz, that is it contains wavelengths short enough to interact with the anatomical characteristics of the outer ears. Scientific findings further suggest that spectral patterns like peaks and notches may also be exploited monaurally, albeit an a priori-knowledge at the central-auditive level concerning the corresponding transfer functions and relevant real-world sounds is required. Binaural spectral cues are more likely to play a major role in localization. They are derived from another transfer function, the so-called Interaural Transfer Function (ITF), being the ratio of the ATFs at the two ears. The contributions of all these cues may sometimes not be enough to prevent the listener from opting for the wrong direction. But things can be eased by allowing head-movements: More than 60 years ago science reasoned that small head movements could provide the information necessary to resolve most of the ambiguities. Recent studies have proved that these findings have been accurate all along.     

The Effect of Ear Canal Orientation on Tympanic Membrane Motion and the Sound Field Near the Tympanic Membrane

The contribution of human ear canal orientation to tympanic membrane (TM) surface motion and sound pressure distribution near the TM surface is investigated by using an artificial ear canal (aEC) similar in dimensions to the natural human ear canal. The aEC replaced the bony ear canal of cadaveric human temporal bones. The radial orientation of the aEC relative to the manubrium of the TM was varied. Tones of 0.2 to 18.4 kHz delivered through the aEC induced surface motions of the TM that were quantified using stroboscopic holography; the distribution of sound in the plane of the tympanic ring P_TR was measured with a probe tube microphone. The results suggest that the ear canal orientation has no substantial effect on TM surface motions, but P_TR at frequencies above 10 kHz is influenced by the ear canal orientation. The complex TM surface motion patterns observed at frequencies above a few kilohertz are not correlated with simpler variations in P_TR distribution at the same frequencies, suggesting that the complex sound-induced TM motions are more related to the TM mechanical properties, shape, and boundary conditions rather than to spatial variations in the acoustic stimulus.     

Interaural intensity differences in the cat: changes in sound pressure level at the two ears associated with azimuthal displacements in the frontal horizontal plane

The changes in sound pressure level (SPL) at the two ears produced by azimuthal displacements of a free-field sound source in the interaural horizontal plane in frontal space were measured in anesthetized cats and the resultant interaural intensity differences (IIDs) were derived by subtraction. The mean curves at different frequencies indicate that azimuthal displacements from the median sagittal plane are generally associated with increases in SPL at the near (directly irradiated) ear and decreases at the far (shadowed) ear. Detailed measurements at 1 degree and 2 degrees azimuthal steps in some animals revealed that at frequencies above 6-8 kHz there were marked variations in tympanic SPL at the far ear at lateral azimuths; these variations are apparently produced by interference between sets of 'creeping' surface waves propagated around the head in each direction. Near-ear changes were generally non-monotonic, becoming smaller or, in some cases, negative at large azimuthal displacements. At some frequencies above 8-10 kHz, functions for individual animals exhibited sharp decreases in level at the near ear (and consequently negative IIDs) at small azimuthal displacements. The general characteristics of the near-ear changes are in accord with previous evidence on the directionality of pinna amplification effects, although the sharp notches at some frequencies and azimuths presumably reflect more complex resonance and interference phenomena. Differences in the IID-azimuth relationship at different frequencies correlate well with data on the acuity of localization of high-frequency tones by cats.     

Brain Stem Potentials Evoked by Binaural Click Stimuli with Differences in Interaural Time and Intensity

Auditory-evoked brain stem potentials were recorded from 12 adults with normal hearing using click stimuli with differences in interaural time and intensity. Almost independent superimposed Jewett V peaks were produced, whose latency and amplitude depended on the parameters of the stimulus applied to either ear. This indicates that separate binaural information for the evaluation of sound source direction is still available at the brain stem level where wave Voriginates. We demonstrate that the normal nonlinear latency/intensity function may be responsible for the subjective compensation of time and intensity differences, since the well-known trading functions show similar intensity-dependent gradients.     

Otoacoustic emissions in 28 young adults exposed to amplified music.

Popular concern about widespread damage to the hearing from exposure to amplified music continues, although there has been little firm evidence of permanent effects in casual listeners. Measurement of transient evoked otoacoustic emissions (TEOAEs) provides a sensitive technique for testing outer hair cell (OHC) function, and was used in this study of 28 young adults aged 18-25 years, whose only significant source of noise exposure was loud music, to look for evidence of poorer cochlear function in those of greater exposure; they provided 27 right ears and 27 left ears suitable for measurement of TEOAE strength. Estimates of subjects' total noise dose were obtained from self-reports of the duration and intensity of their exposure to music and other sources of noise. Ears with greater exposure to loud music showed significantly weaker TEOAEs than less exposed ears in response to a 4 kHz tone burst, or in response to a saturating (82 dBSPL) click if the response was treated with a high-frequency bandpass filter (2-4 kHz) (p<0.05). Differences between more exposed and less exposed groups of ears were most marked in the 2 kHz half-octave band for right ears, and in the 2.8 kHz half-octave band for left ears. A hypothesis is proposed that weakness in TEOAEs as a result of exposure to loud music is seen first in the 2 kHz region of the emission spectrum, and later at higher frequencies; and that for a given amount of exposure, TEOAE weakness (or OHC damage) is more advanced in left ears than in right.     

The localisation of spectrally restricted sounds by human listeners

The two principal binaural cues to sound location are interaural time differences (ITDs), which are thought to be dominant at low frequencies, and interaural level differences (ILDs), which are thought to dominate at mid to high frequencies. The outer ear also filters the sound in a location dependent manner and provides spectral cues to location. In these experiments we have examined the relative contribution of these cues to the auditory localisation performance by humans. Six subjects localised sounds by pointing their face toward the perceived location of stimuli presented in complete darkness in an anechoic chamber. Control stimuli were spectrally flat (400 Hz to 16 kHz), while the relative contribution of location cues in the low frequency channels was determined using noise high passed at 2 kHz and in the high frequency channels using stimuli low passed at 2 kHz. The removal of frequencies below 2 kHz had little effect on either the pattern of systematic errors or the distribution of localisation estimates with the exception of an increase in the size of the standard deviations associated with a few rear locations. This suggests considerable redundancy in the auditory localisation information contained within a broadband sound. In contrast, restricting the target spectrum to frequencies below 2 kHz resulted in a large increase in the cone-of-confusion errors as well as a subject dependent biasing of the front-to-back or back-to-front confusions. These biases and the reduction in localisation accuracy for high pass stimuli at some posterior locations are consistent with a contribution of spectral information at low frequencies.