Localization of aerial pure tones by pinnipeds

In this study, minimum audible angles (MAAs) of aerial pure tones were measured in and compared between a northern elephant seal (Mirounga angustirostris), a harbor seal (Phoca vitulina), and a California sea lion (Zalophus californianus). Testing was conducted between 0.8 and 16 kHz in the elephant seal and 0.8 and 20 kHz in the harbor seal and sea lion in a hemi-anechoic chamber using a left/right psychophysical procedure. Performance for the same frequencies was also quantified for discrete speaker separation of 5 degrees from the mid-line. For all subjects, MAAs ranged from approximately 3 degrees to 15 degrees and were generally equal to or larger than those previously measured in the same subjects with a broadband signal. Performance at 5 degrees ranged from chance to 97% correct, depending on frequency and subject. Poorest performance in the sea lion and harbor seal occurred at intermediate frequencies, which is consistent with the duplex theory of sound localization. In contrast, the elephant seal's poorest performance occurred at higher frequencies. The elephant seal's result suggests an inferior ability to utilize interaural level differences and is perhaps related to best hearing sensitivity shifted toward lower frequencies in this species relative to other pinnipeds.     

Sound localization of frequency-modulated sinusoids by Old World monkeys

Directional hearing acuity, as measured by the minimum audible angle (MAA), was determined in four Old World monkeys, Macaca radiata. The acoustic stimuli were linear changes in frequency (sweeps) for different frequency ranges and sweep rates. The sweeps ranged between 0.5 and 1.3 kHz, at two durations, 100 and 200 ms. In upsweeps which began at 0.5 kHz and were 200 ms in duration, MAA decreased as sweep rate and frequency range increased. These thresholds were compared to MAAs of sweeps which traversed the same range of frequencies but at a different rate, to MAAs of sweeps with identical rates but over different frequency ranges, and to the MAAs of downsweeps. These comparisons indicated that range, and not sweep rate, exerts the greatest effect on the MAA. Interaural phase differences derived from the upper limits of the frequency range are discussed as potential FM localization cues.     

Underwater auditory localization by a swimming harbor seal (Phoca vitulina)

The underwater sound localization acuity of a swimming harbor seal (Phoca vitulina) was measured in the horizontal plane at 13 different positions. The stimulus was either a double sound (two 6-kHz pure tones lasting 0.5 s separated by an interval of 0.2 s) or a single continuous sound of 1.2 s. Testing was conducted in a 10-m-diam underwater half circle arena with hidden loudspeakers installed at the exterior perimeter. The animal was trained to swim along the diameter of the half circle and to change its course towards the sound source as soon as the signal was given. The seal indicated the sound source by touching its assumed position at the board of the half circle. The deviation of the seals choice from the actual sound source was measured by means of video analysis. In trials with the double sound the seal localized the sound sources with a mean deviation of 2.8 degrees and in trials with the single sound with a mean deviation of 4.5 degrees. In a second experiment minimum audible angles of the stationary animal were found to be 9.8 degrees in front and 9.7 degrees in the back of the seal's head.     

The bat head-related transfer function reveals binaural cues for sound localization in azimuth and elevation

Directional properties of the sound transformation at the ear of four intact echolocating bats, Eptesicus fuscus, were investigated via measurements of the head-related transfer function (HRTF). Contributions of external ear structures to directional features of the transfer functions were examined by remeasuring the HRTF in the absence of the pinna and tragus. The investigation mainly focused on the interactions between the spatial and the spectral features in the bat HRTF. The pinna provides gain and shapes these features over a large frequency band (20-90 kHz), and the tragus contributes gain and directionality at the high frequencies (60 to 90 kHz). Analysis of the spatial and spectral characteristics of the bat HRTF reveals that both interaural level differences (ILD) and monaural spectral features are subject to changes in sound source azimuth and elevation. Consequently, localization cues for horizontal and vertical components of the sound source location interact. Availability of multiple cues about sound source azimuth and elevation should enhance information to support reliable sound localization. These findings stress the importance of the acoustic information received at the two ears for sound localization of sonar target position in both azimuth and elevation.     

Infants' localization of sounds in the median sagittal plane: effects of signal frequency

The purpose of this research was to determine if infants, like adults, show differential localization performance in the median sagittal plane (MSP) as a function of the spectrum of the signal. Infants 6-18 months of age were seated in a dark room facing an array of nine loudspeakers, with one loudspeaker positioned at ear level, 0 degrees, and four each positioned above and below ear level at 4 degrees, 8 degrees, 12 degrees, and 16 degrees. A two-alternative, forced-choice procedure was used in which a sequence of noise bursts was presented at 0 degrees and then shifted vertically, above or below 0 degrees, and continued to be presented until the infant made a directional head and/or eye movement; correct responses were visually reinforced. For each of three bandpass noise conditions (less than 4 kHz, 4-8 kHz, 8-12 kHz), minimum audible angle (MAA) for each listener, i.e., the smallest of the four angular shifts in vertical sound location that the listener could reliably detect, was estimated. Results indicated that MAA systematically decreased with increasing age, revealing an increasingly finer partitioning of auditory space. Moreover, performance at each age revealed the importance of high frequencies for localization in the MSP. Infants did not reliably localize the low-pass signal (less than 4 kHz) and showed the best performance to the signal comprising the highest frequencies (8-12 kHz). These findings reveal systematic age-related improvements in sound localization abilities during infancy, and suggest that spectral cues similar to those for adults operate for infants in vertical localization.     

Lateralization of acoustic signals by dichotically listening budgerigars (Melopsittacus undulatus)

Sound localization allows humans and animals to determine the direction of objects to seek or avoid and indicates the appropriate position to direct visual attention. Interaural time differences (ITDs) and interaural level differences (ILDs) are two primary cues that humans use to localize or lateralize sound sources. There is limited information about behavioral cue sensitivity in animals, especially animals with poor sound localization acuity and small heads, like budgerigars. ITD and ILD thresholds were measured behaviorally in dichotically listening budgerigars equipped with headphones in an identification task. Budgerigars were less sensitive than humans and cats, and more similar to rabbits, barn owls, and monkeys, in their abilities to lateralize dichotic signals. Threshold ITDs were relatively constant for pure tones below 4 kHz, and were immeasurable at higher frequencies. Threshold ILDs were relatively constant over a wide range of frequencies, similar to humans. Thresholds in both experiments were best for broadband noise stimuli. These lateralization results are generally consistent with the free field localization abilities of these birds, and add support to the idea that budgerigars may be able to enhance their cues to directional hearing (e.g., via connected interaural pathways) beyond what would be expected based on head size.     

The acoustical cues to sound location in the rat: measurements of directional transfer functions

The acoustical cues for sound location are generated by spatial- and frequency-dependent filtering of propagating sound waves by the head and external ears. Although rats have been a common model system for anatomy, physiology, and psychophysics of localization, there have been few studies of the acoustical cues available to rats. Here, directional transfer functions (DTFs), the directional components of the head-related transfer functions, were measured in six adult rats. The cues to location were computed from the DTFs. In the frontal hemisphere, spectral notches were present for frequencies from approximately 16 to 30 kHz; in general, the frequency corresponding to the notch increased with increases in source elevation and in azimuth toward the ipsilateral ear. The maximum high-frequency envelope-based interaural time differences (ITDs) were 130 mus, whereas low-frequency (<3.5 kHz) fine-structure ITDs were 160 mus; both types of ITDs were larger than predicted from spherical head models. Interaural level differences (ILDs) strongly depended on location and frequency. Maximum ILDs were <10 dB for frequencies <8 kHz and were as large as 20-40 dB for frequencies >20 kHz. Removal of the pinna eliminated the spectral notches, reduced the acoustic gain and ILDs, altered the acoustical axis, and reduced the ITDs.     

Auditory lateralization in monkeys: an examination of two cues serving directional hearing.

In assigning binaural ongoing time differences (phase) as the cue for localization of low frequencies, and binaural intensity differences as the cue for localization of high frequencies, the duplex theory has successfully accounted for human directional hearing of tones. Sensitivity of monkeys to these cues was examined in two experiments. The dependencies on frequency of interaural intensity difference thresholds (lateralization experiment I) and time difference thresholds (lateralization experiment II) were determined behaviorally on three monkeys (M. nemestrina). The range of frequencies was from 125 Hz to 8 kHz in experiment I and from 250 Hz to 2 kHz in experiment II. The results indicate that the duplex theory is applicable to monkeys. However, monkeys are less sensitive than man to both binaural cues. The shortest time disparity monkeys discriminate is 42 microseconds at 1.5 kHz and the smallest intensity difference is 3.5 dB at 500 Hz. Good agreement between the present findings and localization measurements [C. H. Brown et al., J. Acoust. Soc. Am. 63, 1484-1492 (1978)] suggests: (a) that monkeys utilize time disparity cues through higher frequencies than man; and (b) that inaccurate localization by monkeys at high frequencies reflects decreasing sensitivity to interaural intensity cues.     

The effect of aging on horizontal plane sound localization

An experiment was conducted to determine the effect of aging on sound localization. Seven groups of 16 subjects, aged 10-81 years, were tested. Sound localization was assessed using six different arrays of four or eight loudspeakers that surrounded the subject in the horizontal plane, at a distance of 1 m. For two 4-speaker arrays, one loudspeaker was positioned in each spatial quadrant, on either side of the midline or the interaural axis, respectively. For four 8-speaker arrays, two loudspeakers were positioned in each quadrant, one close to the midline and the second separated from the first by 15 degrees, 30 degrees, 45 degrees, or 60 degrees. Three different 300-ms stimuli were localized: two one-third-octave noise bands, centered at 0.5 and 4 kHz, and broadband noise. The stimulus level (75 dB SPL) was well above hearing threshold for all subjects tested. Over the age range studied, percent-correct sound-source identification judgments decreased by 12%-15%. Performance decrements were apparent as early as the third decade of life. Broadband noise was easiest to localize (both binaural and spectral cues were available), and the 0.5-kHz noise band, the most difficult to localize (primarily interaural temporal difference cue available). Accuracy was relatively higher in front of than behind the head, and errors were largely front/back mirror image reversals. A left-sided superiority was evident until the fifth decade of life. The results support the conclusions that the processing of spectral information becomes progressively less efficient with aging, and is generally worse for sources on the right side of space.     

Near-threshold equal-loudness contours for harbor seals (Phoca vitulina) derived from reaction times during underwater audiometry: a preliminary study

Equal-loudness functions describe relationships between the frequencies of sounds and their perceived loudness. This pilot study investigated the possibility of deriving equal-loudness contours based on the assumption that sounds of equal perceived loudness elicit equal reaction times (RTs). During a psychoacoustic underwater hearing study, the responses of two young female harbor seals to tonal signals between 0.125 and 100 kHz were filmed. Frame-by-frame analysis was used to quantify RT (the time between the onset of the sound stimulus and the onset of movement of the seal away from the listening station). Near-threshold equal-latency contours, as surrogates for equal-loudness contours, were estimated from RT-level functions fitted to mean RT data. The closer the received sound pressure level was to the 50% detection hearing threshold, the more slowly the animals reacted to the signal (RT range: 188-982 ms). Equal-latency contours were calculated relative to the RTs shown by each seal at sound levels of 0, 10, and 20 dB above the detection threshold at 1 kHz. Fifty percent detection thresholds are obtained with well-trained subjects actively listening for faint familiar sounds. When calculating audibility ranges of sounds for harbor seals in nature, it may be appropriate to consider levels 20 dB above this threshold.     

人工头近场头相关传输函数及其特性

采用球形正十二面体声源及其空间定位系统,测量并建立了KEMAR人工头的近场头相关传输函数(HRTF)数据库。基于数据库分析了近场HRTF在频域和时域随声源距离变化的规律;讨论了用近场HRTF算得的双耳声级差(TLD)和双耳时间差(ITD)所包含的声源距离定位信息。结果表明,测量系统和所得数据具有较好的重复性和准确性,保留了1 kHz以下的低频定位信息。并且,近场HRTF幅度谱和ILD随声源距离的变化明显;用相关法算得2 kHz以下频段的ITD随声源距离略有变化。本文数据库及其分析结果将为声源距离定位的应用提供基础。      

Resolution in azimuth sound localization in the Mongolian gerbil (Meriones unguiculatus)

Minimum resolvable angles (MRAs) for sound localization in azimuth in the gerbil were determined in a behavioral study using tones, 300-Hz bands of noise centered at frequencies between 500 Hz and 8 kHz and broad-band noise of on average 60 dB SPL overall level. Using the method of constant stimuli, seven gerbils were trained in a two-alternative-forced-choice procedure to indicate if sounds were presented to them from the left or from the right by choosing the left or right arm of a Y-shaped cage. The MRA is the minimum angle between two loudspeaker locations that the gerbils discriminated. Animals were either stimulated from the front (N=4) or from the back (N=3). The MRA for broad-band noise randomly varying in level by +/- 6 dB was 23 degrees and 45 degrees for gerbils stimulated from the front or back, respectively. Generally a gerbil's MRA for tones declined up to 2 kHz reaching 20 degrees and 31 degrees for gerbils stimulated from the front or back, respectively, and the MRA was generally increased above this frequency. Results for narrow-band noise stimuli were similar. Results are discussed with respect to the available interaural cues and physiological mechanisms of sound localization in the gerbil.     

Receiving beam patterns in the horizontal plane of a harbor porpoise (Phocoena phocoena)

Receiving beam patterns of a harbor porpoise were measured in the horizontal plane, using narrow-band frequency modulated signals with center frequencies of 16, 64, and 100 kHz. Total signal duration was 1000 ms, including a 200 ms rise time and 300 ms fall time. The harbor porpoise was trained to participate in a psychophysical test and stationed itself horizontally in a specific direction in the center of a 16-m-diameter circle consisting of 16 equally-spaced underwater transducers. The animal's head and the transducers were in the same horizontal plane, 1.5 m below the water surface. The go/no-go response paradigm was used; the animal left the listening station when it heard a sound signal. The method of constants was applied. For each transducer the 50% detection threshold amplitude was determined in 16 trials per amplitude, for each of the three frequencies. The beam patterns were not symmetrical with respect to the midline of the animal's body, but had a deflection of 3-7 degrees to the right. The receiving beam pattern narrowed with increasing frequency. Assuming that the pattern is rotation-symmetrical according to an average of the horizontal beam pattern halves, the receiving directivity indices are 4.3 at 16 kHz, 6.0 at 64 kHz, and 11.7 dB at 100 kHz. The receiving directivity indices of the porpoise were lower than those measured for bottlenose dolphins. This means that harbor porpoises have wider receiving beam patterns than bottlenose dolphins for the same frequencies. Directivity of hearing improves the signal-to-noise ratio and thus is a tool for a better detection of certain signals in a given ambient noise condition.     

Acoustical cues for sound localization by the Mongolian gerbil, Meriones unguiculatus

The present study measured the head-related transfer functions (HRTFs) of the Mongolian gerbil for various sound-source directions, and explored acoustical cues for sound localization that could be available to the animals. The HRTF exhibited spectral notches for frequencies above 25 kHz. The notch frequency varied systematically with source direction, and thereby characterized the source directions well. The frequency dependence of the acoustical axis, the direction for which the HRTF amplitude was maximal, was relatively irregular and inconsistent between ears and animals. The frequency-by-frequency plot of the interaural level difference (ILD) exhibited positive and negative peaks, with maximum values of 30 dB at around 30 kHz. The ILD peak frequency had a relatively irregular spatial distribution, implying a poor sound localization cue. The binaural acoustical axis (the direction with the maximum ILD magnitude) showed relatively orderly clustering around certain frequencies, the pattern being fairly consistent among animals. The interaural time differences (ITDs) were also measured and fell in a +/- 120 micros range. When two different animal postures were compared (i.e., the animal was standing on its hind legs and prone), small but consistent differences were found for the lower rear directions on the HRTF amplitudes, the ILDs, and the ITDs.     

Minimum audible movement angle in the horizontal plane as a function of stimulus frequency and bandwidth, source azimuth, and velocity.

Minimum audible movement angles (MAMAs) were measured in the horizontal plane for four normal-hearing adult subjects in a darkened anechoic chamber. On each trial, a single stimulus was presented, and the subject had to say whether it came from a stationary loudspeaker or from a loudspeaker that was moving at a constant angular velocity around him. Thresholds were established by adaptively varying stimulus duration. In experiment 1, MAMAs were measured as a function of center frequency (500-5000 Hz), velocity (10 degrees-180 degrees/s), and direction of motion (left versus right). There was no effect of direction of motion. MAMAs increased with velocity, from an average of 8.8 degrees of arc for a target moving at 10 degrees/s to an average of 20.2 degrees of arc for a target moving at 180 degrees/s. MAMAs were higher for a 3000-Hz tone than for tones of lower or higher frequencies, as has been previously reported [D. R. Perrott and J. Tucker, J. Acoust. Soc. Am. 83, 1522-1527 (1988)]. In experiment 2, minimum audible angles (MAAs) were measured with sequentially presented stationary tone pulses (500-5000 Hz), and were shown to exhibit the same dependence on signal frequency that the MAMAs showed (average MAA at 3000 Hz: 8.4 degrees; average MAA at the other frequencies: 3.4 degrees). In experiment 3, MAMAs and MAAs were measured as a function of stimulus bandwidth (centered at 3000 Hz) and listening azimuth (0 degrees vs 60 degrees). Average MAAs decreased monotonically as stimulus bandwidth increased from 0 Hz to wideband (from 8.4 degrees to 1.2 degrees at 0 degrees azimuth; from 11.3 degrees to 1.5 degrees at 60 degrees azimuth). As in experiment 1, MAMAs increased with stimulus velocity, from values comparable to the MAAs for the slowest-velocity (10 degrees/s) targets to 70 degrees of arc or more in the poorest condition (third-octave band of noise presented at a velocity of 180 degrees/s and an azimuth of 60 degrees). MAMAs obtained in the slower-velocity conditions depended in the same way on stimulus bandwidth and listening azimuth that MAAs depended on these variables. In no case was the MAMA ever smaller than the MAA. It is hypothesized that a minimum integration time is required to achieve optimal performance in a dynamic spatial resolution task. Average estimates of this minimum time based on the current data vary from 336 ms (for targets presented at midline) to 1116 ms (for narrow-band targets presented at 60 degrees azimuth).(ABSTRACT TRUNCATED AT 400 WORDS)     

The benefit of practice for sound localization without sight

An experiment was designed to determine whether normally sighted human subjects would be able to adapt to the handicapping effects of sudden deprivation of visual cues on horizontal plane sound localization. Two groups of sighted normal-hearing young adults participated. One group was allowed the benefit of sight. The other group was blindfolded. Measurements of accuracy and the time to respond were made daily over the course of five consecutive days, in a semi-reverberant sound proof booth that modeled listening in a small office. Sound localization was assessed using an array of eight speakers that surrounded the subject in space. Each day, one block of 120 trials was presented for each of three stimuli, two one-third octave noise bands, centred at 0.5 and 4 kHz, and broadband noise, to assess the utilization of interaural temporal difference cues, interaural level difference cues and binaural and spectral cues in combination. Blindfolded subjects were relatively less accurate than sighted subjects. Both groups showed gains with practice, the blindfolded group to a greater degree, largely due to improvements in the use of spectral cues. The blindfolded group took longer to respond than the sighted group, but showed greater decrements in response time with practice.     

Directional dependence of interaural envelope delays

Interaural envelope delays were measured in six human subjects as a function of the location of a movable sound source, bandpassed between 3 and 16 kHz. A total of 324 source locations were tested in horizontal and vertical increments of 10 degrees. A method is described for estimating the complex directional transfer function of the external ear, independent of the position of the recording microphone in the ear canal. To compute interaural envelope delays, directional transfer functions from the left and right ears were convolved with a critical-band filter, the envelopes were computed, and the envelopes were cross correlated. Interaural envelope delays, as well as interaural group delays, varied somewhat with the center frequency of the critical-band filter and with the vertical location of the sound source. Nevertheless, to a first approximation, envelope delays measured in the ear canals increased monotonically with increasing angle of incidence relative to the median plane, as they would for two microphones on the surface of a rigid sphere. The results are discussed in relation to the possible contribution of interaural envelope delays to sound localization behavior.     

The auditory periphery of the ferret. I: Directional response properties and the pattern of interaural level differences

The transformations of sound by the auditory periphery of the ferret have been investigated using an impulse response technique for a large number of sound locations surrounding the animal. Individual frequencies were extracted from the detailed spectral transformation functions (STFs) obtained for each stimulus location and, using sophisticated spatial interpolation routines, were used to calculate the directional response of the periphery at that frequency. The strength of the directional response was directly related to the analysis frequency. Furthermore, as the analysis frequency was increased to 20 kHz, the orientation of the directional response increased in elevation from the horizon (E0 degrees) to about E30 degrees, while the azimuthal location remained fairly constant at 30 degrees to 40 degrees from the midline. For analysis frequencies above 20 kHz, the response became increasingly directional toward the ipsilateral interaural axis. The interaural level differences (ILDs) were also calculated for all animals studied. ILDs increased from around 5 to 25 dB over the range of frequencies from 3-24 kHz. The two-dimensional patterns of iso-ILD contours were roughly concentric and centered on the interaural axis for frequencies below 16 kHz. For higher frequencies, there was a tendency for the ILD contours to be centered on more anterior and inferior locations. The increased directionality of the auditory periphery with increasing analysis frequency, together with the presence of sharp nulls in the response at high analysis frequencies, is consistent with a diffractive effect produced by the aperture of the pinna. However, this simple model does not predict the directional responses over the low to middle frequency range.     

Narrow-band sound localization related to external ear acoustics.

Human subjects localized brief 1/6-oct bandpassed noise bursts that were centered at 6, 8, 10, and 12 kHz. All testing was done under binaural conditions. The horizontal component of subjects' responses was accurate, comparable to that for broadband localization, but the vertical and front/back components exhibited systematic errors. Specifically, responses tended to cluster within restricted ranges that were specific for each center frequency. The directional transfer functions of the subjects' external ears were measured for 360 horizontal and vertical locations. The spectra of the sounds that were present in the subjects' ear canals, the "proximal stimulus" spectra, were computed by combining the spectra of the narrow-band sound sources with the directional transfer functions for particular stimulus locations. Subjects consistently localized sounds to regions within which the associated directional transfer function correlated most closely with the proximal stimulus spectrum. A quantitative model was constructed that successfully predicted subjects' responses based on interaural level difference and spectral cues. A test of the model, using techniques adapted from signal detection theory, indicated that subjects tend to use interaural level difference and spectral shape cues independently, limited only by a slight spatial correlation of the two cues. A testing procedure is described that provides a quantitative comparison of various predictive models of sound localization.     

A localization algorithm based on head-related transfer functions

Two sound localization algorithms based on the head-related transfer function were developed. Each of them uses the interaural time delay, interaural level difference, and monaural spectral cues to estimate the location of a sound source. Given that most localization algorithms will be required to function in background noise, the localization performance of one of the algorithms was tested at signal-to-noise ratios (SNRs) from 40 to -40 dB. Stimuli included ten real-world, broadband sounds located at 5 degrees intervals in azimuth and at 0 degrees elevation. Both two- and four-microphone versions of the algorithm were implemented to localize sounds to 5 degrees precision. The two-microphone version of the algorithm exhibited less than 2 degrees mean localization error at SNRs of 20 dB and greater, and the four-microphone version committed approximately 1 degrees mean error at SNRs of 10 dB or greater. Potential enhancements and applications of the algorithm are discussed.