Source Azimuth Research Articles

Reproduction of sound

In a sound reproduction system which enables the listener to distinguish between signals from in front and behind as well as signals on the left and the right, only two independent transmission channels are employed. The contributions to the signals in the two transmission channels relating to a sound source at a particular azimuth have the same amplitude and frequency and differ in phase by an amount indicating the azimuth of such source.

Barn owls can use ongoing‐time disparities to determine the azimuth of a sound source

A class of auditory neurons in the owl's nucleus mesencephalicus lateralis pars dorsalis (MLD), a midbrain auditory nucleus, have small spatial receptive fields. We sought our relationships between the spatial receptive fields of these neurons (measured under free‐field conditions) and sensitivities to binaural disparities (presented dichotically). To switch easily between free‐field and dichotic stimulation we constructed threaded, miniature earphones, which could be inserted into threaded rings implanted into the owl's ear canals. Neurons with spatial receptive fields were found to be sensitive to a narrow range of ongoing‐time disparities in the biologically significant range of tens of microseconds. There was a strong correlation between the azimuth of the spatial receptive fields and the magnitude of the ongoing‐time disparities. Freely behaving owls were presented with similar ongoing‐time disparities through identical earphones. They reacted to the stimuli in a manner consistent with the conclusion that ongoing‐time disparities were used to ascertain the azimuth of a sound source.

Hectometer and Kilometer Solar Observations

Three dimensional “snapshots” of the large scale solar magnetic field topology as well as the solar wind electron density distribution from about 0.1 to 1 AU are obtained by tracking traveling solar radio bursts at hectometer and kilometer wavelengths with instruments aborad the ISEE-3 satellite and the HELIOS-2 solar probe. Both instruments observe in the frequency range from 30 kHz to 1 MHz and both are equipped with dipole antennas located in the vehicle spin plane. ISEE-3 also has a dipole along the spin axis and the signals from the two ISEE-3 antennas are combined to give the azimuth and elevation angles of the radio source. Triangulation between HELIOS-2 and ISEE-3 provides the additional observation necessary to uniquely determine the position of the radio source in space at each observing frequency. The techniques will be outlined, and illustrated by an example of the three dimensional field geometry and electron density distribution determined by the observations.

Microseisms in geothermal exploration—studies in Grass Valley, Nevada

Frequency(f)‐wavenumber(k) spectra of seismic noise in the bands 1 ⩽ f ⩽ 10 Hz in frequency and |k| ⩽ 35.7 cycles/km in wavenumber, measured at several places in Grass Valley, Nevada, exhibit numerous features which can be correlated with variations in surface geology and sources associated with hot spring activity. Exploration techniques for geothermal reservoirs, based upon the spatial distribution of the amplitude and frequency characteristics of short‐period seismic noise, are applied and evaluated in a field program at this potential geothermal area. A detailed investigation of the spatial and temporal characteristics of the noise field was made to guide subsequent data acquisition and processing. Contour maps of normalized noise level derived from judiciously sampled data are dominated by the hot spring noise source and the generally high noise levels outlining the regions of thick alluvium. Major faults are evident when they produce a shallow lateral contrast in rock properties. Conventional seismic noise mapping techniques cannot differentiate noise anomalies due to buried seismic sources from those due to shallow geologic effects. The noise radiating from a deep reservoir ought to be evident as body waves of high‐phase velocity with time‐invariant source azimuth. A small two‐dimensional (2-D) array was placed at 16 locations in the region to map propagation parameters. The f‐k spectra reveal shallow local sources, but no evidence for a significant body wave component in the noise field was found. With proper data sampling, array processing provides a powerful method for mapping the horizontal component of the vector wavenumber of the noise field. This information, along with the accurate velocity structure, will allow ray tracing to locate a source region of radiating microseisms. In Grass Valley, and probably in most areas of sedimentary cover, the 2–10 Hz microseismic field is predominantly fundamental‐mode Rayleigh waves controlled by the very shallow structure.

Mechanisms of sound localization in the barn owl (Tyto alba)

1. We investigated the mechanisms by which the barn owl (Tyto alba) determines the azimuth and elevation of a sound source. Our measure of localizing ability was the accuracy with which the owl oriented its head to a sound source. 2. When localizing tonal signals, the owl committed the smallest errors at frequencies between 4 and 8 kHz. The azimuthal component of these errors was frequency independent from 1 to 8 kHz, but the elevational component increased dramatically for frequencies below 4 kHz. 3. The owl's mean error when localizing wide band noise was nearly three times less than its mean error when localizing the optimal frequency for tonal localization (6 kHz). 4. Occluding the right ear caused the owl to orient below and to the left of the sound source; occluding the left ear caused it to orient above and to the right of the sound source. 5. With ruff feathers (facial ruff) removed, the owl continued to localize sounds accurately in azimuth, but failed to localize sounds in elevation. 6. We conclude from these results that the barn owl uses interaural comparisons of sound spectrum to determine the elevation of a sound source. Both interaural onset time and interaural spectrum are used to identify the azimuth of the sound source. If onset time is not available (as in a continuous sound), the owl can derive the azimuth of the source from interaural spectrum alone, but its spatial resolution is poorer.

The effect of interaural electroacoustic hearing aid properties on sound localization abilities in normal and hearing‐impaired listeners

The use and benefit of binaural amplification in the hearing impaired has generated considerable conflict. One easily measured benefit of binaural hearing includes improved sound localization. The normal binaural auditory system has the capacity to determine with considerable accuracy the azimuth of a sound source. Binaural hearing aid fitting procedures are typically accomplished with an independent concern for each ear. The interaural intensity, phase characteristics, and time of arrival cues processed by hearing aids undoubtedly affect the quality of the binaural fitting, The purpose of this study was to investigate the dependence between binaural hearing aids and in their electroacoustic functioning by determining minimal audible angles (MAA) and center position (CP) shifts in a sound localization task. Six normal and two hearing‐impaired adults responded to tonal and click stimuli using four selected pairs of hearing aids. These pairs were selected for matched electroacoustic characteristics: interaural phase distortion, and interaural transient distortion. Results of this study revealed smaller MAAs and CP shifts with the hearing aid pair matched closely in electroacoustic characteristics, whereas dissimilar pairs had larger MAAs and greater CP shifts. The results of this study provide information concerning binaural interactions and hearing aid fitting procedures with implications for improved binaural hearing in the hearing impaired.

The effects of microphone spacing on auditory localization.

The localization ability of 10 normally hearing adults was determined under varying microphone separations and varying sound source azimuths. The stimuli (white noise busts) were prerecorded, after being transduced through 'body' hearing aids and then played to the subjects over headphones. Results indicated that there was an improvement in localization ability for all azimuth conditions when the microphones were spaced wider than 12.7 cm apart (15.2-30.5 cm). The smaller the separations (5.5-12.7 cm), the poorer the localization. Localization was always poorer at 30 degrees azimuth (the smallest used) than at any of the other azimuths (0 degree, 30 degrees, 60 degrees, 90 degrees right and left), regardless of microphone spacing. Implications are made about the relation of these findings to the use of binaural body aids on infants and young children.

P- and SV-wave corner frequencies over low-loss paths: A discriminant for earthquake source theories?

Seismic data recorded at Jamestown, California, provide P- and SV-wave spectral corner frequencies for 18 trans-Sierra Nevada events in the range 3.2≤ML≤4.0. These include most of the available earthquakes from 06 July 1974 to 31 August 1975. The ratio R of P- to SV-wave corner frequency was 0.96±0.17 for these events, which sample a wide range of source azimuths and focal mechanisms, Assumptions of a flat far-field source spectrum and frequency-independent attenuation in the 1-20 Hz band lead to Qβ not less than 480. Many recent source theories predict factor-of-two or more variation of R with azimuth, and significant deviation of its average value from unity; we conclude that such source models do not apply to these earthquakes. Previous studies of R by MOLNAR et al. (1973), STUMP (1974), and BAKUN et al. (1975), also based on excellent data, find different results. We interpret this discrepancy not as a contradiction, but as an indication that R may be an extremely useful source-model discriminant, its variation indicating that different models need be applied to different source regions. R may lead to a clearer understanding of the physical processes associated with earthquakes; for example, the only source models that predict a value of unity for R at all azimuths are those in which the corner frequency measures the time duration of motion at the source, thus indicating that this duration is as long as the rupture propagation time across the source.

Dynamic theory of sound-source localization.

Under idealizations about the nature of the path taken by sounds to each of a listener's two ears, and the shape of the listener's head, it is shown that both the azimuth and the range of a sound source can be expressed in terms of the rate at which certain sound stimulus parameters change when sound-source azimuth varies. It is further shown that these rates of change can be approximated by a listener if he rotates his head on its vertical axis in ways, and to extents, that are appropriate. The conclusion, then, is that a listener can calculate the azimuth and range of a sound source wholly on the basis of his interaction with the sound stimulus, without any prior knowledge of the sound source. The problem of localization error is discussed, particularly in relation to the extent to which our idealizations depart from reality, and our results are briefly related to those of other researchers in the field of sound-source localization.

The role of shadows in directional training and homing of pigeons, Columba livia

AbstractSome of the hypotheses of how homing pigeons are able to find their way home would require them to make precise measurements of the location of the sun. The purpose of this study was to determine if homing pigeons can measure the altitude and azimuth of a light source accurately enough to use the hypothesized navigational systems. Also the specific cues the pigeons used for the discriminations were analyzed.A psychophysical technique in which the pigeon controls the movement of the light (or of itself) toward or away from a trained direction was used. The ability to perform this task gives evidence on how accurately homing pigeons can measure the location of a light source such as the sun.Five pigeons were trained and tested. They were able to discriminate differences of as little as 3.4° in azimuth of the light source; 8° in altitude. It was determined that shadow patterns in the apparatus were used in all the discriminations. The role shadows may have played in the directional training done in the past, and the role shadows may play in the natural homing process of pigeons are discussed.

The Precision of a Human Underwater Sound Localization Response

The precision of a human underwater sound localization response was defined as the correspondence between the objective azimuth of the sound source and the azimuth indicated by a pointer controller by the subject. It was determined that the pointing response had an average error of 5.98 when the subjects pointed to a visual target located in the frontal plane. When the subjects were required to locate an acoustic target located at some point in 360°, the average error was 19.2°. Front to back (or back to front) reversals occurred at 0° and 180°±45° and amounted to 15.1% of the total responses. A replication of the visual target localization in 360° yielded an average error of 6.98° After 240 training trials in which the subject was allowed to visually correct his auditory localization, the average error in 360° was 13.1° Reversals were reduced to 7.1% of the total responses.

Intensity changes at the ear as a function of the azimuth of a tone source: a comparative study.

Sound intensity of a tone (4, 10, and 20 kHz) was measured at the external auditory meatus as a function of the azimuth (at every 15°) of the sound source. The observations were made under free-field conditions, and man, squirrel monkey, rat, and bat were used as subjects. At 4 kHz, rat and man showed consistent variation of intensity as a function of azimuth. Only irregular variations were obtained in monkey and bat. Interaural-intensity differences (IIDS) at 4 kHz were estimated to be sufficient to support localization of a tone at a minimum angle of 2.1° (rat) and 1.125° (man) from the midline. At 10 kHz, rat, bat, and monkey showed consistent variations of intensity with azimuth of at least 15 dB. IIDs at 10 kHz were estimated to be sufficient to support the localization of a tone at a minimum angle of 1.125° (monkey), 2.0° (bat), and 1.5° (rat) from the midline. At 20 kHz, consistent intensity variations of at least 14 dB were obtained in rat, bat, and monkey. IIDs were estimated as sufficient to support localization at a minimum angle of 1.5° (monkey), 2.25° (bat), and 3.6° (rat).

Binaural record of cochlear potentials in the guinea-pig and directional hearing.

Several experiments involving binaural records of cochlear potentials in the guinea-pig are described. The conductive properties of the two ears were studied as a function of frequency and intensity. The variations of the responses were observed when the azimuth of the sound source was modified, and also, in the case of bone-conducted tones, when the point of application of the receiver on the cranium was changed. In another series of experiments, the possible role of the ossicular muscles contractions in directional hearing was investigated on wakeful animals.

Effects of interaural time delays on masking by two competing signals.

Interference with thresholds for spondees and intelligibility of monosyllabic words (at 60 dB SPL) produced by combining modulated white noise (4 mod/sec, − 10 dB interburst ratio) and connected speech as simultaneous maskers was studied under a variety of interaural listening conditions at each of three masker levels (66, 72, and 78 dB sound-pressure level). Monaural performance in the presence of both maskers (CmNmSm) was 7.8–10.5 dB poorer than with either masker alone (NmSm or CmSm). Thus, the two maskers produced added interference substantially in excess of the approximately 3-dB increase to be expected on the basis of the summing of their powers alone. Very little release from masking was achieved in the transition from monaural (CmNmSm) to homophasic listening (C0N0S0). During antiphasic (CτNπS0) presentation, approximately 5 dB of release from masking occurred (re C0N0S0). A little less than half as much release from masking appeared when the maskers were given 0.8 msec interaural time delays, while the test material remained in phase at the two ears. Performance with opposing interaural delays (C0.8N 0.8S0) was no better than with parallel time delays (C0.8N0.8S0). Since neither of these conditions yielded as large masking-level difference as did the antiphasic, even though the sources of signals were lateralized (localized) much more sharply by the subject during time-delay conditions, this study offers further evidence that localization (separation of source azimuths) and the capacity to understand speech against competing sounds are separate capacities. Various implications of the finding are discussed.

Rôle of Stimulus Frequency in the Apparent Location of Tone Bursts

Loudspeakers were concealed behind a drape arranged in a semicircle with a radius of 5 ft. Listeners, seated at its center, were asked to repeat the location of tone bursts By giving the number on the drape behind which the sound appeared. Stimulus frequency ranged from 0.5 to 8 kcps; burst duration was 10 msec; rise-decay time was 5 msec. Head movements were minimized by a device attached to the listener's chair. The results showed that the apparent location of the intermediate frequencies (2–4 kcps) were displaced toward the midline. The magnitude of displacement increased with increases of the azimuth of the sound source. Augmented interaural intensity differences by partially occluding the ear opposite the sound source served to decrease the constant error of location, but did not eliminate it. When bands of noise bursts instead of tone bursts were used, listeners still displaced toward the midline that band consisting of the intermediate frequencies. The adequacy of interaural tone and intensity difference cues in accounting for these data is discussed.

DISCRIMINATION ABILITY OF THE SOUND FROM FRONT AND REAR SOURCES(DFR)

1) Amoung the earlier studies on the directional hearing, there have been few on the discrimination ability of the sound from front and rear sources(DFR).Foundamental study on this ability was performed to establish the test method, and various kinds of hearing disorders were tested. 2) The method was as follows: Two loudspeakers were set in front of and in the back of the subject at the same distance. Pure tone, white noise and bandpass-noise of the same loudness were given from the two speakers.The subjects were told to answer where the sounds came from. 3) When a subject kept his head still. i) The wider the frequency range of the testnoise, the better the DFR. ii) To elicit good DFR, the duration and the intensity of the testnoise should be more than 0.25sec and 40db(SL) respectively. DFR was not significantly influenced by the rise and decay time of the testnoise. iii) Concerning the azimuth of the sound sources, the best DFR was obtained when the sources was 30° to 45° apart from the median sagittal plane. iv) When earplugs were inserted in the both earcanals, DFR was disturbed, but the effect of auditory fatigue was not so remarkable as earplugs. 4) When the head of subject was moving, narrow bandpassnoise was also well discriminated as whhite noise. 5) DFR of the subjects with hard deafness was oor in proportion to the degree of hearing loss. 6) DFR was poor in the presence of central disorders, even when the hearing was normal. The last result indicates the possible usefulness of he test as a diagnostic aid of central disorders.