Directional sensitivity of sound-pressure levels in the human ear canal

Abstract

Changes in sound pressures measured in the ear canal are reported for broadband sound sources positioned at various locations about the subject. These location-dependent pressures are one source of acoustical cues for sound localization by human listeners. Sound source locations were tested with horizontal and vertical resolution of 10 degrees. Sound levels were measured with miniature microphones placed inside the two ear canals. Although the measured amplitude spectra varied with the position of the microphone in the ear canal, it is shown that the directional sensitivity at any particular frequency of the broadband stimulus is independent of microphone position anywhere within the ear canal. At any given frequency, the distribution of sound pressures as a function of sound source location formed a characteristic spatial pattern comprising one or two discrete areas from which sound sources produced maximum levels in the ear canal. The locations of these discrete areas varied in horizontal and vertical location according to sound frequency. For example, around 8 kHz, two areas of maximum sensitivity typically were found that were located laterally and were separated from each other vertically, whereas, around 12 kHz, two such areas were found located on the horizontal plane and separated horizontally. The spatial patterns of sound levels were remarkably similar among different subjects, although some frequency scaling was required to accommodate for differences in the subjects' physical sizes. Interaural differences in sound-pressure level (ILDs) at frequencies below about 8 kHz tended to increase monotonically with increasing distance of the sound source from the frontal midline and tended to be relatively constant as a function of vertical source location. At higher frequencies, however, ILDs varied both with the horizontal and with the vertical location of the sound source. At some frequencies, asymmetries between the left and right ears in a given subject resulted in substantial ILDs even for midline sound sources. These results indicate the types of horizontal and vertical spatial information that are available from sound level cues over various ranges of frequency and, within a small subject population, indicate the nature of intersubject variability.

Stretched and upside-down maps of auditory space in the optic tectum of blind-reared owls; acoustic basis and behavioral correlates

“…Intensity cues are highly frequency dependent for all species (e.g., Harrison and Downey, 1970;Knudsen, 1980;Palmer and King, 1985;Middlebrooks et al, 1989;Musicant et al, 1990), and therefore the problems involved in constructing these maps are comparable to those involved in constructing the representation of elevation in the barn owl. Figure 14 and Table 4 demonstrate that elevational tuning errors in barn owls correlate with the frequency dependence of the intensity cues, and we expect that in other species the importance of vision for calibrating different portions of the auditory map may also vary with the frequency dependence of the cues.…”

“…8) 12A; 13C,D) and therefore are predictable. In species with symmetrical ears, these properties apply to binaural cues corresponding with sources located along the midsagittal plane (Moore and Irvine, 1979;Middlebrooks et al, 1989;Musicant et al, 1990). The consistency of cue values through development and across generations could permit genetically determined mechanisms to establish connections between neurons coding for 0-dB IID and 0-psec ITD with the representation of frontal space in the tectum.…”

“…In animals with symmetrical ears, the correspondence of the sign of the IID with the general direction of the sound source is neither uncertain nor frequency dependent: left-ear-louder IIDs always correspond with locations to the left, and right-earlouder IIDs always correspond with locations to the right (Harrison and Downey, 1970;Palmer and King, 1985;Middlebrooks et al, 1989;Musicant et al, 1990). Consequently, in such species the orientation of the IID map could be specified by visionindependent mechanisms that would prevent the IID map from flipping in blind-reared individuals.…”

Relearning Auditory Spectral Cues for Locations Inside and Outside the Visual Field

“…2G) particularly for lateral angles associated with the pinna axis over the mid to high frequencies at lateral angles between 30°and 60°( e.g. Middlebrooks et al 1989). So as to compare the acoustic effects produced by our moulds with data obtained in their work, we have calculated the SI according to van Wanrooij and van Opstal (2005).…”

Can measures of sound localization acuity be related to the precision of absolute location estimates?

“…Again, these could arise purely from motor errors; alternatively, they could at least partially come about because the auditory space map itself is compressed. The latter scenario could result from limitations in the encoding of binaural cues, for example, because symmetrical ears are most sensitive to interaural differences in the frontal hemifield (Sandel et al, 1955;Middlebrooks et al, 1989). Adding to this, the extent of these underestimations can vary between adjacent targets and these deviations can be large relative to distribution variance.…”