Directional properties of bat ears for target localization

Among several bat acoustic capabilities, it is reported that the error they
show in discriminating the direction of a target in the elevation plane is within
a few degrees. Many researchers have investigated bat echolocation and attempted to explain which features their accuracy relies on. Some works have
pointed out the importance of beam patterns associated to the ears of the bats:
indeed, ear shapes are very complex and different across species. This allows
bats to significantly discriminate among the directions the echoes of their calls
come from. Also, binaural cues for sound localization have been found to be
determinant for some bat species.
A method to estimate target orientation based on the beam patterns of
the receivers and their binaural geometric relationship is developed for a sonar
system emulating bats echolocation: a broadband signal is produced in the ultrasonic range and the echoes are collected by two receivers. The location of
the target with respect to the bat is given by spherical coordinates including
the distance from the bat and the orientation (defined by azimuth and elevation
angles). While distance of the target from the sonar system is recovered through
the time from the emission to the reception of the signal, the orientation is estimated from knowledge of the beam profiles of the receivers. The ratios between received and original signals’ spectra are calculated for the left and the right receiver across frequencies after compensating for distance effects. Orientations ensuring attenuation values in the beam patterns closest to the ratios are selected.
Among these, one orientation for the left receiver and one for the right,
fulfilling appropriate mutual geometric relations given by the geometry of the
situation, are selected, these two defining the direction of the signal echo.
The beam patterns associated to the left and the right ear of the Phyllostomus
discolor bat are used within a limited range of orientations. A model of
the emission of the signal and its reception is simulated for a set of target orientations spanning the beam pattern domain. Additionally, several noise levels
are considered. Estimates of directions of the echoes are performed and the
error for each of them with respect to the true orientation is calculated. Most
error values are comparable to those reported in the literature and associated
to some bat species. The beam patterns have very irregular shapes over the
set of orientations that change with the frequency range: indeed, the accuracy
of the method in estimating the true orientation depends on the choice of the
frequency range and on the area of orientations over which the beam pattern is
defined.
This approach makes it possible to investigate which acoustic features ensure
good orientation estimates in order to explain why bats have such complex
beam patterns filtering the echoes. As this method relies only on knowledge
of the receivers’ beam patterns and geometric relations based on the binaural
configuration, it appears feasible that bats utilise this effect.