THREE SONARS IN ONE BAT: LAYERED BIOSONAR IMAGES WITH UNIFIED SPATIAL DIMENSIONS

摘要

The big brown bat (Eptesicus fuscus) is an insectivorous species that uses echolocation to perceive objects in its surroundings. Big brown bats transmit frequency-modulated (FM) sounds containing several harmonics that, collectively, span frequencies from about 20 to 110 kHz. Echolocation sounds have very high sound pressures, about 100-130 dB SPL (p-to-p) close to the open mouth. Broadcast durations are short and are adjusted to approximately match the two-way travel time of echoes from the target of immediate interest so that echoes do not overlap broadcasts. Durations vary from about 10-20 ms when the bat is flying in the open, to 2-5 ms when the bat is approaching within about 1 meter of objects, and to about 0.5 ms in the terminal part of an interception. Interpulse intervals are scaled to the larger scope of the environment, ordinarily extending to at least 50-100 ms so that echoes from all objects in the scene have returned before the next sound is sent out, with the caveat that sounds often are grouped into pairs or triplets, called strobe groups" created by emitting sounds at alternating short and long intervals, which presumably serves to organize the bat's perception of auditory target "scenes." Figure 1 illustrates spectrograms for a series of biosonar sounds recorded during the late approach and terminal stages of target interception by a big brown bat. The 1st harmonic (FM1) sweeps from about 55 kHz down to 23 kHz, and the 2nd harmonic (FM2) sweeps from about 80-90 kHz down to 50 kHz (Figure 1). A short segment from the 3rd harmonic (FM3) usually is present, too, sweeping down from about 100 to 80 kHz. The harmonics give the spectrograms in Figure 1 a fingerprint-like appearance that bats use for recognition of their echoes.