Spatially dependent acoustic cues generated by the external ear of the big brown bat, Eptesicus fuscus

Abstract

To measure the directionality of the external ear of the echolocating bat, Eptesicus fuscus, the left or right eardrum of a dead bat was replaced by a microphone which recorded signals received from a sound source that was moved around the stationary head. The test signal was a 0.5-ms FM sweep from 100 kHz to 10 kHz (covering all frequencies in the bat's biosonar sounds). Notches and peaks in transfer functions for 7 tested ears varied systematically with changes in elevation. For the most prominent notch, center frequency decreased from about 50 kHz for elevations at or near the horizontal to 30-40 kHz for elevations 30 degrees-40 degrees below the horizontal. A second notch shifted from about 85 kHz to 70 kHz over these same elevations. Above the horizontal, a peak that flanks these notches changed in amplitude by 15 dB with changes in elevation. Removal of the tragus from the external ear disrupted the systematic movement of notch frequencies with elevation but did not disrupt changes in the peak's amplitude. Smaller changes in notch frequency also occurred with changes in azimuth, so monaural notch information alone cannot determine the position of sound sources away from the median plane. However, because bats routinely keep the head pointed at the target's azimuth, median-plane localization occurs with monaural cues delivered to the two ears. Corresponding changes with elevation occurred in the impulse-response, which consists of a series of 3-6 peaks spaced 10-20 microseconds apart. The time separation of two prominent impulse peaks systematically increased from 22-26 microseconds above the horizontal to about 36-40 microseconds below the horizontal, and removal of the tragus disrupted this time shift below the horizontal.

Quantitative approaches to sensory information encoding by bat noseleaves and pinnae

“…The creation of strong sidelobes by pinna features has been demonstrated in two different vespertilionid species: In the big brown bat (Eptesicus fuscus, Palisot de Beauvois, 1796), numerical analysis of the acoustic function of the tragus has supported the conclusion that the tragus creates a pronounced notch and sidelobe in elevation (Müller 2004). This finding aligns well with behavioral experiments (Lawrence and Simmons 1982) and measurements from the bat pinna in the same species (Wotton et al 1995;Aytekin et al 2004). Similarly, a flap on the interior surface of the pinna was found to be responsible for the formation of a pronounced sidelobe in the brown long-eared bat (Plecotus auritus, L., 1758) (Müller et al 2008).…”

Ear Deformations Give Bats a Physical Mechanism for Fast Adaptation of Ultrasonic Beam Patterns

“…1, bottom row) tend to have prominent side lobes that change direction in a frequency-dependent fashion. Previously published bat biosonar beam patterns of LDC [13][14][15][16] and HDC bats [17] support the existence of these differences. The beam patterns of HDC bats are well suited for on-axis target detection or identification but are not well adapted for estimating target direction based on spectral signatures.…”

Spatial memory and stereotypy of flight paths by big brown bats in cluttered surroundings

“…The operating range of the big brown bat's echolocation system is limited, by spherical spreading and atmospheric attenuation of ultrasound, to an effective range of about 5m for detecting insects (Kick and Simmons, 1984). The effective angular field-of-view is less limited by the transmit and receive beam patterns, which are very broad (140deg at 25kHz to 60deg at 80kHz, −6dB) (Aytekin et al, 2004;Ghose and Moss, 2003;Hartley and Suthers, 1989;Jen and Chen, 1988;Wotton et al, 1995), than by auditory computations that create a narrower (20-25deg) frontal zone of protection from clutter masking (Bates et al, 2011;Sümer et al, 2009). In sparsely cluttered environments, E. fuscus scans its surroundings by aiming its sonar beam at relevant objects (Surlykke et al, 2009).…”