Beampattern Complexity And Time-Variance In Flying Bats

Emission beampatterns,i.e.,ultrasound amplitude as a function of direction and frequency,could be an important characteristic of a bat's biosonar since they determine which part of an environment is illuminated with a given frequency and at a given time.Numerical results have predicted intricate geometrical features such as multiple peaks and notches in bat biosonar beampatteris.Furthermore,it has been shown that bats with nasal emission change the shapes of their emission baffles,"noseleaves",during pulse emission,and pulses that were accompanied by unequivocal noseleaf motions showed a significantly larger variability across time than those that were not.Hipposideros pratti is one of the species of the Hipposderidae that emit biosonar pulses nasally and diffract the outgoing wave-packets with elaborate noseleaves.The work presented here is aimed at understanding if and how these two aspects(beam geometry and emission dynamics)are integrated into the bats' biosonar behaviors on the wing.It is the first time to study the geometrical characteristics of the biosonar beampatterns of the flying Hipposideros pratti bats.This study not only provides a better understanding of the biosonar system of Hipposideros pratti bats,but also inspires people's understanding of bat biosonar navigation strategies.Furthermore,if possible,the noseleaves motion can also be combined with the emission beam,more interesting findings of bat biosonar system may be uncovered.Moreover,from the engineering perspective,the knowledge of bat biosonar system could promote the developments of bionics,for example,provideing novel ideas and solutions for man-made sonar,antenna,and micro air vehicle(MAY).The main work and contributions of the thesis are the following:1.Designed and built an experimental platform for biosonar beampattern measurements of flying bats.The experimental platform contains an ultrasonic microphone array,binocular camera,and a anechoic flight tunnel which bats can fly freely.Except for study of biosonar beampattern,this experimental platform will promote more research about bat biosonar.2.Based on the platform,the ultrasound data and image data of three bats were successfully collected.Then,acoustics signal processing,computer vision,and interpolation were used to reconstruct the bat biosonar beampattern.The results show both simple and complex structure of bat biosonar beampatterns.Moreover,the time-variance of bat biosonar beampatterns is also obvious.3.Corroborated the reliability of reconstructed bat beampattern by using a reference.A loudspeaker was placed in the same position as the bat to serve as a static reference because of the known simple structure of loudspeaker beampattern.From the contrast of the loudspeaker beampattern and bat biosonar beampattern,the bat biosonar beampatterns are more complex and time-variant.Furthermore,linear discriminant analysis,entropy and standard deviation were also introduced to quantitatively investigate the complexities of bat beampatterns and loudspeaker beampatterns.Linear discriminant analysis shows bat beampatterns are different from loudspeaker beampatterns;Entropy analysis indicates bat beampatterns has more information than loudspeaker beampatterns;Standard deviation corroborates bat beampatterns are more complex than loudspeaker beampatterns.