Jason E. Gaudette

Echolocating Big Brown Bats, Eptesicus fuscus, Modulate Pulse Intervals to Overcome Range Ambiguity in Cluttered Surroundings.

Big brown bats (Eptesicus fuscus) emit trains of brief, wideband frequency-modulated (FM) echolocation sounds and use echoes of these sounds to orient, find insects, and guide flight through vegetation. They are observed to emit sounds that alternate between short and long inter-pulse intervals (IPIs), forming sonar sound groups. The occurrence of these strobe groups has been linked to flight in cluttered acoustic environments, but how exactly bats use sonar sound groups to orient and navigate is still a mystery. Here, the production of sound groups during clutter navigation was examined. Controlled flight experiments were conducted where the proximity of the nearest obstacles was systematically decreased while the extended scene was kept constant. Four bats flew along a corridor of varying widths (100, 70, and 40 cm) bounded by rows of vertically hanging plastic chains while in-flight echolocation calls were recorded. Bats shortened their IPIs for more rapid spatial sampling and also grouped their sounds more tightly when flying in narrower corridors. Bats emitted echolocation calls with progressively shorter IPIs over the course of a flight, and began their flights by emitting shorter starting IPI calls when clutter was denser. The percentage of sound groups containing 3 or more calls increased with increasing clutter proximity. Moreover, IPI sequences having internal structure become more pronounced when corridor width narrows. A novel metric for analyzing the temporal organization of sound sequences was developed, and the results indicate that the time interval between echolocation calls depends heavily on the preceding time interval. The occurrence of specific IPI patterns were dependent upon clutter, which suggests that sonar sound grouping may be an adaptive strategy for coping with pulse-echo ambiguity in cluttered surroundings.

Multi-component separation and analysis of bat echolocation calls

The vast majority of animal vocalizations contain multiple frequency modulated (FM) components with varying amounts of non-linear modulation and harmonic instability. This is especially true of biosonar sounds where precise time-frequency templates are essential for neural information processing of echoes. Understanding the dynamic waveform design by bats and other echolocating animals may help to improve the efficacy of man-made sonar through biomimetic design. Bats are known to adapt their call structure based on the echolocation task, proximity to nearby objects, and density of acoustic clutter. To interpret the significance of these changes, a method was developed for component separation and analysis of biosonar waveforms. Techniques for imaging in the time-frequency plane are typically limited due to the uncertainty principle and interference cross terms. This problem is addressed by extending the use of the fractional Fourier transform to isolate each non-linear component for separate analysis. Once separated, empirical mode decomposition can be used to further examine each component. The Hilbert transform may then successfully extract detailed time-frequency information from each isolated component. This multi-component analysis method is applied to the sonar signals of four species of bats recorded in-flight by radiotelemetry along with a comparison of other common time-frequency representations.

Mouth gape angle has little effect on the transmitted signals of big brown bats (Eptesicus fuscus)

Bats perform high-resolution echolocation by comparing temporal and spectral features of their transmitted pulses to the received echoes. In complex environments with moving prey, dynamically adapting the transmitted pulses can increase the probability of successful target representation and interception. This study further investigates the adaptive vocal-motor strategies of big brown bats (Eptesicus fuscus). During stationary target detection experiments, echolocation sounds were simultaneously recorded with high-speed, infrared video to examine the relationship of mouth position and movement to pulse characteristics among bats. All three bats produced strobe groups, but the proportion and frequency characteristics of the strobe group pulses differed for individual bats. Additionally, mouth gape angle had little effect on the emitted pulse characteristics, which suggests that laryngeal mechanisms drive changes in emitted pulses.

Modeling of bio-inspired broadband sonar for high-resolution angular imaging

Echolocating mammals perceive images of targets with hyper-resolution and navigate seamlessly through obstacles in complex acoustic environments. The biological solution to imaging with sound is vastly different from man-made sonar. The most prominent difference is that instead of imaging with narrow beams, bats ensonify a large spatial region and exploit broadband echo information to acoustically focus with about one degree of angular resolution. Angular localization may therefore be redefined as a spectral pattern matching problem. By imaging with wider beams, this remarkable performance requires only a single broadband transmitter and two receive elements. Our computational modeling work has led to new insight into the salient spatial information encoded by the bat’s auditory system. Although theoretically not required, spatial localization performance increases with the aid of highly complex baffle structures such as those found in biological sonar. Replicating these bio-inspired baffle structures and...

Large reconfigurable microphone array for transmit beam pattern measurements of echolocating bats

Measurements of the transmit beam patterns in bats have previously been limited to a single cross-sectional plane or averaged over multiple in-flight approaches with sparse microphone arrays. No high-resolution measurements have been published to date of individual transmitted beams jointly in azimuth and elevation. Toward this goal, a high density microphone array was designed and constructed using low-cost ultrasonic microphones and custom electronic circuitry. The planar array is 1.83 meters wide by 1.42 meters tall with sensors positioned on a 2.54 cm square grid. The system can record up to 228 channels simultaneously at a 500 kHz sampling rate. Big brown bats (Eptesicus fuscus) were trained to echolocate pairs of virtual targets in a two-alternative forced choice discrimination task while their signals were being recorded by the array. Visualizations of the beam patterns during the task will be presented along with some advanced signal processing techniques used in the analysis. [funded by ONR and N...

Acoustic tracking of bats in clutter environments using microphone arrays

The big brown bat, Eptesicus fuscus, uses echolocation for foraging and orientation. Bats can change the echolocation calls dependent on the environments. Therefore, it is necessary to clarify the changes of acoustic characteristics of these calls. In this study, the flight path of the bat were tracked by computing the time differences of arrivals (TDOA) at the microphone array system in the flight room. The acoustic patterns of echolocation calls could be calculated from the measured call data at each microphone by compensating the spread and absorption loss. The head aim and beam pattern at each harmonics were computed from these acoustic patterns of echolocation calls. It was examined whether these acoustics beam patterns were dependent on clutter environment, that is, density of chains. [This research was supported by ONR, NSF, and JST, CREST.]

High resolution acoustic measurement system and beam pattern reconstruction method for bat echolocation emissions

Measurements of the transmit beam patterns emitted by echolocating bats have previously been limited to cross-sectional planes or averaged over multiple signals using sparse microphone arrays. To date, no high-resolution measurements of individual bat transmit beams have been reported in the literature. Recent studies indicate that bats may change the time-frequency structure of their calls depending on the task, and suggest that their beam patterns are more dynamic than previously thought. To investigate beam pattern dynamics in a variety of bat species, a high-density reconfigurable microphone array was designed and constructed using low-cost ultrasonic microphones and custom electronic circuitry. The planar array is 1.83 m wide by 1.42 m tall with microphones positioned on a 2.54 cm square grid. The system can capture up to 228 channels simultaneously at a 500 kHz sampling rate. Beam patterns are reconstructed in azimuth, elevation, and frequency for visualization and further analysis. Validation of the array measurement system and post-processing functions is shown by reconstructing the beam pattern of a transducer with a fixed circular aperture and comparing the result with a theoretical model. To demonstrate the system in use, transmit beam patterns of the big brown bat, Eptesicus fuscus, are shown.

Modeling of precise onset spike timing for echolocation in the big brown bat, Eptesicus fuscus.

The neuronal circuits enabling highly accurate sonar performance in bats are not well understood. In this research, a computational neuroscience model was developed based on Meddis’ auditory peripheral model and a biologically plausible network of integrate‐and‐fire neurons. The stimuli presented to the model include acoustic simulations of target echoes using wideband FM echolocation calls recorded from Eptesicus fuscus. Synaptic connectivity of the neural network was varied to explore its effect on the precision of onset spike timing. Results show that precisely timed spike codes can relay sufficient information of echo time‐frequency representations to account for performance observed in laboratory experiments. This research represents a portion of our overarching goal to reverse engineer the bat’s critical auditory functions into a refined sonar signal processing algorithm. A necessary first component of this process is to understand the functionality of relevant circuitry in the auditory midbrain. Su...

Sonar processing by the spectrogram correlation and transformation model of biosonar

Echolocating big brown bats emit frequency-modulated (FM) biosonar sounds and perceive target range from echo delays through spectrogram correlation (SC) and target shape from interference nulls in echo spectra through spectrogram transformation (ST). Combined, the SCAT model is a computationally unified auditory description of biosonar as a real-time process. We developed a Matlab implementation of SCAT and tested it with a succession of simulated bat-like FM signals (chirps), each followed by one or more FM echoes that have realistic delay and spectral characteristics. The model simulates neural response latencies in frequency-tuned delay-lines that use coincidence detections for target ranging by SC. For ST, a novel, deconvolution-like network transforms echo spectra into images of the target’s glints by detecting coincidences between spikes that represent spectral nulls in parallel channels tuned to null frequencies. Experiments show that dolphins likely separate ST into two operations—MaPS for short glint separations (macro power spectral features, 80 µs). The ST deconvolution network models MiPS. The highly distributed character of the model favors real-time operation, an important goal for bioinspired sonar development. [Work supported by ONR.]Echolocating big brown bats emit frequency-modulated (FM) biosonar sounds and perceive target range from echo delays through spectrogram correlation (SC) and target shape from interference nulls in echo spectra through spectrogram transformation (ST). Combined, the SCAT model is a computationally unified auditory description of biosonar as a real-time process. We developed a Matlab implementation of SCAT and tested it with a succession of simulated bat-like FM signals (chirps), each followed by one or more FM echoes that have realistic delay and spectral characteristics. The model simulates neural response latencies in frequency-tuned delay-lines that use coincidence detections for target ranging by SC. For ST, a novel, deconvolution-like network transforms echo spectra into images of the target’s glints by detecting coincidences between spikes that represent spectral nulls in parallel channels tuned to null frequencies. Experiments show that dolphins likely separate ST into two operations—MaPS for short ...

Bio-inspired broadband sonar array prototypes and underwater experiments for two- and three-dimensional acoustic imaging applications

Underwater sonar imaging relies upon the information contained in time correlation delays across numerous channels. Designing higher resolution imaging systems currently requires increasing the array aperture to wavelength ratio (L / λ), where L is the effective array length and λ is the acoustic wavelength in the medium. This fundamental constraint leads engineers down the path of adding a significant number of channels to a sonar design, which in turn increases array complexity and cost. Our research in bio-inspired sonar has revealed an approach that circumvents this constraint by exploiting bandwidth in addition to time-delay for the angular imaging process. Presented will be the results of a 2-channel underwater prototype tested in an acoustic tank and design work toward a 3-channel prototype for extending imaging to elevation angles. This work represents ongoing efforts toward developing a compact, low-cost broadband underwater sonar for near- to mid-range imaging and classification. Future integration with an autonomous underwater vehicle will demonstrate this technology for simple obstacle detection and avoidance of complex objects. [Work supported by ONR and internal investments by NUWC Division Newport.]Underwater sonar imaging relies upon the information contained in time correlation delays across numerous channels. Designing higher resolution imaging systems currently requires increasing the array aperture to wavelength ratio (L / λ), where L is the effective array length and λ is the acoustic wavelength in the medium. This fundamental constraint leads engineers down the path of adding a significant number of channels to a sonar design, which in turn increases array complexity and cost. Our research in bio-inspired sonar has revealed an approach that circumvents this constraint by exploiting bandwidth in addition to time-delay for the angular imaging process. Presented will be the results of a 2-channel underwater prototype tested in an acoustic tank and design work toward a 3-channel prototype for extending imaging to elevation angles. This work represents ongoing efforts toward developing a compact, low-cost broadband underwater sonar for near- to mid-range imaging and classification. Future integrat...