Doctoral dissertation

• 1.
  Umadi, Ravi (2025). Echolocation Dynamics in Active Foraging and Pursuit TUM School of Life Sciences. Submitted

  Abstract

  Echolocating bats rely on self-generated biosonar signals to perceive and navigate their environments. While traditional models treat echolocation as a rigid, reflexive behaviour with fixed emitter and receiver geometries, empirical evidence suggests a high degree of plasticity in both signal generation and spatial sampling. This thesis explores the dynamic sonar strategies employed by the omnivorous bat Phyllostomus discolor, focusing on how emitter-receiver decoupling, deformable noseleaf morphology, and context-sensitive signal timing enhance acoustic sensing flexibility. I first present measured beam geometric and a computational beam model that treats the bat's noseleaf as a two-point source rather than a conventional piston emitter. This approach reveals that small deformations in the noseleaf can result in substantial changes to the spatial profile of the sonar beam, including shifts in beam direction and width. My results show that such modulation is functionally significant, allowing the bat to alter its acoustic gaze without body reorientation. Subsequently, I examine the degree of coordination between emission and reception axes during fixed-ears and free-moving conditions. Using high-resolution motion capture and stereo microphone arrays, I demonstrate that P. discolor exhibits fast, independent pinna movements that enable acoustic sampling across a wide spatial field. This emitter-receiver decoupling enhances spatial resolution and increases the likelihood of detecting novel or peripheral targets, supporting a broader sensory field during foraging and navigation. In the final study, I analyse changes in sonar signal structure during target approach. I show that bats regulate their call rate and duration based not only on distance to target but also on task urgency and context variability. The terminal buzz, traditionally viewed as a motor constraint, is instead framed here as a responsive sensory strategy that modulates information update rate dynamically. Together, these findings reveal a sophisticated system of acoustic sensing grounded in morphological flexibility, sensorimotor coordination, and adaptive control. By integrating physical modelling, behavioural experiments, and signal analysis, my work contributes to a growing view of echolocation as an active and embodied perceptual process. These insights have implications for the evolution of biosonar systems and offer promising design principles for artificial sensing technologies, particularly in robotics and autonomous navigation.

Methods

• 1.
  Umadi, Ravi (2025). Embedded Ultrasonics - A Microcontroller-Based Multichannel Ultrasound Recorder for Behavioural Field Studies Submitted [link]

  Abstract

  1. Behavioural studies investigating acoustic communication in animals—particularly echolocating bats—require transducers and recording systems that are lightweight, power-efficient, and easy to deploy. However, capturing high-frequency calls typically demands specialised, high-cost equipment and laptops, which limit portability and scalability in field settings. Advances in embedded microcontrollers and high-fidelity MEMS microphones offer an underexplored opportunity for building compact, affordable, and open-source recorders. These platforms provide design flexibility and a growing support community, but are seldom adapted for multi-channel ultrasonic recording in behavioural research. 2. To address this gap, I developed Batsy4-Pro, a 4-channel ultrasonic recorder based on the Teensy 4.1 microcontroller. The system uses WM8782 ADCs to acquire synchronised 192 kHz audio streams and records them to a microSD card. The firmware is written in C++ on Arduino IDE and allows flexible configuration of recording parameters, buffering schemes, and triggering logic. The system weighs under 150 g and runs on a 5 V DC power supply, enabling untethered field deployment. 3. The system also integrates real-time heterodyne monitoring using a PCM5102A DAC, allowing users to audibly monitor bat activity during field experiments, reducing the need for additional equipment and helping field researchers decide when to initiate recordings. 4. Using a custom-built microphone array, I validated its recording fidelity using both synthetic bat call playbacks under controlled conditions and free-flying bat vocalisations in a natural foraging corridor. Recordings consistently showed high signal-to-noise ratios (>40 dB), suitable for accurate call detection and spatial localisation. 5. Batsy4-Pro offers an accessible and extensible tool for ultrasonic recording in behavioural, ecological, and neuroethological research, supporting studies where portability, ease of use, and field-readiness are essential. The open-source code and modular design facilitate community-driven development and promote the system's adoption, enabling the evolution of experimental protocols and recording paradigms, as well as the exploration of novel approaches to behavioural experiments in natural settings.

• 2.
  Umadi, Ravi (2025). ESPERDYNE - A Dual-Band Heterodyne Monitor and Ultrasound Recorder for Bioacoustic Field Surveys. Accepted at Methods in Ecology and Evolution [link]

  Abstract

  1. Background. Ultrasonic monitoring is essential for ecological studies of bats and other animals, yet high-performance field devices remain prohibitively expensive and inaccessible-particularly in biodiversity-rich regions with limited research infrastructure. Existing low-cost options often lack real-time listening and require a complex setup. There remains a critical need for versatile, affordable, and field-ready tools that support acoustic behavioural research, educational and conservation outreach. 2. New Tool. I introduce Esperdyne, an open-source, dual-channel ultrasound monitoring and recording system based on the ESP32-S3 microcontroller. With a component cost under 75 euro, Esperdyne combines real-time heterodyne monitoring, stereo recording from a retroactive ring buffer, and an intuitive rotary-based user interface with OLED display. It supports full-duplex 192 kHz audio, dual-band tuning for simultaneous FM/CF monitoring, and real-time playback via headphones or a speaker. All audio processing-including adjustable carrier frequency mixing, gain control, and file-saving logic-is implemented without reliance on fixed-rate audio libraries. 3. Applications. Esperdyne has been tested in field conditions and shown to reliably detect high-SNR calls and harmonics from free-flying bats. A companion MATLAB tool Bat Reviewer supports rapid inspection, playback, and export of selected recordings. Together, these tools enable portable, solo-operated acoustic surveys with minimal training. Beyond ecological research, Esperdyne is suitable for education, outreach, and preliminary field assessments in remote or resource-constrained settings. Its modular design encourages hardware customisation and firmware extension by interdisciplinary teams. 4. Availability and Implementation. Full hardware schematics, firmware, and software tools are publicly available. The system can be built using hobbyist-accessible components and standard Arduino tooling. By sharing this system openly, I aim to lower technical barriers and foster broader participation in ultrasound-based biodiversity monitoring and conservation. Esperdyne demonstrates how microcontroller-based platforms can bridge gaps between affordability, usability, and scientific capability-supporting global efforts in soundscape ecology.

Research

• 1.
  Umadi, Ravi (August 25, 2025). Widefield Acoustics Heuristic - Advancing Microphone Array Design for Accurate Spatial Tracking of Echolocating Bats BMC Ecology and Evolution [link]

  Abstract

  Accurate three-dimensional localisation of ultrasonic bat calls is essential for advancing behavioural and ecological research. I present a comprehensive, open-source simulation framework—Array WAH —for designing, evaluating, and optimising microphone arrays tailored to bioacoustic tracking. The tool incorporates biologically realistic signal generation, frequency-dependent propagation, and advanced Time Difference of Arrival (TDoA) localisation algorithms, enabling precise quantification of both positional and angular accuracy. The framework supports both frequency-modulated (FM) and constant-frequency (CF) call types, the latter characteristic of Hipposiderid and Rhinolophid bats, which are particularly prone to localisation errors due to their long-duration emissions. A key innovation is the integration of source motion modelling during call emission, which introduces Doppler-based time warping and phase shifts across microphones—an important and often overlooked source of error in source localisation. I systematically compare four array geometries—a planar square, a pyramid, a tetrahedron, and an octahedron—across a volumetric spatial grid. The tetrahedral and octahedral configurations demonstrate superior localisation robustness, while planar arrays exhibit limited angular resolution. My simulations reveal that spatial resolution is fundamentally constrained by array geometry and the signal structure, with typical localisation error ranging between 5-10 cm at 0.5 m arm lengths. By providing a flexible, extensible, and user-friendly simulation environment, Array WAH supports task-specific design and deployment of compact, field-deployable localisation systems. It is especially valuable for investigating the acoustic behaviour of free-flying bats under naturalistic conditions, and complements emerging low-power multichannel ultrasonic recorders for field deployment and method validation.

• 2.
  Umadi, Ravi (2025). Swarm Cohesion in Bats Emerges from Stable Temporal Loops Under Review [link]

  Abstract

  Swarming in echolocating bats presents a compelling example of decentralised coordination driven by acoustic sensing. Unlike visually guided animals, bats navigate and maintain cohesion in dense groups using self-generated sonar signals. I present a biologically grounded simulation framework in which agents operate asynchronously using a closed-loop control policy based on local echo delays. Building on the theory of biosonar responsivity and the deduction of temporal precision in bat echolocation, my model predicts a tight coupling between echo delay, call duration, and call rate, and demonstrates that local, echo-timed interactions suffice to generate stable, self-organised swarm behaviour. To test these predictions, I systematically varied initial velocity and responsivity coefficient, kr, across a simulation grid and analysed the resulting call dynamics and collision risks. An information propagation model was derived, estimating the effective spatiotemporal speed of behavioural updates across the swarm. I also developed a decay-based model of perturbation attenuation to quantify the spatial limit of influence for local disturbances. Together, these analyses reveal a trade-off between responsiveness and stability - lower kr yields faster information flow but higher collision risk in denser regions, while higher kr promotes stability but reduces reactivity. My results align closely with recent empirical findings and provide a generative explanation for swarm cohesion based on the temporal precision of echolocation. These insights offer a promising design framework for decentralised control in bioinspired robotic swarms, where stability, autonomy, and resilience emerge from local sensing and feedback, rather than explicit synchrony or global coordination.

• 3.
  Umadi, Ravi (2025). Temporal Feasibility Constraints on Wingbeat-Call Synchrony in Actively Echolocating Bats. Under Review [link]

  Abstract

  1. Echolocating bats coordinate sound production, echo reception, and flight within a closed sensorimotor loop that operates under finite propagation delays and bounded response times. Although call timing and wingbeat synchrony have been described across many behavioural contexts, it remains unclear when such coordination is temporally feasible and when it must necessarily break down. Here, I develop a constraint-based analysis that formalises how temporal feasibility limits shape behavioural coordination during active echolocation. 2. Building on the responsivity framework, the model explicitly represents the ordering and timing dependencies between call emission, echo delay, sensory processing, and subsequent action, and derives conditions under which call timing can remain phase-locked to a cyclic motor rhythm such as the wingbeat. Analysis of these constraints reveals distinct coordination regimes: permissive regimes in which phase locking can emerge without dedicated coupling, and constrained regimes in which progressive phase slip or decoupling becomes unavoidable as sensory demand increases. 3. Monte Carlo simulations show that transitions between synchrony and asynchrony arise as necessary consequences of first-principles timing constraints and bounded motor dynamics, rather than from changes in behavioural strategy. Increased motor flexibility shifts, but does not eliminate, the boundaries of synchrony-permissive regimes. Empirical observations from field studies are discussed as illustrative examples of these regimes, highlighting how apparent coupling and decoupling can both emerge from the same underlying control architecture. 4. Together, this work identifies temporal feasibility as a governing constraint on echolocation behaviour, clarifies when buffered pseudo&[mdash]closed-loop operation can masquerade as feedback control, and generates testable predictions for when and why wingbeat&[mdash]call synchrony should fail in ecological contexts such as prey pursuit. More broadly, the framework situates bat echolocation within a general class of active sensing systems shaped by delayed feedback and bounded response dynamics.

• 4.
  Umadi, Ravi and Firzlaff, Uwe (2025). Biosonar Responsivity Sets the Stage for the Terminal Buzz Under Review [link]

  Abstract

  Echolocating bats dynamically adjust their sonar signals during prey pursuit, yet the mechanistic limits that govern these rapid transitions have remained unclear. Here, we introduce the responsivity framework, a predictive model that formalises the scaling between echo delay, call rate, and relative velocity through a single parameter-the responsivity coefficient kr. From this relation, we derive biologically interpretable quantities such as the reaction window Tb and the buzz-readiness threshold, marking the onset of a high-gain sensorimotor regime preceding the terminal buzz. Simulations of bat-prey interactions, incorporating both stationary and motile targets, reproduced systematic velocity-call-rate trade-offs and realistic behavioural profiles, from which distances, velocities, and reaction times could be inferred using call timing alone. Internal consistency checks confirmed that the framework's analytical identities for distance and velocity hold across sequences, while spatio-temporal maps revealed how Tb contracts with increasing kr and velocity, defining the biophysical limit of temporal control. Comparisons with high-resolution field recordings showed that observed call-rate dynamics followed the predicted trends, with variability arising from environmental context and localisation uncertainty. By linking simple acoustic observables to a broad set of derived parameters, the responsivity framework provides a mechanistic and predictive tool for interpreting echolocation behaviour. It explains variable buzz lengths and reaction limits consistent with experimental observations. It establishes a general principle: sequential adaptive behaviours unfold under constraints set by the speed of regulatory feedback. While demonstrated in bat biosonar, this principle offers broader relevance to understanding adaptive control and sensory-motor integration across biological systems.

• 5.
  Umadi, Ravi (2025). Oscillating Ears Dynamically Transform Echoes in Constant-Frequency Bats In Revisions [link]

  Abstract

  Constant-frequency (CF) bats exhibit rapid oscillations of their external ears. Yet, the functional role of these movements has remained unresolved since their initial documentation over half a century ago. Although recent studies have demonstrated that pinna motion generates Doppler shifts, they do not explain why ear oscillations intensify at close range or how these dynamics contribute to echo perception. In this study, I investigate the hypothesis that oscillatory ear movements enhance echo information during CF echolocation. Using a simplified receiver-motion model, I examine how time-varying pinna pose reshapes the temporal and spectral structure of returning echoes. I show that ear oscillations inject dynamic transformations into the received signal, producing multiple informative views of the same echo and increasing both temporal contrast and spectral diversity around the CF carrier. These transformations are strongest under behavioural conditions in which target-state uncertainty is expected to be high, offering a potential functional explanation for the long-standing observation that ear-oscillation rate increases as bats approach a target. The results suggest that oscillatory ear movements act as an adaptive, receiver-side mechanism that enhances echo information during CF echolocation, complementing the well-known emitter-side adaptations of high-duty-cycle biosonar.