Nearfield and farfield measurements of dolphin echolocation beam patterns: No evidence of focusing

Exploring the directivities of whistle in the Indo-Pacific humpback dolphin (Sousa chinensis) and their dependency on the whistles' frequency contour

Directional communication plays a pivotal role in enabling odontocetes to maintain group coordination and social interactions. The fundamental frequency, number of harmonics, and their relative energy distribution in whistles exhibit temporal variation. This study investigated the whistles produced by the Indo-Pacific humpback dolphins (Sousa chinensis) in Xiamen Bay, China. Using computed tomography scanning data, we developed a numerical model of the species and used finite element modeling to examine the beam patterns at both fundamental and harmonic frequencies of whistles, ranging from 3.9 to 64.9 kHz, which corresponds to directivity indices (DIs) between 2.2 and 16.2 dB. We weighted the beams at the fundamental frequencies and harmonics based on their energy distribution to derive composite beam patterns at specific time stamps, allowing us to investigate temporal variations in the corresponding DI within individual whistles. The time-varying properties of DIs were analyzed for various whistle types, including constant, upsweep, downsweep, convex, and sine. Given that harmonics are integer multiples of the fundamental frequency, their contours exhibit similar shapes, whereas the composite DI showed more complexity. These findings indicate that the proportion of energy between the fundamental frequency and harmonics is a key determinant of whistle directivity in Indo-Pacific humpback dolphins.

The effects of range and echo-phase on range resolution in bottlenose dolphins (Tursiops truncatus) performing a successive comparison taska)

Echolocating bats and dolphins use biosonar to determine target range, but differences in range discrimination thresholds have been reported for the two species. Whether these differences represent a true difference in their sensory system capability is unknown. Here, the dolphin's range discrimination threshold as a function of absolute range and echo-phase was investigated. Using phantom echoes, the dolphins were trained to echo-inspect two simulated targets and indicate the closer target by pressing a paddle. One target was presented at a time, requiring the dolphin to hold the initial range in memory as they compared it to the second target. Range was simulated by manipulating echo-delay while the received echo levels, relative to the dolphins' clicks, were held constant. Range discrimination thresholds were determined at seven different ranges from 1.75 to 20 m. In contrast to bats, range discrimination thresholds increased from 4 to 75 cm, across the entire ranges tested. To investigate the acoustic features used more directly, discrimination thresholds were determined when the echo was given a random phase shift (±180°). Results for the constant-phase versus the random-phase echo were quantitatively similar, suggesting that dolphins used the envelope of the echo waveform to determine the difference in range.

A dolphin-inspired compact sonar for underwater acoustic imaging

Underwater imaging sonars are widely used for oceanic exploration but are bulky and expensive for some applications. The sonar system of dolphins, which uses sound pulses called clicks to investigate their environment, offers superior shape discrimination capability compared to human-derived imaging sonars of similar size and frequency. In order to gain better understanding of dolphin sonar imaging, we train a dolphin to acoustically interrogate certain objects and match them visually. We record the echoes the dolphin receives and are able to extract object shape information from these recordings. We find that infusing prior information into the processing, specifically the sparsity of the shapes, yields a clearer interpretation of the echoes than conventional signal processing. We subsequently develop a biomimetic sonar system that combines sparsity-aware signal processing with high-frequency broadband click signals similar to that of dolphins, emitted by an array of transmitters. Our findings offer insights and tools towards compact higher resolution sonar imaging technologies.

Investigation on whistle directivity in the Indo-Pacific humpback dolphin (Sousa chinensis) through numerical modeling

Odontocetes have evolved special acoustic structures in the forehead to modulate echolocation and communication signals into directional beams to facilitate feeding and social behaviors. Whistle directivity was addressed for the Indo-Pacific humpback dolphin (Sousa chinensis) by developing numerical models in the current paper. Directivity was first examined at the fundamental frequency 5 kHz, and simulations were then extended to the harmonics of 10, 15, 20, 25, and 30 kHz. At 5 kHz, the -3 dB beam widths in the vertical and horizontal planes were 149.3° and 119.4°, corresponding to the directivity indexes (DIs) of 4.4 and 5.4 dB, respectively. More importantly, we incorporated directivity of the fundamental frequency and harmonics to produce an overall beam, resulting in -3 dB beam widths of 77.2° and 62.9° and DIs of 8.2 and 9.7 dB in the vertical and horizontal planes, respectively. Harmonics can enhance the directivity of fundamental frequency by 3.8 and 4.3 dB, respectively. These results suggested the transmission system can modulate whistles into directional projection, and harmonics can improve DI.

Numerical-modeling-based investigation of sound transmission and reception in the short-finned pilot whale (Globicephala macrorhynchus)

The sound-transmission, beam-formation, and sound-reception processes of a short-finned pilot whale (Globicephala macrorhynchus) were investigated using computed tomography (CT) scanning and numerical simulation. The results showed that sound propagations in the forehead were modulated by the upper jaw, air components, and soft tissues, which attributed to the beam formation in the external acoustic field. These structures owned different acoustic impedance and formed a multiphasic sound transmission system that can modulate sounds into a beam. The reception pathways composed of the solid mandible and acoustic fats in the lower head conducted sounds into the tympano-periotic complex. In the simulations, sounds were emitted in the forehead transmission system and propagated into water to interrogate a steel cylinder. The resulting echoes can be interpreted from multiple perspectives, including amplitude, waveform, and spectrum, to obtain the acoustic cues of the steel cylinder. By taking the short-finned pilot whale as an example, this study provides meaningful information to further deepen our understanding of biosonar system operations, and may expand sound-reception theory in odontocetes.

How Loud Can you go? Physical and Physiological Constraints to Producing High Sound Pressures in Animal Vocalizations

Sound is vital for communication and navigation across the animal kingdom and sound communication is unrivaled in accuracy and information richness over long distances both in air and water. The source level (SL) of the sound is a key factor in determining the range at which animals can communicate and the range at which echolocators can operate their biosonar. Here we compile, standardize and compare measurements of the loudest animals both in air and water. In air we find a remarkable similarity in the highest SLs produced across the different taxa. Within all taxa we find species that produce sound above 100 dBpeak re 20 μPa at 1 m, and a few bird and mammal species have SLs as high as 125 dBpeak re 20 μPa at 1 m. We next used pulsating sphere and piston models to estimate the maximum sound pressures generated in the radiated sound field. These data suggest that the loudest species within all taxa converge upon maximum pressures of 140–150 dBpeak re 20 μPa in air. In water, the toothed whales produce by far the loudest SLs up to 240 dBpeak re 1 μPa at 1 m. We discuss possible physical limitations to the production, radiation and propagation of high sound pressures. Furthermore, we discuss physiological limitations to the wide variety of sound generating mechanisms that have evolved in air and water of which many are still not well-understood or even unknown. We propose that in air, non-linear sound propagation forms a limit to producing louder sounds. While non-linear sound propagation may play a role in water as well, both sperm whale and pistol shrimp reach another physical limit of sound production, the cavitation limit in water. Taken together, our data suggests that both in air and water, animals evolved that produce sound so loud that they are pushing against physical rather than physiological limits of sound production, radiation and propagation.

Echolocation click parameters and biosonar behaviour of the dwarf sperm whale (Kogia sima)

Dwarf sperm whales (Kogia sima) are small toothed whales that produce narrow-band high-frequency (NBHF) echolocation clicks. Such NBHF clicks, subject to high levels of acoustic absorption, are usually produced by small, shallow-diving odontocetes, such as porpoises, in keeping with their short-range echolocation and fast click rates. Here, we sought to address the problem of how the little-studied and deep-diving Kogia can hunt with NBHF clicks in the deep sea. Specifically, we tested the hypotheses that Kogia produce NBHF clicks with longer inter-click intervals (ICIs), higher directionality and higher source levels (SLs) compared with other NBHF species. We did this by deploying an autonomous deep-water vertical hydrophone array in the Bahamas, where no other NBHF species are present, and by taking opportunistic recordings of a close-range Kogia sima in a South African harbour. Parameters from on-axis clicks (n=46) in the deep revealed very narrow-band clicks (root mean squared bandwidth, BW_RMS, of 3±1 kHz), with SLs of up to 197 dB re. 1 µPa peak-to-peak (μPa_pp) at 1 m, and a half-power beamwidth of 8.8 deg. Their ICIs (mode of 245 ms) were much longer than those of porpoises (<100 ms), suggesting an inspection range that is longer than detection ranges of single prey, perhaps to facilitate auditory streaming of a complex echo scene. On-axis clicks in the shallow harbour (n=870) had ICIs and SLs in keeping with source parameters of other NBHF cetaceans. Thus, in the deep, dwarf sperm whales use a directional, but short-range echolocation system with moderate SLs, suggesting a reliable mesopelagic prey habitat.

Possible limitations of dolphin echolocation: a simulation study based on a cross-modal matching experiment

Dolphins use their biosonar to discriminate objects with different features through the returning echoes. Cross-modal matching experiments were conducted with a resident bottlenose dolphin (Tursiops aduncus). Four types of objects composed of different materials (water-filled PVC pipes, air-filled PVC pipes, foam ball arrays, and PVC pipes wrapped in closed-cell foam) were used in the experiments, respectively. The size and position of the objects remained the same in each case. The data collected in the experiment showed that the dolphin’s matching accuracy was significantly different across the cases. To gain insight into the underlying mechanism in the experiments, we used finite element methods to construct two-dimensional target detection models of an echolocating dolphin in the vertical plane, based on computed tomography scan data. The acoustic processes of the click’s interaction with the objects and the surrounding media in the four cases were simulated and compared. The simulation results provide some possible explanations for why the dolphin performed differently when discriminating the objects that only differed in material composition in the previous matching experiments.

Echolocation click parameters of short-finned pilot whales (Globicephala macrorhynchus) in the wild

Short-finned pilot whales (Globicephala macrorhynchus) are large, deep-diving predators with diverse foraging strategies, but little is known about their echolocation. To quantify the source properties of short-finned pilot whale clicks, we made 15 deployments off the coast of Tenerife of a deep-water hydrophone array consisting of seven autonomous time-synced hydrophone recorders (SoundTraps), enabling acoustic localization and quantification of click source parameters. Of 8185 recorded pilot whale clicks, 47 were classified as being recorded on-axis, with a mean peak-to-peak source level (SL) of 181 ± 7 dB re 1 μPa, a centroid frequency of 40 ± 4 kHz, and a duration of 57 ± 23 μs. A fit to a piston model yielded an estimated half-power (-3 dB) beam width of 13.7° [95% confidence interval (CI) 13.2°-14.5°] and a mean directivity index (DI) of 22.6 dB (95% CI 22.5-22.9 dB). These measured SLs and DIs are surprisingly low for a deep-diving toothed whale, suggesting we sampled the short-finned pilot whales in a context with little need for operating a long-range biosonar. The substantial spectral overlap with beaked whale clicks emitted in similar deep-water habitats implies that pilot whale clicks may constitute a common source of false detections in beaked whale passive acoustic monitoring efforts.

Effects of dolphin hearing bandwidth on biosonar click emissions

Differences in odontocete biosonar emissions have been reported for animals with hearing loss compared to those with normal hearing. For example, some animals with high-frequency hearing loss have been observed to lower the dominant frequencies of biosonar signals to better match a reduced audible frequency range. However, these observations have been limited to only a few individuals and there has been no systematic effort to examine how animals with varying degrees of hearing loss might alter biosonar click properties. In the present study, relationships between age, biosonar click emissions, auditory evoked potentials (AEPs), and hearing bandwidth were studied in 16 bottlenose dolphins (Tursiops truncatus) of various ages and hearing capabilities. Underwater hearing thresholds were estimated by measuring steady-state AEPs to sinusoidal amplitude modulated tones at frequencies from 23 to 152 kHz. Input-output functions were generated at each tested frequency and used to calculate frequency-specific thresholds and the upper-frequency limit of hearing for each subject. Click emissions were measured during a biosonar aspect change detection task using a physical target. Relationships between hearing capabilities and the acoustic parameters of biosonar signals are described here and compared to previous experiments with fewer subjects.

Modeling of the near to far acoustic fields of an echolocating bottlenose dolphin and harbor porpoise

Echolocation signals emitted by odontocetes can be roughly classified into three broad categories: broadband echolocation signals, narrowband high-frequency echolocation signals, and frequency modulated clicks. Previous measurements of broadband echolocation signal propagation in the bottlenose dolphin (Tursiops truncatus) did not find any evidence of focusing as the signals travel from the near-field to far-field. Finite element analysis (FEA) of high-resolution computed tomography scan data was used to examine signal propagation of broadband echolocation signals of dolphins and narrowband echolocation signals of porpoises. The FEA results were used to simulate the propagation of clicks from phonic lips, traveling through the forehead, and finally transmission into the water. Biosonar beam formation in the near-field and far-field, including the amplitude contours for the two species, was determined. The finite element model result for the simulated amplitude contour in the horizontal plane was consistent with prior direct measurement results for Tursiops, validating the model. Furthermore, the simulated far-field transmission beam patterns in both the vertical and horizontal planes were also qualitatively consistent with results measured from live animals. This study indicates that there is no evidence of convergence for either Tursiops or Phocoena as the sound propagates from the near-field to the far-field.

Finite element simulation of broadband biosonar signal propagation in the near- and far-field of an echolocating Atlantic bottlenose dolphin (Tursiops truncatus)

Bottlenose dolphins project broadband echolocation signals for detecting and locating prey and predators, and for spatial orientation. There are many unknowns concerning the specifics of biosonar signal production and propagation in the head of dolphins and this manuscript represents an effort to address this topic. A two-dimensional finite element model was constructed using high resolution CT scan data. The model simulated the acoustic processes in the vertical plane of the biosonar signal emitted from the phonic lips and propagated into the water through the animal's head. The acoustic field on the animal's forehead and the farfield transmission beam pattern of the echolocating dolphin were determined. The simulation results and prior acoustic measurements were qualitatively extremely consistent. The role of the main structures on the sound propagation pathway such as the air sacs, melon, and connective tissue was investigated. Furthermore, an investigation of the driving force at the phonic lips for dolphins that emit broadband echolocation signals and porpoises that emit narrowband echolocation signals suggested that the driving force is different for the two types of biosonar. Finally, the results provide a visual understanding of the sound transmission in dolphin's biosonar.

Automated classification of dolphin echolocation click types from the Gulf of Mexico

Delphinids produce large numbers of short duration, broadband echolocation clicks which may be useful for species classification in passive acoustic monitoring efforts. A challenge in echolocation click classification is to overcome the many sources of variability to recognize underlying patterns across many detections. An automated unsupervised network-based classification method was developed to simulate the approach a human analyst uses when categorizing click types: Clusters of similar clicks were identified by incorporating multiple click characteristics (spectral shape and inter-click interval distributions) to distinguish within-type from between-type variation, and identify distinct, persistent click types. Once click types were established, an algorithm for classifying novel detections using existing clusters was tested. The automated classification method was applied to a dataset of 52 million clicks detected across five monitoring sites over two years in the Gulf of Mexico (GOM). Seven distinct click types were identified, one of which is known to be associated with an acoustically identifiable delphinid (Risso’s dolphin) and six of which are not yet identified. All types occurred at multiple monitoring locations, but the relative occurrence of types varied, particularly between continental shelf and slope locations. Automatically-identified click types from autonomous seafloor recorders without verifiable species identification were compared with clicks detected on sea-surface towed hydrophone arrays in the presence of visually identified delphinid species. These comparisons suggest potential species identities for the animals producing some echolocation click types. The network-based classification method presented here is effective for rapid, unsupervised delphinid click classification across large datasets in which the click types may not be known a priori.

Health of marine mammal populations is often considered an indicator of overall marine ecosystem health and resilience, particularly in highly-impacted regions such as the Gulf of Mexico. Marine mammal populations are difficult to monitor given the many challenges of observing animals at sea (e.g. weather, limited daylight, ocean conditions, and expense). An increasingly common approach is the use of underwater acoustic sensors capable of recording marine mammal calls at remote locations for months at a time. Acoustic sensors generate large datasets in which dolphin echolocation clicks are commonly present. Dolphins are the most diverse family of marine mammals, and distinguishing click characteristics have only been described for a small subset of species. We developed a workflow to automatically identify distinct dolphin click types within large datasets without prior knowledge of their distinguishing features. Our algorithm then recognizes these click types in novel recording data across a range of monitoring locations. Known species-specific click types emerge from the data using this approach, as well as new click types potentially associated with additional species. This technique is a key step toward determining species identification for passive acoustic monitoring of offshore populations of dolphins and other toothed whales under a big data paradigm.

The echolocation transmission beam of free-ranging Indo-Pacific humpback dolphins (Sousa chinensis)

While the transmission beam of odontocetes has been described in a number of studies, the majority of them that have measured the transmission beam in two dimensions were focused on captive animals. Within the current study, a dedicated cross hydrophone array with nine elements was used to investigate the echolocation transmission beam of free-ranging Indo-Pacific humpback dolphins. A total of 265 on-axis clicks were analyzed, from which the apparent peak to peak source levels ranged between 168 to 207 dB (mean 184.5 dB ± 6.6 dB). The 3-dB beam width along the horizontal and vertical plane was 9.6° and 7.4°, respectively. Measured separately, the directivity index of the horizontal and vertical plane was 12.6 and 13.5 dB, respectively, and the overall directivity index (both planes combined) was 29.5 dB. The beam shape was slightly asymmetrical along the horizontal and vertical axis. Compared to other species, the characteristics of the transmitting beam of Indo-Pacific humpback dolphins were relatively close to the bottlenose dolphin (Tursiops truncatus), likely due to the similarity in the peak frequency and waveform of echolocation clicks and comparable body sizes of the two species.

Biosonar signal propagation in the harbor porpoise's (Phocoena phocoena) head: The role of various structures in the formation of the vertical beam

Harbor porpoises (Phocoena phocoena) use narrow band echolocation signals for detecting and locating prey and for spatial orientation. In this study, acoustic impedance values of tissues in the porpoise's head were calculated from computer tomography (CT) scan and the corresponding Hounsfield Units. A two-dimensional finite element model of the acoustic impedance was constructed based on CT scan data to simulate the acoustic propagation through the animal's head. The far field transmission beam pattern in the vertical plane and the waveforms of the receiving points around the forehead were compared with prior measurement results, the simulation results were qualitatively consistent with the measurement results. The role of the main structures in the head such as the air sacs, melon and skull in the acoustic propagation was investigated. The results showed that air sacs and skull are the major components to form the vertical beam. Additionally, both beam patterns and sound pressure of the sound waves through four positions deep inside the melon were demonstrated to show the role of the melon in the biosonar sound propagation processes in the vertical plane.