Dolphin Echolocation Clicks Research Articles

Acoustic diversity, bearing estimation, and localization of sound sources in shallow water and deep ocean off the U.S. Northeast coast using a coherent hydrophone array

An in-house developed 160-element large-aperture hydrophone array from Northeastern University was utilized for ocean monitoring in both the shallow waters of the Great South Channel (GSC) and the deep-water area off the continental slope south of Rhode Island in September 2021. Utilizing passive ocean acoustic waveguide remote sensing (POAWRS) technology, the system enabled real-time, broad-area surveillance of the oceanic environment. A novel multistage clustering was introduced for annotating the vast volume of underwater acoustic data, drawing inspiration from the human cognitive process of categorizing sounds. Initially, sounds are separated into broad groups, akin to a first human listening pass. Subsequent stages of clustering refine these categories, utilizing different sets of features to achieve a granularity of annotation that closely mirrors expert human annotation processes. It was found that the acoustic landscape of the GSC was primarily composed of diverse marine mammal sounds, such as fin whale 20 Hz pulses, humpback whale songs, minky whale buzz sequences, sperm whale and dolphin echolocation clicks, and unidentified whale calls. Conversely, the deep-water area off the continental slope featured predominantly fish sounds, chorus-like sounds, and dolphin vocalizations. Detailed analyses of the most prevalent vocalizations are presented, including their bearing-time trajectories and localizations.

A deep dive into the ecology of fear

The largest migration on Earth occurs on a daily basis in the ocean as creatures move up near the surface during the night to feed and descend to the relative safety of the dark depths during the day. This phenomenon is thought to reflect feeding opportunities and predator avoidance according to light levels, but the dynamics of this have rarely been studied. Samuel Urmy and Kelly Benoit-Bird from the Monterey Bay Aquarium Research Institute, USA, inspired by the originally terrestrial concept of ‘landscapes of fear’, sought to investigate if the fear of predation shapes the ecological process of daily migrations.Urmy and Benoit-Bird used an upwards-facing echosounder, which produces sonar pulses and records the returning echoes to detect and visualize schools of fish, squid and plankton, to provide information about their daily vertical movement patterns. Additionally, an underwater microphone next to the echosounder listened for the hunting sounds made by nearby toothed whale predators, such as Risso's dolphins (Grampus griseus) and Pacific white-sided dolphins (Lagenorhynchus obliquidens). Recording the echolocation clicks gave the researchers an indication of the presence of a predator, allowing them to investigate the interactions between the predators and the prey in their vicinity. Successfully collecting data beneath almost 1 km of seawater for more than a year is no easy feat, so the team sent down an unmanned robot to survey the area with video to visually confirm the identities of the different species that they were picking up with the echosounder.Scrutinizing a whole year of deep-sea data, the researchers observed many examples of prey taking evasive action to avoid predators. In one example, a bout of dolphin echolocation clicks triggered a school of fish to cluster together and dive for safety moments later. While such a diving response physically increases the distance between the fish and the predator, the fish's vertical plunge also results in weaker echoes returning from the vulnerable fish, reducing the opportunity for the hunting predator to successfully home in on their fishy prey. In addition, the team could use the distinctive echo signatures produced by different species – such as larger Pacific hake (Merluccius productus) and smaller anchovies (Engraulis mordax) – to provide a best guess for which species were appearing in the echosounder traces when video data were not available. This was especially useful for identifying instances of predation that did not involve an echolocating species, such as an occasion when a school of predatory fish approached a cloud of zooplankton and the tiny creatures dived to avoid the hunters. The scientists also discovered that it sometimes took clusters of zooplankton weeks to return to their usual depth after an encounter with a hungry predator. This shaping of a prey animal's behaviour and movement, even in the absence of a predator, provides evidence that the concept of the ‘ecology of fear’ is indeed applicable in the 3D marine realm and complements recent research on marine ‘soundscapes of fear’, where prey fall silent to avoid detection by predators that could be nearby.The vertical movement of animals in the ocean plays a major part in nutrient recycling, and Urmy and Benoit-Bird demonstrate that the lasting behavioural responses of prey to hunting predators potentially has a major influence on this flow of nutrients. The effect of predators on the behaviour of their prey can thus be more lingering than we had thought. Zooplankton layers in the open ocean are therefore not just governed by forces of nature or their particular lifestyles, but by the ecology of fear too.

Characterization and comparison of echolocation clicks of white-beaked dolphins (Lagenorhynchus albirostris) off the Northumberland coast, UK.

Odontocetes produce ultrasonic clicks for navigation and foraging. These are commonly categorized as regular or buzz clicks based on the inter-click interval. Buzz clicks are linked to foraging behaviors and may be subdivided into slow buzz clicks for prey chase, and regular buzz clicks for prey capture. This study recorded these three click types produced by white-beaked dolphins (Lagenorhynchus albirostris) off the Northumberland coast, UK. Acoustic parameters (including duration, centroid frequency, and root-mean-squared bandwidth) were calculated and compared across the three click types. The results showed that the regular clicks had shorter durations and higher frequencies than both the buzz click types. The regular buzz clicks had longer durations, lower frequencies, and narrower bandwidths than the slow buzz clicks. Additionally, regardless of click type, about 30% of the clicks had high-frequency (200-250 kHz) secondary peaks and >90% of the clicks displayed spectral peak and notch patterns between 20 and 80 kHz. These findings are useful for future quantitative assessment of the echolocation performance of white-beaked dolphins in the wild. The patterns of spectral peaks and notches identified may facilitate for acoustic identification of this species.

Echolocation clicks of free-ranging Irrawaddy dolphins (Orcaella brevirostris) in Trat Bay, the eastern Gulf of Thailand.

Little is known about the vocalizations of Irrawaddy dolphins (Orcaella brevirostris) in the Gulf of Thailand. The present study first described the echolocation clicks of Irrawaddy dolphins in Trat Bay, in the eastern Gulf of Thailand, using a broadband hydrophone system. Over 2 h of acoustic recordings were collected during 14-day study periods in December 2017 and December 2018. Several criteria were used to judge if a click was on axis or as close to the acoustic axis as possible. To calculate the distance of dolphins, a low-budget localization method based on arrival time differences between the direct and indirect signals was used in the present study. The clicks had a mean peak-to-peak source level of 192 ± 3 dB re 1 μPa, an energy flux density source level of 131 ± 3 dB re 1 μPa2s, a mean centroid frequency of 98 ± 10 kHz, a mean duration of 16 ± 2 μs, and a -3 dB bandwidth of 79 ± 13 kHz. The click parameters of the Irrawaddy dolphins in Trat Bay were slightly different from the clicks recorded from the dolphins in Sundarbans, Bangladesh. The present study provided a basic description of the click characteristics of Irrawaddy dolphins in Trat Bay, which could contribute to the management and conservation strategies for local Irrawaddy dolphins, and a basic reference for the proper input parameters in passive acoustic monitoring and detection.

Echolocation behavior of the Icelandic white-beaked (Lagenorhyncus albirostris) dolphins: Now and then

First studies of the echolocation behaviour of free-ranging white-beaked dolphins (Lagenorhyncus albirostris) were conducted in Faxafloi Bay in the Southwestern part of Iceland in the years 1997 to 1999. However, the sighting rate of white-beaked dolphins has decreased in that area since then and the current studies were conducted in the Northeastern part of Iceland. The aim of this study was to investigate the difference between normal clicks compared to clicks from buzz sequences. The recordings of the Icelandic white-beaked dolphins were conducted using a vertical linear 16-hydrophone array in Skjalfandi Bay, Northeastern Iceland during August 2015 and June 2016. The hydrophones were connected to NI-Boards and to a laptop computer on board using a sample rate of 1 MHz per channel. The group size of the dolphins varied from three individuals up to 30 animals in the area during the recordings. The dolphin echolocation clicks were recorded and it was possible to track individuals and to estimate beam-patt...

Spatiotemporal variation in habitat utilization of humpback dolphins (Sousa chinensis) potentially affected by freshwater discharge

Dynamics of habitat utilization of marine mammals are important for the conservation management of coastal ecosystems. Previous studies have shown that seasonal changes of humpback dolphin distribution are associated with seasonal changes of upstream rainfall. However, temporal variations of dolphin behavior at an estuarine habitat remain unclear. In this study, echolocation clicks of humpback dolphins were recorded at the Xinhuwei River Estuary, western Taiwan, between July 2009 and November 2015. Sound source bearing angles to identify different sound sources, intensity, and inter-click intervals (ICIs) as an index of the sensing distance were measured to investigate the sensing and movement of dolphin groups. Humpback dolphins searched for shorter sensing distances and moved back and forth near the estuary during spring and summer, which suggests foraging-related behaviors were much frequently observed during wet seasons. However, humpback dolphins changed to focus on longer detection ranges (ICIs changed from 40 to 50 ms to 50 to 70 ms) and move in straight forward when upstream rainfall increased during this period. The present result indicated the dynamics of habitat utilization of humpback dolphins at an estuary is likely driven by the distribution and abundance of prey resource which is influenced by the temporal change of river runoff.

Short-term enhancement and suppression of dolphin auditory evoked responses following echolocation click emission.

Biosonar gain control mechanisms in a bottlenose dolphin were investigated by measuring the auditory steady-state response (ASSR) to an external tone while the animal echolocated. The dolphin performed an echo change-detection task that utilized electronically synthesized echoes with echo delays corresponding to 25- and 50-m target range. During the task, amplitude modulated tones with carrier frequencies from 25 to 125 kHz were continuously presented and the instantaneous electroencephalogram stored for later analysis. ASSRs were extracted from the electroencephalogram by synchronously averaging time epochs temporally aligned with the onset of the external tone modulation cycle nearest to each of the dolphin's echolocation clicks. Results showed an overall suppression of the ASSR amplitude for tones with frequencies near the click center frequencies. A larger, temporary suppression of the ASSR amplitude was also measured at frequencies above 40-50 kHz, while a temporary enhancement was observed at lower frequencies. Temporal patterns for ASSR enhancement or suppression were frequency-, level-, and range-dependent, with recovery to pre-click values occurring within the two-way travel time. Suppressive effects fit the patterns expected from forward masking by the emitted biosonar pulse, while the specific mechanisms responsible for the frequency-dependent enhancement are unknown.

There must be mucus: Using a lumped-parameter model to simulate the “thump” and “ring” of a bottlenose dolphin echolocation click

Bottlenose dolphin echolocation clicks display a great diversity in temporal and spectral structure, with both unimodal and bimodal spectra observed (Houser et al., JASA, 1999). Wavelet scalograms applied to data collected by the Navy Marine Mammal Program and the Bioacoustic Measuring Tool (BMT) (Martin et al., JASA, 2005) show that echolocation clicks can display two distinct phases: an initial “thump,” followed by an extended “ring” that is adequately modeled by a damped harmonic oscillator. The thump and ring can display either similar or different spectral characteristics, giving rise to a unimodal or bimodal spectrum. A three-mass lumped parameter model, adapted from the speech processing and terrestrial bioacoustics literature, has been used to simulate the oscillation and collision of the dorsal bursae in a dolphin’s nasal passage. The three-mass model reproduces many of the time and frequency domain features of entire click trains as well as individual clicks, including unimodal and bimodal spectra. A key insight of the models is that some slight adhesion between the faces of the colliding bursue seems necessary in order to reproduce the high-frequency click structure. A viscoelastic mucus coating could provide one possible mechanism for this required adhesion force. [Data provided by Steve Martin, NMMF.]

Amazon river dolphins (Inia geoffrensis) use a high-frequency short-range biosonar.

Toothed whales produce echolocation clicks with source parameters related to body size; however, it may be equally important to consider the influence of habitat, as suggested by studies on echolocating bats. A few toothed whale species have fully adapted to river systems, where sonar operation is likely to result in higher clutter and reverberation levels than those experienced by most toothed whales at sea because of the shallow water and dense vegetation. To test the hypothesis that habitat shapes the evolution of toothed whale biosonar parameters by promoting simpler auditory scenes to interpret in acoustically complex habitats, echolocation clicks of wild Amazon river dolphins were recorded using a vertical seven-hydrophone array. We identified 404 on-axis biosonar clicks having a mean SLpp of 190.3 ± 6.1 dB re. 1 µPa, mean SLEFD of 132.1 ± 6.0 dB re. 1 µPa(2)s, mean Fc of 101.2 ± 10.5 kHz, mean BWRMS of 29.3 ± 4.3 kHz and mean ICI of 35.1 ± 17.9 ms. Piston fit modelling resulted in an estimated half-power beamwidth of 10.2 deg (95% CI: 9.6-10.5 deg) and directivity index of 25.2 dB (95% CI: 24.9-25.7 dB). These results support the hypothesis that river-dwelling toothed whales operate their biosonars at lower amplitude and higher sampling rates than similar-sized marine species without sacrificing high directivity, in order to provide high update rates in acoustically complex habitats and simplify auditory scenes through reduced clutter and reverberation levels. We conclude that habitat, along with body size, is an important evolutionary driver of source parameters in toothed whale biosonars.

Do spectral features of Risso's dolphin echolocation clicks vary geographically?

The ability to classify odontocetes to species and population from acoustic recordings leads to improvements in stock identification, abundance and density estimation, and habitat-based density modeling, which are crucial for conservation and management. Risso's dolphins off Southern California have distinctive peaks and valleys in their echolocation clicks, which allow researchers to easily distinguish them from other species in passive acoustic recordings. However, Risso's dolphin echolocation clicks from other geographic areas have not been described and it remains unknown whether they have similarly distinctive click spectra and whether stocks are acoustically distinct. We investigate the potential for using acoustics to identify populations by quantifying the acoustic structure of Risso's dolphin echolocation clicks recorded over wide-ranging geographic regions including the U.S. waters of the North Atlantic Ocean (north and south of Cape Hatteras), Gulf of Mexico, and North Pacific Ocean (Eastern Tropical Pacific and Southern California). Several distinctive peak and valley patterns are found and we evaluate these in light of variability within individuals, groups, and regions as these acoustic differences may indicate differences in age, sex, or stock composition, foraging behavior, or ambient noise environment of the dolphin schools.

PARDON ME? HEARING LOSS IN WILD DOLPHINS

At some point in our lives most of us will have resorted to shouting during a conversation with an elderly relative. Unfortunately, as we age, sounds have to be louder to be heard and high-pitched sounds are no longer perceived. But are humans alone in suffering auditory decline with age? Certainly in captive dolphins, older members of the family do show signs of age-related hearing loss, but the same has never been observed in the wild. However, when a male Indo-Pacific humpback dolphin approximately 40 years old was rescued after it had stranded itself in an inland river near Foshan, China, in March 2012, Songhai Li, from Sanya Institute of Deep-sea Science and Engineering of the Chinese Academy of Sciences, China, and his colleagues seized the opportunity to find out (p. 4144).The team began by giving the rescued dolphin a hearing test. To do this, three EEG (electroencephalograph) suction-cup electrodes were placed on the dolphin's head and back, to measure electrical activity in response to a range of different sounds. The researchers varied both the frequency (kHz) of the sounds and the loudness to find the lowest threshold of hearing, namely the frequency of the quietest detected sound. For the 40-year-old dolphin this corresponded to a sound at 38 kHz. By comparing this with a hearing test from a younger 13-year-old male dolphin that had been stranded in a similar spot back in August 2007, the team could see that in the younger dolphin this threshold was at a higher frequency. Overall, the results showed that the older dolphin had more difficulty hearing high-frequency sounds, with a cut-off about 30–40 kHz lower than for the younger dolphin.The team next went on to record the dolphin's echolocation clicks using a hydrophone. Overall, compared with the younger dolphin, the older dolphin's peak and centre click frequencies were about 16 kHz lower. This shift suggests that the older dolphin shifted its echolocation clicks towards frequencies that it is able to hear. Both the decreased sensitivity to high-frequency calls and the compensation in vocalisations strongly suggest that wild dolphins also suffer from hearing loss.

Using T‐PODs to assess variations in the occurrence of coastal bottlenose dolphins and harbour porpoises

Abstract Assessments of anthropogenic impacts on cetaceans are often constrained by limited data on the extent to which these species use particular areas. Timing porpoise detectors (T‐PODs) are autonomous data recorders for detecting cetacean echolocation clicks, potentially providing cost‐effective opportunities for monitoring cetacean occurrence. The performance of T‐PODs was assessed in three areas off the Scottish east coast, where the relative occurrence of bottlenose dolphins and harbour porpoises was known to differ. Land‐based observations in one area compared visual and acoustic detections of dolphins, while direct hydrophone recordings of dolphin echolocation clicks were compared with T‐POD detections during boat surveys. Land‐based surveys recorded 89 groups of dolphins within 900 m of the T‐POD. All groups spending >30 min in the area were detected on the T‐POD, and the probability of detection declined in relation to distance from the recording site. The number of dolphin clicks recorded on the independent hydrophone system was significantly related to the number detected by a T‐POD. Between pairs of T‐PODs, there was also significant correlation with the numbers of clicks recorded in each hour, both for channels set to detect bottlenose dolphins and for channels set to detect harbour porpoises. Year‐round deployments of paired T‐PODs detected significant geographical variation in detections for both bottlenose dolphins and harbour porpoises. This pattern reflected published data from visual surveys, where dolphins occurred most regularly within the Moray Firth Special Area of Conservation, and porpoises were sighted more regularly in offshore waters. T‐PODs do not detect all cetaceans in the area, and care must be taken when interpreting data from mixed species communities. Nevertheless, these results confirm that T‐PODs provide an effective method for monitoring the occurrence of bottlenose dolphins and harbour porpoises, and provide excellent potential for collecting baseline data from poorly studied areas and monitoring long‐term temporal change in key areas of interest. Copyright © 2009 John Wiley & Sons, Ltd.

Functional bandwidth of an echolocating Atlantic bottlenose dolphin (Tursiops truncatus)

The frequency band that an Atlantic bottlenose dolphin (Tursiops truncatus) used to perform an echolocation target discrimination task was determined using computer simulated phantom targets. The dolphin was trained to discriminate frequency filtered phantom targets from unfiltered ones in a go/no-go paradigm. The dolphin's performance indicated perception of echo alteration only when applied filters interfered with the frequency band between 29 and 42 kHz. The dolphin did not behaviorally convey perception of applied filters that affected frequencies outside this functional bandwidth, such as a low pass 43 kHz or a high pass 28 kHz filter. The upper limit of the functional bandwidth at 42 kHz corresponded with the dolphin's upper hearing limit of 45 kHz, as determined through auditory evoked potential measurements. The lower limit of the functional bandwidth corresponded to a drop in intensity below 30 kHz within the dolphin's echolocation clicks. The randomized presentation of different filters showed that the dolphin paid attention to the entire 29-42 kHz band for each trial, not just subsets. The absence of temporal cues between some of the targets the dolphin could discriminate indicated that in these cases the target discrimination cues were based solely on the frequency content.

Patterns of coastal use by Hawaiian spinner dolphins (Stenella longirostris) observed using passive acoustic monitoring.

Spinner dolphins are the most commonly sighted cetacean in coastal Hawaiian waters and are an important higher‐level trophic component of the near‐shore ecosystem. Establishing their long‐term patterns of occurrence is important both for their conservation and for a better understanding of the relationships between dolphins, their prey, and other members of the ecosystem. Here we report on an effort to use ecological acoustic recorders (EARs) to monitor the presence of spinner dolphins along the leeward coast of the island of Oahu, Hawaii. The EAR is a bottom‐moored recorder with a bandwidth of 30 kHz, which allows the detection of both dolphin whistles and echolocation clicks. Eight EARs were deployed along the coast at depths ranging from 15 to 50 m. Five units were deployed in an array, providing information about the EAR’s detection range. The other three units were placed at sites along the coast commonly frequented by spinner dolphins. The results reveal distinct patterns of preference in habitat use, both for daytime resting behavior and night‐time foraging. The recordings also provide a measure of vessel traffic at locations important to spinner dolphins. Combined, these data demonstrate the value of passive acoustic methods for monitoring cetacean populations and their habitat over extended periods.

Narrow band high frequency clicks from Hourglass dolphins (Lagenorhynchus cruciger)

The source properties of toothed whale echolocation clicks are essential for evaluating the performance of biosonar systems. However, such data only exist for a small number of free ranging toothed whale species. Here we present a quantitative analysis of the first recordings of echolocation sounds from hour glass dolphins (Lagenorhynchus cruciger) made in the Drake Strait. Recordings were made with a vertical array of four broad‐band, calibrated hydrophones that allows for acoustic localization and source parameter derivation. Only apparent on‐axis clicks were used for analysis. Hour glass dolphins produce clicks with a mean peak and centroid frequency around 128kHz, a narrow, mean RMS bandwidth of 10kHz with no energy below 100kHz and a mean signal duration of 161μs. Echolocation clicks of hour glass dolphins are thus of the high frequency narrow band‐width type (NBHF type) found in porpoises, a few delphinids of the Cephalorhynchus genus and the pygmy sperm whale Kogia breviceps, but are strikingly different from other dolphins. Acoustic convergence from vastly different genera and ecotypes suggest that evolution of NBHF clicks are adaptations to special auditory processing or acoustic crypsis rather than to habitat.

Dr. Blue at the Underwater Sound Reference Division

At the Underwater Sound Reference Division Dr. Blue had many varied interests including acoustic metrology, acoustic materials research, enginering acoustics, underwater transducers, and bioacoustics. He performed the first thorough study on the reproducibility of calibrations of standard hydrophones. He supported the development of traveling-wave tubes and large pressure vessels for calibrating Navy sonar transducers and for the measurement of acoustic materials. The Acoustic Pressue Tank Facility is a 30<th>000-gal acoustic test vessel ideally suited for acoustic measurements at hydrostatic pressures up to 2700 psi and at frequencies as low as 1.0 kHz and can accommodate acoustic devices up to 6 ft in diameter. Dr. Blue supported research for improved measurement techniques for determining the properties of acoustic materials. Dr. Blue actively supported the development of new elastomeric materials used in the construction of underwater transducers. The Sonar Transducer Reliability Improvement Program was an extinsive program to properly specify a sonar transducer for Navy applications. He first suggested that coupler reciprocity might be used to perform ultra-precise calibrations of hydrophones. He also supported research in manatee vocalizations and dolphin echolocation clicks. Low-noise hydrophones with noise levels better than sea-state zero were developed for general use.

Target Strength of a Nylon Monofilament and an Acoustically Enhanced Gillnet: Predictions of Biosonar Detection Ranges

Thousands of marine mammals die each year in fisheries-related entanglements. A substantial number of these animals entangle themselves in gillnets. Two populations in immediate danger are the coastal stock of the mid-Atlantic bottlenose dolphin, Tursiops truncatus, and the Gulf of Maine harbour porpoise, Phocoena phocoena. We investigated the efficacy of using an alternative net material made with barium sulphate hypothesized to be acoustically more reflective than traditional nets. By using simulated dolphin echolocation clicks, the target strength of the experimental net was compared with the target strength of a similar gauge nylon net. Results demonstrated that at angles greater than normal incidence, but less than 40°, the new barium sulphate net was acoustically more reflective than the nylon net; however, there was no significant difference in the target strength of the two nets at 0°. At angles greater than 40°, both nets were difficult to discern from background noise. Target strengths of the nets were used to calculate detection ranges for T. truncatus and P. phocoena. Both species should be able to detect the experimental nets at a distance greater than the nylon nets. For T. truncatus, this distance may be enough to reduce entanglement; however, because of P. phocoena’s lower source level echolocation signals, they may not detect either net with echolocation in time to avoid contact.

Active classification of submerged shells insonified by dolphin ‘‘clicks.’’

Many echoes returned by various submerged shells when they are insonified by dolphin echolocation clicks have been examined. The details of the experimental tests the database collected have been described elsewhere. The signal processing analysis of the returned signatures attempted to understand how the animal performed his amazing target classification feats. The echoes were examined in the time, the frequency, and the joint time–frequency domains, and it has precisely been shown how specific features observable in these domains are directly related to the physical characteristics of the shells. The processing capitalizes on certain basic resonance features contained in the echoes, and from them it determines the size, shape, wall thickness, and the material composition of the shell and its filler substance. In the same way that these features give us the identifying characteristics of the targets with hardly any calculation required, it is believed that they also give them to the dolphin. This may explain the secret of their unique success. Emphasized here are features in the combined time–frequency domain, extracted by a variety of Wigner-type distributions belonging to the bilinear (or Cohen) class. The automation of the processing does not seem to be beyond present capabilities.

Biological sources of noise in coastal waters

Underwater biological sounds are transient events. Durations range from microseconds to tens of seconds, frequencies from ≤10 Hz to ≥100 kHz, and source pressure levels up to ≥220 dB re : 1 μPa at 1 m. While each sound is produced by one individual, many sounds occur in choruses that may involve thousands of simultaneous sources. Some crustaceans produce loud impulsive noise in the 2–20 kHz range. These animals are common enough in many shallow areas to dominate the ambient near 10 kHz. Evening fish choruses seasonally dominate the ambient in many shallow waters in the 100–1000 Hz frequency range. While whales and dolphins are less numerous than the animals listed above, many species produce sounds of sufficient intensity to affect the ambient. For example, during their winter breeding season fin whales produce series of 20-Hz pulses lasting 1 s at source levels of 180 dB re : 1 μPa at 1 m. Dolphin echolocation clicks may reach source pressure levels ≥220 dB with spectral peaks 50–150 kHz. Dolphins are also skilled mimics of manmade sounds, which may lead to unpredicted interference.

The sonar system of the dolphins

The theory that the echolocation clicks of dolphins and whales are produced in the region of nasal air sack of the cranial organ known as the melon is physiologically untenable. Both experimental and anatomical findings prove conclusively that the larynx is the only source of sound.