Underwater Sound Localization Research Articles

Effects of sea surface waves and wind-generated bubbles on underwater sound source localization in the Persian Gulf

Effects of sea surface waves and wind-generated bubbles on underwater sound source localization in the Persian Gulf

Research on Underwater Sound Source Localization Based on NLM-EEMD and FCM-Generalized Quadratic Correlation

The accuracy of underwater sound source localization depends on the preprocessing effect of signal denoising and delay estimation performance. In order to meet the needs of high-precision underwater positioning, this article studies a time delay estimation method based on nonlocal mean-enforced mean empirical mode decomposition (NLM-EEMD) autocorrelation denoising preprocessing and fuzzy C-means clustering (FCM)-generalized quadratic for underwater sound source localization technology. The theories such as NLM, EEMD, FCM, and generalized quadratic correlation delay estimation are studied in detail, and the speech signal of the test library in the TIMIT standard library is selected for simulation analysis, which verifies the correctness of the delay estimation method. Experiments were carried out in a larger outdoor pool. The analysis results show that the NLM-EEMD adaptive denoising method effectively reduces noise interference and improves the quality of EEMD decomposition and the FCM-generalized quadratic correlation method increases the correlation between two signals. The time delay estimation accuracy is improved, and the positioning accuracy is 48.66% higher than that of the direct generalized cross-correlation method.

Underwater sound localization abilities of harbor seals (Phoca vitulina) for high-frequency noise band stimuli in the median plane.

Pinnipeds use a variety of acoustic information underwater for social interactions, hunting, and predator avoidance. Thus, the ability to accurately localize a sound source in the environment can have a clear survival value. Nonetheless, the sound localization mechanisms for seals underwater still have to be clarified, especially for sounds received in the median plane. In this study, the sound localization abilities of five harbor seals for high-frequency noise band stimuli were measured underwater in the median plane. The seals' minimum audible angles (MAAs) were determined for two different high-frequency noise band stimuli using a two-alternative forced-choice procedure. Noise 1 had a frequency range between 8 and 16 kHz. Noise 2 contained frequencies between 14 and 16 kHz. Psychoacoustic results for the tested harbor seals show that the seals were able to localize these stimuli in the median plane underwater with MAAs between 5.1° and 17.5°. The results suggest that spectral cues improve the seals' ability to localize high-frequency sound signals in the median plane.

Modeling underwater hearing and sound localization in the frog Xenopus laevis.

Animals that are small compared to sound wavelengths face the challenge of localizing a sound source since the main cues to sound direction-interaural time differences (ITD) and interaural level differences (ILD)-both depend on size. Remarkably, the majority of terrestrial vertebrates possess internally coupled ears (ICE) with an air-filled cavity connecting the two eardrums and producing an inherently directional middle-ear system. Underwater, longer wavelengths and faster sound-speed reduce both ITD and ILD cues. Nonetheless, many animals communicate through and localize underwater sound. Here, a typical representative equipped with ICE is studied: the fully aquatic clawed frog Xenopus laevis. It is shown that two factors improve underwater sound-localization quality. First, inflated lungs function as Helmholtz resonator and generate directional amplitude differences between eardrum vibrations in the high-frequency (1.7-2.2 kHz) and low-frequency (0.8-1.2 kHz) range of the male advertisement calls. Though the externally arriving ILDs practically vanish, the perceived internal level differences are appreciable, more than 10 dB. As opposed to, e.g., lizards with thin and flexible eardrums, plate-like eardrums are shown to be Xenopus' second key to successfully handling aquatic surroundings. Based on ICE, both plate-like eardrums and inflated lungs functioning as Helmholtz resonators explain the phonotaxis performance of Xenopus.

An Underwater Sound Localization Technique Based on the Combination of Chaotic Electronic Signal and Sensor Array

An Underwater Sound Localization Technique Based on the Combination of Chaotic Electronic Signal and Sensor Array

Underwater sound localization of pure tones in the median plane by harbor seals (Phoca vitulina).

In an underwater environment the physical characteristics of sound propagation differ considerably from those in air. For this reason, sound localization underwater is associated with difficulties, especially in the median plane. It was the approach of the present study to investigate whether harbor seals (Phoca vitulina) are able to determine the direction of a tonal signal form above or below in the underwater environment. Minimum audible angles (MAAs) or the angular range in which the animals could localize a pure tone stimulus in the vertical plane were obtained for frequencies from 0.35 up to 16 kHz. Testing was conducted with four male harbor seals in a semi-circle area of 6 m in diameter in about 2.5 m depth, by using a two alternative forced choice method. The results show that harbor seals are able to localize a pure tone in the median plane under water with a high performance for low frequency stimuli between 350 Hz and 2 kHz with MAAs ranging from below 2.5° up to about 25°. For higher frequencies the animals show strong individual differences.

Modeling the utility of binaural cues for underwater sound localization

The binaural cues used by terrestrial animals for sound localization in azimuth may not always suffice for accurate sound localization underwater. The purpose of this research was to examine the theoretical limits of interaural timing and level differences available underwater using computational and physical models. A paired-hydrophone system was used to record sounds transmitted underwater and recordings were analyzed using neural networks calibrated to reflect the auditory capabilities of terrestrial mammals. Estimates of source direction based on temporal differences were most accurate for frequencies between 0.5 and 1.75 kHz, with greater resolution toward the midline (2°), and lower resolution toward the periphery (9°). Level cues also changed systematically with source azimuth, even at lower frequencies than expected from theoretical calculations, suggesting that binaural mechanical coupling (e.g., through bone conduction) might, in principle, facilitate underwater sound localization. Overall, the relatively limited ability of the model to estimate source position using temporal and level difference cues underwater suggests that animals such as whales may use additional cues to accurately localize conspecifics and predators at long distances.

Manatee hearing and sound localization can help navigate noisy shallow waters and cocktail events, no Lombard's needed

Simultaneous masking procedures were used to measure the hearing and underwater sound localization abilities of West Indian manatees. Auditory detection thresholds of pulsed, no-pulsed pure tones, and complex sounds were measured against white noise using forced-choice paradigms. Auditory thresholds as a function of intensity, center frequency, bandwidth, pulse rate, and spectral characteristics were measured. Resulting critical ratios for pure tone measurements demonstrated manatees have acute frequency filtering abilities compared with humans and other marine mammals. Signal repetition rate along with amplitude and frequency modulation providing temporal contrasts against aperiodic background noise and lowered detection thresholds. FM signal detection thresholds were measured at or below ambient background levels. Results with species specific calls and boat noise suggested loudness summation across distant critical bands, as well as FM and amplitude modulation reduced the masking effects observed with pure tones. Manatees are well adapted for hearing and locating high frequency sounds in noisy shallow water habitats where physical boundary and near surface phenomena impede the propagation of low frequencies. Narrow critical bands and selective perception of pulsed signals may be adaptations for detecting and locating species-specific vocalizations. High frequency harmonics in manatee calls provide essential directional cues between mothers and calves.

Identifying signature whistles from recordings of groups of unrestrained bottlenose dolphins (Tursiops truncatus)

AbstractBottlenose dolphins (Tursiops truncatus) have individually distinctive signature whistles. Each individual dolphin develops its own unique frequency modulation pattern and uses it to broadcast its identity. However, underwater sound localization is challenging, and researchers have had difficulties identifying signature whistles. The traditional method to identify them involved isolating individuals. In this context, the signature whistle is the most commonly produced whistle type of an animal. However, most studies on wild dolphins cannot isolate animals. We present a novel method, SIGnature IDentification (SIGID), that can identify signature whistles in recordings of groups of dolphins recorded via a single hydrophone. We found that signature whistles tend to be delivered in bouts with whistles of the same type occurring within 1–10 s of each other. Nonsignature whistles occur with longer or shorter interwhistle intervals, and this distinction can be used to identify signature whistles in a recording. We tested this method on recordings from wild and captive bottlenose dolphins and show thresholds needed to identify signature whistles reliably. SIGID will facilitate the study of signature whistle use in the wild, signature whistle diversity between different populations, and potentially allow signature whistles to be used in mark‐recapture studies.

A re‐examination of human underwater sound localization abilities in the azimuth.

Divers are frequently exposed to underwater sounds. The subjective impression of the diving community is that sound localization underwater is extremely difficult. However, Feinstein [1973a, 1973b] found underwater minimum audible angles (MAAs) to be approximately 10 deg. To the extent that underwater MAAs reflect the general performance of the binaural system, this would suggest that humans should be reasonably effective at underwater sound localization. A re‐examination of the underwater MAAs, with greater subject and environment control, was performed. The present work indicates underwater MAAs at approximately 20–30 deg, significantly poorer than previous findings. [Work supported by the ONR.]

Phase Encoding in the Mauthner System: Implications in Left–Right Sound Source Discrimination

The paired teleost Mauthner (M)-cells and their associated network serve as an excellent system to study the biophysical basis of decision making. In teleosts, an abrupt sound evokes an M-spike, triggering a C-start escape that is usually directed away from a sound source. The response latency is minimized by electrical synapses between auditory afferents and the M-cell lateral dendrite. Here, we demonstrate that the electrical synapses also mediate phase encoding. Ramped sound pressure waves (150-250 Hz) evoked electrotonic postsynaptic potentials in the M-cell locked to two diametrically opposed phase angles that were frequency dependent but intensity independent. Phase encoding was also evident at the behavioral level underwater, because the stimuli evoked directional C-starts with an onset that was phase locked to the sound wave. In interneurons inhibitory to the M-cell, these same stimuli also evoked phase-locked electrotonic postsynaptic potentials and action potentials. The resulting chemical and electrical (i.e., field effect) inhibitions functioned tonically and phasically, respectively. Phase encoding could be important in underwater sound source localization, which is thought to require a neural computation involving a phase comparison between the pressure and the directional particle motion components of sound. This computation may be implemented by an interplay between phase-dependent afferent excitation and feedforward inhibition that activates the appropriate M-cell and directs the C-start away from the sound source.

Underwater localization of pure tones by harbor seals (Phoca vitulina)

The underwater sound localization acuity of harbor seals (Phoca vitulina) was measured in the horizontal plane. Minimum audible angles (MAAs) of pure tones were determined as a function of frequency from 0.2 to 16 kHz for two seals. Testing was conducted in a 10-m-diam underwater half circle using a right/left psychophysical procedure. The results indicate that for both harbor seals, MAAs were large at high frequencies (13.5 degrees and 17.4 degrees at 16 kHz), transitional at intermediate frequencies (9.6 degrees and 10.1 degrees at 4 kHz), and particularly small at low frequencies (3.2 degrees and 3.1 degrees at 0.2 kHz). Harbor seals seem to be able to utilize both binaural cues, interaural time differences (ITDs) and interaural intensity differences (IIDs), but a significant decrease in the sound localization acuity with increasing frequency suggests that IID cues may not be as robust as ITD cues under water. These results suggest that the harbor seal can be regarded as a low-frequency specialist. Additionally, to obtain a MAA more representative of the species, the horizontal underwater MAA of six adult harbor seals was measured at 2 kHz under identical conditions. The MAAs of the six animals ranged from 8.8 degrees to 11.7 degrees , resulting in a mean MAA of 10.3 degrees .

Electrophysiological Assessment of Temporal Resolution in Pinnipeds

Studies of auditory temporal processing in marine mammals have traditionally focused on the highly refined temporal resolution capabilities of dolphins and other odontocete cetaceans. However, a recent electrophysiological investigation of manatee (Trichechus manatus) hearing has shown their temporal resolution to be better than expected, leading to speculation that enhanced temporal processing capabilities are adaptive for underwater sound localization. This study measured evoked responses from several California sea lions (Zalophus californianus), a harbor seal (Phoca vitulina), and a northern elephant seal (Mirounga angustirostris) to determine how well the auditory systems of these amphibious mammals resolve rhythmic stimuli. Trains of broadband clicks were presented in air at repetition rates from 125 to 1,500 s -1 , and the averaged evoked responses elicited by these stimuli were recorded from the skin. Rate-following responses were detected in the sea lions at rates up to 1,000 s -1 , with an estimated upper limit of temporal resolution between 875 to 1,000 s -1 . This upper limit is better than previously anticipated and was further substantiated by limited testing with the harbor seal and northern elephant seal. While these findings might support an underwater sound localization hypothesis, measurements comparable to those of the pinnipeds were also obtained in a phylogenetically similar terrestrial mammal: a domestic dog (Canis familiaris). It is therefore possible that increased temporal resolution in pinnipeds and other nonecholocating marine mammals is not a result of the evolutionary pressure of an aquatic environment.

Electrophysiological investigation of temporal resolution in three pinniped species: Adaptive implications

Electrophysiological studies of auditory temporal processing in marine mammals have traditionally focused on the role of highly refined temporal resolution in dolphin echolocation. Studies in manatees, however, have found their temporal resolution to be better than expected, leading to speculation that such capabilities are an adaptation for underwater sound localization. This study measured the ability of auditory brainstem responses to follow rhythmic click stimuli in California sea lions (Zalophus californianus), harbor seals (Phoca vitulina), and northern elephant seals (Mirounga angustirostris). Trains of 640-s clicks were presented in air at repetition rates of 125–1500 per second and averaged rate-following responses were recorded. Rate-following responses were detected in both the harbor seal and the sea lion at rates up to 1000 clicks per second, indicating that pinnipeds, like manatees, possess temporal resolution greater than humans but inferior to dolphins. While this finding might support an underwater sound localization hypothesis, comparable results were obtained in preliminary testing of a dog (Canis familiaris), suggesting that increased temporal resolution in pinnipeds may not be the result of the evolutionary pressure of an aquatic environment, but rather a result of increased high-frequency hearing essential to mammalian sound localization. [Work supported by NOPP, ONR, and NMFS.]

Underwater auditory localization by harbor seals (Phoca vitulina)

The underwater sound localization acuity of a swimming harbor seal was measured in the horizontal plane. The stimulus was either a double sound (two 6-kHz pure tones lasting 0.5 s separated by an interval of 0.2 s) or a single continuous sound of 1.2 s. Testing was conducted in a 10-m-diameter underwater half-circle with hidden loudspeakers. The animal was trained to swim along the diameter of the half-circle and to change its course towards the sound source right after the signal was given. The seal indicated the sound source by touching its assumed position at the half-circle. The deviation of the seal’s choice from the actual sound source was measured by means of video analysis. In trials with the double sound the seal localized the sound sources with a mean deviation of 2.8° and in trials with the single sound with a mean deviation of 4.5°. In a second experiment minimum audible angles of the animal were determined as a function of frequency (2, 4, 6, 8, and 16 kHz resulting in MAAs of 9.4°, 9.6°, 9.8°, 13.0°, and 13.5°, respectively). Intraspecific differences of the MAA will be presented for five additional harbor seals.

Underwater auditory localization by a swimming harbor seal (Phoca vitulina)

The underwater sound localization acuity of a swimming harbor seal (Phoca vitulina) was measured in the horizontal plane at 13 different positions. The stimulus was either a double sound (two 6-kHz pure tones lasting 0.5 s separated by an interval of 0.2 s) or a single continuous sound of 1.2 s. Testing was conducted in a 10-m-diam underwater half circle arena with hidden loudspeakers installed at the exterior perimeter. The animal was trained to swim along the diameter of the half circle and to change its course towards the sound source as soon as the signal was given. The seal indicated the sound source by touching its assumed position at the board of the half circle. The deviation of the seals choice from the actual sound source was measured by means of video analysis. In trials with the double sound the seal localized the sound sources with a mean deviation of 2.8 degrees and in trials with the single sound with a mean deviation of 4.5 degrees. In a second experiment minimum audible angles of the stationary animal were found to be 9.8 degrees in front and 9.7 degrees in the back of the seal's head.

Underwater Hearing and Sound Localization with and without an Air Interface

Underwater hearing acuity and sound localization are improved by the presence of an air interface around the pinnae and inside the external ear canals. Hearing threshold and the ability to localize sound sources are reduced underwater. The resonance frequency of the external ear is lowered when the external ear canal is filled with water, and the impedance-matching ability of the middle ear is significantly reduced due to elevation of the ambient pressure, the water-mass load on the tympanic membrane, and the addition of a fluid-air interface during submersion. Sound lateralization on land is largely explained by the mechanisms of interaural intensity differences and interaural temporal or phase differences. During submersion, these differences are largely lost due to the increase in underwater sound velocity and cancellation of the head's acoustic shadow effect because of the similarity between the impedance of the skull and the surrounding water. Ten scuba divers wearing a regular opaque face mask or an opaque ProEar 2000 (Safe Dive, Ltd., Hofit, Israel) mask that enables the presence of air at ambient pressure in and around the ear made a dive to a depth of 3 m in the open sea. Four underwater speakers arranged on the horizontal plane at 90-degree intervals and at a distance of 5 m from the diver were used for testing pure-tone hearing thresholds (PTHT), the reception threshold for the recorded sound of a rubber-boat engine, and sound localization. For sound localization, the sound of the rubber boat's engine was randomly delivered by one speaker at a time at 40 dB HL above the recorded sound of a rubber-boat engine, and the diver was asked to point to the sound source. The azimuth was measured by the diver's companion using a navigation board. Underwater PTHT with both masks were significantly higher for frequencies of 250 to 6000 Hz when compared with the thresholds on land (p <0.0001). No differences were found in the PTHT or the reception threshold for the recorded sound of a rubber-boat engine for dry or wet ear conditions. There was no difference in the sound localization error between the regular mask and the ProEar 2000 mask. The presence of air around the pinna and inside the external ear canal did not improve underwater hearing sensitivity or sound localization. These results support the argument that bone conduction plays the main role in underwater hearing.

Underwater sound localization: Role of interaural time and intensity differences

In theory, underwater sound localization is very limited compared with that in air. Most cues are greatly reduced underwater: Interaural time differences are divided by four (due to faster sound propogation in water) and intensity differences are greatly reduced because signal conduction to the inner ear mainly occurs through skull bone condition, making the head transparent. However, preliminary studies indicate some localization abilities in scuba divers. An azimuth identification task was conducted in Lake Leman. At a depth of 10 m, 14 divers indicated which of eight transducers located every 45 was generating a sound (400 Hz, 6 kHz, or white noise). Results indicate: (a) correct estimates were highest for lateral positions (left and right) and poor for median positions (front and back), (b) performance was high for 400 Hz and white noise but low for 6-kHz signals, and (c) head movements did not appear to facilitate localization. Thus the best localization abilities were observed in positions with highest interaural time differences, suggesting that, contrary to theoretical predictions, temporal binaural cues must be at least partially functional. However, the fact that localization was poor for high-frequency signals suggests that interaural intensity differences do not play a significant role.

Left-right discrimination of sound onset by the Mauthner system.

We present a neural model for how the Mauthner system could compute the direction of a transient sound stimulus originating on either the left or right side of a fish. This computation results in an initial orientation of an escape response away from the side of the stimulus. Our idea is based on the phase model of underwater sound localization by fishes. If the phase model is applicable to the Mauthner system, then the problem of sound localization can be reduced to a logical operator, the EXCLUSIVE-NOR (or XNOR). We show how this can be solved by the Mauthner system using afferents that convey separate inputs of sound pressure transduced by the swimbladder (rarefaction and compression) and particle displacement (left and right) from the inner ear. In our model, both pressure components are responsible for bringing the Mauthner cell to threshold. Mauthner firing is gated by the inhibitory PHP neurons receiving specific combinations of pressure and displacement that implement the XNOR logic. We refer to this as the XNOR model. This model is experimentally verifiable and makes specific predictions about the expected acoustic response characteristics of the Mauthner and PHP neurons. Our model places a component of PHP function into a new neuroethological context and may provide insights into the central neurophysiological mechanisms of directional hearing in fishes. In particular, we show how the XNOR model can be applied to predict the activity of diverse neural elements involved in acoustic localization by fishes.

Diving in darkness: whiskers as sense organs of the ringed seal (Phoca hispida saimensis)

Underwater vocalization and the functional structure of different vibrissae of the ringed seal (Phoca hispida saimensis) of Lake Saimaa, Eastern Finland, were studied. These seals live in darkness under the ice cover for several months during the year. It is known that blind seals are managing well in the lake. Visibility under water in some parts of the area where the seals live is only 2 m. It is suggested that echolocation is used in orientation and feeding. The Saimaa seal has click and click trial underwater vocalizations. However, both the frequency and intensity of the vocalization are low compared with, for example, those of dolphins. The structural adaptations for underwater sound localization are also not well developed.The ringed seal has, however, extremely well‐developed vibrissae. The innervation of one vibrissa is more than 10 times greater than normally found in mammals. The main structural deviations from normal mammalian vibrissae are: (1) an upper cavernous sinus, (2) a groove in the wall of the capsule at the level of the lower cavernous sinus, (3) elasticity of the connective tissue bands fixing the hair root to the capsule in the lower cavernous sinus and especially (4) the structure and innervation of the ring sinus area. Sensory elements are situated upon the glassy membrane on the surface of the outer rootsheath and in the basal cell layer of the outer rootsheath which is like a sensory epithelium. Below this epithelium a layer of liquid or gelatinous material and large amounts of glycogen are found. This sensory epithelium is especially well developed in the superciliary vibrissae. These vibrissae are protruded some millimetres when the seals are attentive. It is suggested that the vibrissae also sense sounds, which are transmitted to the sensory elements by tissue conduction through the capsule wall and via the blood sinuses. The seals may possibly detect compressional waves with the vibrissae.