Directional and amplitude characteristics of pulsed call sequences in captive free-swimming Pacific white-sided dolphins (Lagenorhynchus obliquidens).

We investigated the directional properties and gain control of a pulsed call sequence that functions as a contact call in Pacific white-sided dolphins (Lagenorhynchus obliquidens). The pulsed call sequences were stereotyped patterns composed of three or more pulsed call elements and were collected from two dolphins, separated into adjacent pools, and allowed to swim freely. Eight hydrophones and an overhead camera were used to determine the positions and directions of the participants. The mean peak frequency and source levels were 8.4 ± 4.4 (standard deviation)-18.7 ± 12.7 kHz and 160.8 ± 3.8 to 176.4 ± 7.9 dB re 1 µPa (peak-to-peak), respectively, depending on the element types. The elements were omnidirectional, with mean directivity index of 0.9 ± 3.4 dB. The dolphins produced sequences, regardless of their relative position and direction to the lattice, leading to the adjacent pool where the conspecific was housed. They increased the amplitude by 6.5 ± 4.6 dB as the distance from the caller to an arbitrary point in the adjacent pool doubled. These results suggest that callers broadcast pulsed call sequences in a wide direction to reach dispersed conspecifics. However, they can adjust the acoustic active space by controlling the source levels.

Intelligibility of chimeric locally time-reversed speech: Relative contribution of four frequency bands.

Intelligibility of four-band speech stimuli was investigated (n = 18), such that only one of the frequency bands was preserved, whereas other bands were locally time-reversed (segment duration: 75-300 ms), or vice versa. Intelligibility was best retained (82% at 75 ms) when the second lowest band (540-1700 Hz) was preserved. When the same band was degraded, the largest drop (10% at 300 ms) occurred. The lowest and second highest bands contributed similarly less strongly to intelligibility. The highest frequency band contributed least. A close connection between the second lowest frequency band and sonority was suggested.

Intelligibility of chimeric locally time-reversed speech.

The intelligibility of chimeric locally time-reversed speech was investigated. Both (1) the boundary frequency between the temporally degraded band and the non-degraded band and (2) the segment duration were varied. Japanese mora accuracy decreased if the width of the degraded band or the segment duration increased. Nevertheless, the chimeric stimuli were more intelligible than the locally time-reversed controls. The results imply that the auditory system can use both temporally degraded speech information and undamaged speech information over different frequency regions in the processing of the speech signal, if the amplitude envelope in the frequency range of 840-1600 Hz was preserved.

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

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

Individuality embedded in the isolation calls of captive beluga whales (Delphinapterus leucas).

INTRODUCTION: Species with fission-fusion social systems tend to exchange individualized contact calls to maintain group cohesion. Signature whistles by bottlenose dolphins are unique compared to the contact calls of other non-human animals in that they include identity information independent of voice cues. Further, dolphins copy the signatures of conspecifics and use them to label specific individuals. Increasing our knowledge of the contact calls of other cetaceans that have a fluid social structure may thus help us better understand the evolutionary and adaptive significance of all forms of individually distinctive calls. It was recently reported that one type of broadband pulsed sounds (PS1), rather than whistles, may function as individualized contact calls in captive belugas. The objective of this study was to assess the function and individual distinctiveness of PS1 calls in an isolation context. Recordings were made from five captive belugas, including both sexes and various ages. RESULTS: PS1 was the predominant call type (38 % in total) out of five broader sound categories. One sub-adult and three adults had individually distinctive and stereotyped pulse repetition pattern in PS1; one calf showed no clear stereotyped pulse repetition pattern. While visual inspection of the PS1 power spectra uncovered no apparent individual specificity, statistical analyses revealed that both temporal and spectral parameters had inter-individual differences and that there was greater inter-individual than intra-individual variability. Discriminant function analysis based on five temporal and spectral parameters classified PS1 calls into individuals with an overall correct classification rate of 80.5 %, and the most informative parameter was the average Inter-pulse interval, followed by peak frequency. CONCLUSION: These results suggest that belugas use individually distinctive contact calls in an isolation context. If belugas encode signature information in PS1 calls, as seen in bottlenose dolphins, the pulse repetition pattern may be the carrier, as it is individually stereotyped and appears to require vocal development. This idea is supported by the finding that the average inter-pulse interval is the most powerful discriminator in discriminant analysis. Playback experiments will elucidate which parameters are perceived as individual characteristics, and whether one of the parameters functions as a signature.

Echolocation of insects using intermittent frequency-modulated sounds.

Using echolocation influenced by Doppler shift, bats can capture flying insects in real three-dimensional space. On the basis of this principle, a model that estimates object locations using frequency modulated (FM) sound was proposed. However, no investigation was conducted to verify whether the model can localize flying insects from their echoes. This study applied the model to estimate the range and direction of flying insects by extracting temporal changes from the time-frequency pattern and interaural range difference, respectively. The results obtained confirm that a living insect's position can be estimated using this model with echoes measured while emitting intermittent FM sounds.

Echolocation of static and moving objects in two-dimensional space using bat-like frequency-modulation sound.

Bats use frequency-modulated echolocation to identify and capture moving objects in real three-dimensional space. The big brown bat, Eptesicus fuscus, emits linear period modulation sound, and is capable of locating static objects with a range accuracy of less than 1 µs. A previously introduced model can estimate ranges of multiple, static objects using linear frequency modulation (LFM) sound and Gaussian chirplets with a carrier frequency compatible with bat emission sweep rates. The delay time for a single object was estimated with an accuracy of about 1.3 µs by measuring the echo at a low signal-to-noise ratio. This model could estimate the location of each moving object in two-dimensional space. In this study, the linear period modulation sounds, mimicking the emitting pulse of big brown bats, were introduced as the emitted signals. Echoes were measured from moving objects at two receiving points by intermittently emitting these sounds. It was clarified that this model could localize moving objects in two-dimensional space by accurately estimating the object ranges.

Localization and tracking of moving objects in two-dimensional space by echolocation.

Bats use frequency-modulated echolocation to identify and capture moving objects in real three-dimensional space. Experimental evidence indicates that bats are capable of locating static objects with a range accuracy of less than 1 µs. A previously introduced model estimates ranges of multiple, static objects using linear frequency modulation (LFM) sound and Gaussian chirplets with a carrier frequency compatible with bat emission sweep rates. The delay time for a single object was estimated with an accuracy of about 1.3 µs by measuring the echo at a low signal-to-noise ratio (SNR). The range accuracy was dependent not only on the SNR but also the Doppler shift, which was dependent on the movements. However, it was unclear whether this model could estimate the moving object range at each timepoint. In this study, echoes were measured from the rotating pole at two receiving points by intermittently emitting LFM sounds. The model was shown to localize moving objects in two-dimensional space by accurately estimating the object's range at each timepoint.

Reconstruction of the signal produced by a directional sound source from remote multi-microphone recordings.

A mathematical method for reconstructing the signal produced by a directional sound source from knowledge of the same signal in the far field, i.e., microphone recordings, is developed. The key idea is to compute inverse filters that compensate for the directional filtering of the signal by the sound source directivity, using a least-square error optimization strategy. Previous work pointed out how the method strongly depends on arrival times of signal in the microphone recordings. Two strategies are used in this paper for calculating the time shifts that are afterward taken as inputs, together with source directivity, for the reconstruction. The method has been tested in a laboratory environment, where ground truth was available, with a Polaroid transducer as source. The reconstructions are similar with both strategies. The performance of the method also depends on source orientation.

Evaluation of the echolocation model for range estimation of multiple closely spaced objects.

Experimental evidence indicates that bats can use frequency-modulated echolocation to identify objects with an accuracy of less than 1 µs. However, when modeling this process, it is difficult to estimate the delay times of multiple closely spaced objects by analyzing the echo spectrum, because the sequence of delay separations cannot be determined without information on the temporal changes in the interference patterns of the echoes. To extract the temporal changes, Gaussian chirplets with a carrier frequency compatible with bat emission sweep rates are introduced. The delay time for object 1 (T(1)) is estimated from the echo spectrum around the onset time. The T(2) is obtained by adding the T(1) to the delay separation between objects 1 and 2. Further objects are located in sequence by this procedure. Here echoes were measured from single and multiple objects at a low signal-to-noise ratio. It was confirmed that the delay time for a single object could be estimated with an accuracy of about 1.3 µs. The range accuracy was less than 6 µs when the frequency bandwidth was less than 10 kHz. The delay time for multiple closely spaced objects could be estimated with a high range resolution by extracting the interference pattern.

Analysis of the temporal structure of fish echoes using the dolphin broadband sonar signal.

Behavioral experiments indicate that dolphins detect and discriminate prey targets through echolocating broadband sonar signals. The fish echo contains components from multiple reflections, including those from the swim bladder and other organs, and can be used for the identification of fish species and the estimation of fish abundance. In this paper, temporal structures were extracted from fish echoes using the cross-correlation function and the lowpass filter. First, the echo was measured from an anesthetized fish in a water tank. The number, reflector intensity, and echo duration were shown to be dependent on the species, individual, and orientation of the fish. In particular, the echo duration provided useful information on the fish body height and for species identification. Second, the echo was measured from the live fish suspended by nylon monofilament lines in the open sea. It was shown that this duration could be estimated regardless of whether or not the fish were moving.

An echolocation model for range discrimination of multiple closely spaced objects: transformation of spectrogram into the reflected intensity distribution.

Using frequency-modulated echolocation, bats can discriminate the range of objects with an accuracy of less than a millimeter. However, bats' echolocation mechanism is not well understood. The delay separation of three or more closely spaced objects can be determined through analysis of the echo spectrum. However, delay times cannot be properly correlated with objects using only the echo spectrum because the sequence of delay separations cannot be determined without information on temporal changes in the interference pattern of the echoes. To illustrate this, Gaussian chirplets with a carrier frequency compatible with bat emission sweep rates were used. The delay time for object 1, T1, can be estimated from the echo spectrum around the onset time. The delay time for object 2 is obtained by adding T1 to the delay separation between objects 1 and 2 (extracted from the first appearance of interference effects). Further objects can be located in sequence by this same procedure. This model can determine delay times for three or more closely spaced objects with an accuracy of about 1 micros, when all the objects are located within 30 micros of delay separation. This model is applicable for the range discrimination of objects having different reflected intensities and in a noisy environment (0-dB signal-to-noise ratio) while the cross-correlation method is hard to apply to these problems.

An echolocation model for the restoration of an acoustic image from a single-emission echo.

Bats can form a fine acoustic image of an object using frequency-modulated echolocation sound. The acoustic image is an impulse response, known as a reflected-intensity distribution, which is composed of amplitude and phase spectra over a range of frequencies. However, bats detect only the amplitude spectrum due to the low-time resolution of their peripheral auditory system, and the frequency range of emission is restricted. It is therefore necessary to restore the acoustic image from limited information. The amplitude spectrum varies with the changes in the configuration of the reflected-intensity distribution, while the phase spectrum varies with the changes in its configuration and location. Here, by introducing some reasonable constraints, a method is proposed for restoring an acoustic image from the echo. The configuration is extrapolated from the amplitude spectrum of the restricted frequency range by using the continuity condition of the amplitude spectrum at the minimum frequency of the emission and the minimum phase condition. The determination of the location requires extracting the amplitude spectra, which vary with its location. For this purpose, the Gaussian chirplets with a carrier frequency compatible with bat emission sweep rates were used. The location is estimated from the temporal changes of the amplitude spectra.