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47-channel burst-mode recording hydrophone system enabling measurements of the dynamic echolocation behavior of free-swimming dolphins

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Detailed echolocation behavior studies on free-swimming dolphins require a measurement system that incorporates multiple hydrophones (often >16). However, the high data flow rate of previous systems has limited their usefulness since only minute long recordings have been manageable. To address this problem, this report describes a 47-channel burst-mode recording hydrophone system that enables highly resolved full beamwidth measurements on multiple free-swimming dolphins during prolonged recording periods. The system facilitates a wide range of biosonar studies since it eliminates the need to restrict the movement of animals in order to study the fine details of their sonar beams.
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47-channel burst-mode recording hydrophone system enabling
measurements of the dynamic echolocation behavior of
free-swimming dolphins (L)
Josefin Starkhammara
Electrical Measurements, Faculty of Engineering LTH, Lund University, P.O. Box 118,
SE-221 00 Lund, Sweden
Mats Amundin
Kolmarden Wildlife Park, SE-618 92 Kolmarden, Sweden and Department for Physics, Chemistry and
Biology (IFM), Linkoping University, SE-581 83 Linkoping, Sweden
Johan Nilsson and Tomas Jansson
Electrical Measurements, Faculty of Engineering LTH, Lund University, P.O. Box 118, SE-221 00 Lund,
Sweden
Stan A. Kuczaj
Department of Psychology, The University of Southern Mississippi, P.O. Box 5025, Hattiesburg, Mississippi
39406-5025
Monica Almqvist and Hans W. Persson
Electrical Measurements, Faculty of Engineering LTH, Lund University, P.O. Box 118,
SE-221 00 Lund, Sweden
Received 9 February 2009; revised 30 June 2009; accepted 30 June 2009
Detailed echolocation behavior studies on free-swimming dolphins require a measurement system
that incorporates multiple hydrophones often 16. However, the high data flow rate of previous
systems has limited their usefulness since only minute long recordings have been manageable. To
address this problem, this report describes a 47-channel burst-mode recording hydrophone system
that enables highly resolved full beamwidth measurements on multiple free-swimming dolphins
during prolonged recording periods. The system facilitates a wide range of biosonar studies since it
eliminates the need to restrict the movement of animals in order to study the fine details of their
sonar beams. © 2009 Acoustical Society of America. DOI: 10.1121/1.3184536
PACS numbers: 43.80.Ev, 43.80.Ka, 43.60.Qv WWAPages: 959–962
I. INTRODUCTION
The echolocation of dolphins and other odontocetes has
been extensively studied, a primary focus being the beam
axis Nachtigall and Moore, 1988;Thomas and Kastelein,
1990;Au, 1993;Villadsgaard et al., 2007;Kyhn et al.,
2009. Recording scenarios that focus on the beam axis typi-
cally provide accurate and effective measurements. However,
since these situations often require that the echolocating ani-
mal be kept stationary, it is likely that the full dynamics of
the sonar beam has not yet been described. In addition, these
static test conditions are by definition impossible to use in
other important contexts, such as in studies of the spontane-
ous use of echolocation by free-swimming dolphins, object
investigation behavior in groups of dolphins, and calf mim-
icry of their mother’s echolocation clicks. Although detailed
sonar studies have been conducted with free-swimming dol-
phins Sigurdson, 1996;Martin et al., 2005, among others,
these studies used relatively few hydrophones and conse-
quently had limited sonar beam coverage. As a result, much
is known about the beam axis, and little is known about the
rest of the beam.
Recording dolphin sonar in dynamic test conditions re-
quires a system able to deal with varying measurement pa-
rameters, including the animal’s relative distance to the re-
ceivers, the number of animals present, the orientation of the
beam relative to the receivers, and the required time for the
animal to respond to an echolocation task. Consequently, a
measurement system capable of long recording periods, large
beamwidth coverage, and high spatial and temporal resolu-
tions is needed in order to localize the beam axis and to
measure the rest of the beam with high accuracy from free-
swimming dolphins.
Multi-channel sonar recording systems have previously
been reported by Miller and Tyack 1998,Ball and Buck
2005,Starkhammar et al. 2007,Amundin et al. 2008,
and Moore et al. 2008, among others. The most extensive
system developed thus far was created by Moore et al.
2008, who employed 24 hydrophones in an array. It was
designed for high spatial resolution across the array area
since it was used in a study of beamwidth control in a bottle-
nose dolphin Tursiops truncatusand so required measure-
aAuthor to whom correspondence should be addressed. Electronic mail:
josefin.starkhammar@elmat.lth.se
J. Acoust. Soc. Am. 126 3, September 2009 © 2009 Acoustical Society of America 9590001-4966/2009/1263/959/4/$25.00
ments of small spatial alterations of the amplitude distribu-
tion across the whole beam cross-section. The system
recorded data during 5 s intervals with a sample rate of 312.5
kS/s, resulting in a data flow rate of 16 Mbytes/s. Although
this system worked well for its purpose, the extremely high
data flow rate makes it unsuitable for measuring echoloca-
tion behavior in free-swimming dolphins. Using this system,
a single minute of recordings would result in a 1 Gbyte large
file.
A measurement system optimized for dynamic test con-
ditions requires large beamwidth coverage and higher spatial
and temporal resolutions. The system must also be able to
record for longer time periods in order to be useful in the
field. This requires an increase in the number of array ele-
ments, the physical size of the array, and, preferably, also the
sample rate. In addition, the problems associated with ex-
tremely high data flow rates must be solved.
This report describes a system with a measurement ap-
proach optimized for studies of dolphin sonar under dynamic
test conditions. The system uses a larger number of hydro-
phones 47 channels, allows an increased sample rate 1
MHz, and acquires data with lower data flow rates than
previously reported multi-hydrophone systems. This facili-
tates full waveform recordings of the echolocation activity of
dolphins during prolonged time periods and comprehensive
beamwidth coverage, provided that the dolphins are within
reasonable distance from the screen. In addition, the ap-
proach enables real-time analysis and real-time visualization
of data during recordings. This measurement approach and
recording system enables researchers to investigate dolphin
sonar use in a wider range of contexts than has previously
been made possible. In the following sections, the authors
describe this system and provide examples of its potential
uses with free-swimming dolphins.
II. MATERIALS AND METHODS
A new 47-channel dolphin echolocation measurement
system was developed and tested with a group of 19 Atlantic
bottlenose dolphins, housed together in a large open sea pen
at Roatán Institute of Marine Science, Roatán, Honduras. All
dolphins were allowed to swim freely and to explore at will
objects suspended in front of or behind the recording hydro-
phone array. The size of array was 0.750.75 m2. Figure
1Ashows a schematic representation of the experimental
setup, and Fig. 1Bshows a photograph of the setup during
one trial.
Typically, multi-hydrophone arrays used in biosonar ap-
plications produce a considerable amount of data due to the
relatively high sample rate required to reconstruct the full
waveform accurately in post-analysis approximately ten
times the maximum frequency for accurate visualization
Buchla and McLachlan, 1992兲兴. The long recording times
required in test conditions with spontaneously echolocating
dolphins using a 47-element array system would result in
unmanageably large data files after only a few seconds, using
the data acquisition approaches in previous systems. There-
fore, an alternative approach to continuous sampling of all
parallel channels was needed for the 47-element hydrophone
array system.
In order to facilitate longer recordings and to keep the
data flow rate manageable with a high sample rate, the sys-
tem was designed to be triggered by one echolocation click
at a time and to not sample data during the silent periods that
occur between clicks in click trains. Figure 2describes the
basic data acquisition method. Basically, the system only ac-
quires a small pre-set number of samples, containing only
the actual click, when a hydrophone output exceeds the cho-
sen trig level. This is referred to here as burst-mode sampling
as opposed to continuous sampling. The time stamps T1
and T2 in Fig. 2correspond to the start time of each sample
burst and are stored in association with each click.
Successful burst-mode sampling required data acquisi-
tion hardware capable of extremely fast re-triggering of mea-
surements even after the particularly short inter-click inter-
vals 1msthat may occur in click train “buzzes” see
FIG. 1. Color online兲共AA schematic drawing of the system setup in the field test. BAn underwater photograph of the experimental setup.
FIG. 2. Color onlineBasic data acquisition method.
960 J. Acoust. Soc. Am., Vol. 126, No. 3, September 2009 Starkhammar et al.: Letters to the Editor
Herzing, 1996;Herzing and dos Santos, 2004or when mul-
tiple dolphins echolocate concurrently. The total time re-
quired to finish one burst-mode acquisition, re-trig, and start
a new one is referred to here as the system’s rearm time.
In order to capture the echolocation data across the en-
tire array, the system was designed to simultaneously trigger
all channels, regardless of which hydrophone was hit first by
the sonar beam. Signals were acquired from 68 parallel
and synchronized channels using six PXI-digitizer cards NI
PXI-5105, National Instruments, USA, each with eight si-
multaneously sampling analog-to-digital-channels and 12 bit
voltage resolution. One of the digitizer channels was used as
a trig-channel. The 47 remaining channels were wired to the
47 individually pre-amplified hydrophones, all amplified
with either 35 or 50 dB, depending on the measurement situ-
ation. The sum of all the 47 pre-amplified hydrophone sig-
nals was wired to the pre-designated trig-channel using a
separate signal summarizing hardware circuitry.
The software was optimized to ensure fast rearm time
and real-time visualization and analysis of data, aspects that
were typically the most time-consuming as well as important
determinants of the overall system performance. This im-
provement was accomplished with software created in LAB-
VIEW 8.6National Instruments, USAenabling dual-core
operation of the CPU.
III. RESULTS AND DISCUSSION
The echolocation measurements of the free-swimming
dolphins in the group of 19 individuals demonstrated that the
system is capable of measuring the full waveforms of the
spontaneous sonar activity in the group. Measurements were
obtained with all 47 simultaneously sampling hydrophones at
a sample rate of 1 MHz during measurement sessions of
various lengths often 15 min. Each acquisition in these
tests was set to record during a time window of 150
s
around each click event with a pre-trig time period of 40
s
see Fig. 2. The total duration of the measurement sessions
was determined by the tourist activity at the facility. Sessions
were never aborted due to system failure.
Figure 3Ashows the corresponding waveforms of one
single click acquired simultaneously by all 47 hydrophones.
Each position of the numbered elements corresponds spa-
tially to the hydrophones shown in Fig. 1. The level of spa-
tial resolution and comprehensive beamwidth coverage pro-
vides new information concerning the entire cross-section of
the beam during one single click. As an example, Fig. 3B
shows the relative energy distribution of the cross-section in
the beam. The energy in the click is coded as an indentation
of the interpolated three dimensional 3Dsurface where
high energies “push” the surface downward, away from the
echolocating dolphin.
A suspended scuba tank provided a way to further dem-
onstrate the functionality of the system by shadowing the
hydrophones in the center of the screen when a dolphin
echolocated toward the tank from small bearing angles i.e.,
close to the perpendicular to the screen. This shadowing
effect is clearly seen as a ridge in the middle of the beam
energy plot in Fig. 3B.
The presented level of spatial resolution and comprehen-
sive beamwidth coverage give an unprecedented detailed
measure of the spectral content within the cross-section of
the beam Fig. 3C. The minimized data flow rates make it
possible to view the spectral content even in real-time. The
system also allows researchers to study entire echolocation
scan sequences in detail by processing all successive clicks
and then re-playing them at variable frame rates. The reso-
lution of the measurements enables detailed re-plays of the
propagation of every single click across the array, further
facilitating quantified detailed studies of the dynamic varia-
tions in the echolocation behavior of dolphins during pro-
longed periods of time.
Benchmark tests of the system performance showed that
the low data flow makes possible recordings during 20 min
of constantly echolocating animals with inter-click intervals
of 20 ms before the data file size reaches 1 Gbyte and be-
comes unreasonably large for commonly used post-
processing tools such as MATLAB®, The MathWorksInc.,
USA. This is a considerable improvement compared to pre-
viously published systems. The low data flow rate of the
present system 0.83 Mbyte/s under the conditions in the
benchmark testsis even more advantageous in more realis-
tic measurement scenarios, where free-swimming dolphins
echolocate spontaneously and when minute long silent peri-
ods in the recordings are likely to occur.
In conclusion, the presented 47-element hydrophone
system enables recordings with improved spatial and tempo-
ral resolutions of the cross-section of the echolocation beam
FIG. 3. Color onlineVisualization of one echolocation click acquired at the 47 hydrophone positions in the experimental setup. This click can be visualized
by plotting Athe waveforms, Bthe relative energy distribution within the beam, or Cthe color coded frequency distribution within the beam. High click
energy in Bis illustrated by letting it push the interpolated 3D surface downward, away from the echolocating dolphin.
J. Acoust. Soc. Am., Vol. 126, No. 3, September 2009 Starkhammar et al.: Letters to the Editor 961
of free-swimming dolphins. Moreover, the system makes
possible extended recording periods due to the minimized
data flow rate. These features facilitate the reconstruction,
visualization, and re-play of significant aspects of the clicks
during extended echolocation sequences. The system’s abil-
ity to process information from free-swimming dolphins in
groups opens the door to a completely new range of studies,
which will help us to better understand the functions of dol-
phin sonar since it eliminates the need to restrict the move-
ment of animals in order to study the fine details in their
sonar beams.
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962 J. Acoust. Soc. Am., Vol. 126, No. 3, September 2009 Starkhammar et al.: Letters to the Editor
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... This section provides an example of a transient echolocation signal from a beluga whale (Delphinapterus leucas). The signal is sampled with 1 MHz and recorded by one of 47 simultaneously sampling hydrophones as described in [15]. The signal was chosen because it is recorded at the centre of the echolocation beam, based on the peak amplitude level the signal is sample by the hydrophone closest to the centre beam axis of the animal. ...
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The sonar of dolphins has undergone evolutionary re-finement for millions of years and has evolved to be the premier sonar system for short range applications. It far surpasses the capability of technological sonar, i.e. the only sonar system the US Navy has to detect buried mines is a dolphin system. Echolocation experiments with captive animals have revealed much of the basic parameters of the dolphin sonar. Features such as signal characteristics, transmission and reception beam patterns, hearing and internal filtering properties will be discussed. Sonar detection range and discrimination capabilities will also be included. Recent measurements of echolocation signals used by wild dolphins have expanded our understanding of their sonar system and their utilization in the field. A capability to perform time-varying gain has been recently uncovered which is very different than that of a technological sonar. A model of killer whale foraging on Chinook salmon will be examined in order to gain an understanding of the effectiveness of the sonar system in nature. The model will examine foraging in both quiet and noisy environments and will show that the echo levels are more than sufficient for prey detection at relatively long ranges.
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The interpulse‐intervals, 3‐D‐head‐attitudes, azimuths, and pulse waveforms of two Atlantic bottlenose dolphins were recorded while they searched successive circular bottom areas of 80‐yd radii in the open ocean to echolocate bottom objects at depths of 40–50 ft. The animals remained near the center of the area while searching the bottom for 8‐in., half‐tri‐planes on steel plates with both the location of animals and objects determined by a high‐accuracy GPS. The animals were rewarded for correctly reporting the presence or absence of objects that were placed at a random 50% of the stations and, where present, randomly distributed over station area. The derived scan patterns and pulse spectra provide a systematic description of open‐ocean echolocation of bottom objects. As in previous work with water‐column objects in a single field, these animals adapted their spatial distributions of ensonification to their recent experience with detected objects (i.e., the search area). Head orientation and interpulse intervals were correlated with object location on object‐present trials while analysis of the pulses‐on‐object as a function of distance, echo‐to‐pulse delays, and consistency of the pulse spectra showed important differences between the open‐water test scenario and the more traditional tests with animal and targets in static positions.
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In this Letter we describe a beamforming video recorder consisting of a video camera at the center of a 16 hydrophone array. A broadband frequency-domain beamforming algorithm is used to estimate the azimuth and elevation of each detected sound. These estimates are used to generate a visual cue indicating the location of the sound source within the video recording, which is synchronized to the acoustic data. The system provided accurate results in both lab calibrations and a field test. The system allows researchers to correlate the acoustic and physical behaviors of marine mammals during studies of social interactions. (c) 2005 Acoustical Society of America.
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Investigations of communication systems benefit from concurrent observation of vocal and visible behaviors of individual animals. A device has been developed to identify individual vocalizing resident killer whales (Orcinus orca) during focal behavioral observations. The device consists of a 2-m, 15-element hydrophone array, which is easily towed behind a small vessel, on-board multi-channel recorders, and real-time signal processing equipment. Acoustic data from the hydrophones are digitized and processed using broadband frequency-domain beamforming to yield frequency-azimuth (FRAZ) and “directo-gram” displays of arriving sounds. Based upon statistical analysis of independent portions of typical killer whale calls, the precision of the angle-of-arrival estimate ranges from ±0° to ±2.5° with a mean precision of ±1.5°. Echolocation clicks also are resolved precisely with a typical −6 dB mainlobe width of ±2.0°. Careful positioning of the array relative to the animals minimizes the effects of depth ambiguities and allows identification of individual sources in many circumstances. Several strategies for identifying vocalizing individuals are discussed and an example of a successful identification is described. Use of the array with resident killer whales did not interfere with vessel maneuverability, animal tracking, or behavioral sampling of focal individuals. This localization technique has promise for advancing the abilities of researchers to conduct unbiased behavioral and acoustic sampling of individual free-ranging cetaceans.
Article
Dolphins within the Navy Marine Mammal Program use echolocation to effectively locate underwater mines. They currently outperform manmade systems at similar tasks, particularly in cluttered environments and on buried targets. In hopes of improving manmade mine-hunting sonar systems, two instrumentation packages were developed to monitor free-swimming dolphin motion and echolocation during open-water target detection tasks. The biosonar measurement tool (BMT) is carried by a dolphin and monitors underwater position and attitude while simultaneously recording echolocation clicks and returning echoes through high-gain binaural receivers. The instrumented mine simulator (IMS) is a modified bottom target that monitors echolocation signals arriving at the target during ensonification. Dolphin subjects were trained to carry the BMT in open-bay bottom-object target searches in which the IMS could serve as a bottom object. The instrumentation provides detailed data that reveal hereto-unavailable information on the search strategies of free-swimming dolphins conducting open-water, bottom-object search tasks with echolocation.