Echolocation signals of dusky dolphins (Lagenorhynchus
obscurus) in Kaikoura, New Zealand
Whitlow W. L. Aua)
Marine Mammal Research Program, Hawaii Institute of Marine Biology, University of Hawaii,
P.O. Box 1109, Kailua, Hawaii 96734
Bernd Wu ¨rsigb)
Marine Mammal Research Program, Texas A & M University, 4700 Avenue U, Building 303, Galveston,
?Received 29 August 2003; revised 23 January 2004; accepted 28 January 2004?
An array of four hydrophones arranged in a symmetrical star configuration was used to measure the
echolocation signals of the dusky dolphin ?Lagenorhynchus obscurus? near the Kaikoura Peninsula,
New Zealand. Most of the echolocation signals had bi-modal frequency spectra with a
low-frequency peak between 40 and 50 kHz and a high-frequency peak between 80 and 110 kHz.
The low-frequency peak was dominant when the source level was low and the high frequency peak
dominated when the source level was high. The center frequencies in the dusky broadband
echolocation signals are among the highest of dolphins measured in the field. Peak-to-peak source
levels as high as 210 dB re 1 ?Pa were measured, although the average was much lower in value.
The levels of the echolocation signals are about 9–12 dB lower than for the larger white-beaked
dolphin ?Lagenorhynchus albirostris? which belongs to the same genus but is over twice as heavy
as the dusky dolphins. The source level varied in amplitude approximately as a function of the
one-way transmission loss for signals traveling from the animals to the array. The wave form and
spectrum of the echolocation signals were similar to those of other dolphins measured in the field.
© 2004 Acoustical Society of America. ?DOI: 10.1121/1.1690082?
PACS numbers: 43.80.Ev, 43.80.Ka, 43.80.Jz ?FD?
The dusky dolphin ?Lagenorhyncus obscurus? is only
found in the Southern hemisphere inhabiting cold-temperate
coastal waters around New Zealand, western South Africa,
and South America ?Gaskin, 1968; Van Waerebeek et al.,
1995; Van Waerebeek and Wu ¨rsig, 2002?. Dusky dolphins
are relatively small, reaching sexual maturity at about 1.75 m
and rarely exceed 100 kg in weight ?Van Waerebeek and
Wu ¨rsig, 2002?. One area of New Zealand where dusky dol-
phins are commonly found is the waters of Kaikoura in the
South Island. In this area, dusky dolphins occur in groups of
generally 40–200 animals, but can reach to well over 1000
animals during April to May ?Wu ¨rsig et al., 1997?. The in-
teraction of the subtropical convergence and the 1000-m-
deep Kaikoura Canyon create a productive upwelling rela-
tively close to shore ?Robertson et al., 1978?, affecting the
mesopelagic micronekton ?small fishes, squid, and shrimp? in
the diel vertical migrating deep scattering layer. Dusky dol-
phins forage at night on this deep scattering layer ?Wu ¨rsig
et al., 1989? as it migrates vertically from deep waters ??500
m? to within 29–49 m of the surface between 2300 and 0100
h local time ?Benoit-Bird et al., 2004?.
Echolocation no doubt plays an important role in dusky
dolphin natural history, especially when foraging at night for
relatively small prey over the Kaikoura submarine canyon.
Dusky dolphins in other parts of the world ?Wu ¨rsig and Wu ¨r-
sig, 1980?, including some other parts of New Zealand
?Markowitz et al., in press? also prey on schooling fish dur-
ing daylight hours, no doubt utilizing their echolocation ca-
pabilities. However, there is very little information on the
structure and form of echolocation signals utilized by dusky
dolphins, and therefore a description of such signals near the
Kaikoura Peninsula is the subject of this paper.
The echolocation characteristics of delphinid species
have been studied primarily in captivity ?Au, 1993?. Mea-
surements from stationary dolphins in captivity have shown
that echolocation clicks are emitted in a directional beam and
signals measured off-axis are distorted with respect to the
signals measured along the major axis of the beam. There-
fore, it is very difficult to obtain accurate measurements of
free-ranging, fast moving dolphins in the wild. Another com-
plicating factor is associated with the broadband nature of
echolocation signals and the possibility that the center fre-
quency of echolocation clicks may vary with the intensity of
the emitted clicks. Au et al. ?1985? found that higher inten-
sity clicks emitted by a beluga whale ?Delphinapterus leu-
cas? in Hawaii had higher frequencies than the lower inten-
sity clicks used by the same animal in San Diego Bay. Au
et al. ?1995? also found a nearly linear relationship between
the center frequency and source level of a false killer whale
?Pseudorca crassidens?, i.e., the higher the source level, the
higher the center frequency of the emitted clicks. Therefore,
any measurements of the spectra of echolocation clicks
should be accompanied by an estimate of the source levels.
a?Electronic mail: email@example.com
b?Electronic mail: firstname.lastname@example.org
2307J. Acoust. Soc. Am. 115 (5), Pt. 1, May 2004 0001-4966/2004/115(5)/2307/7/$20.00© 2004 Acoustical Society of America
A. Measurement system
A four-hydrophone array with the hydrophones arranged
as a symmetrical star was used to measure the echolocation
signals of the dusky dolphin. Such a sensor geometry was
used successfully by Aubauer ?1995? in tracking echolocat-
ing bats in the field and by Au and various colleagues ?Au
and Benoit-Bird, 2003; Au and Herzing, 2003; Rasmussen
et al., 2002?. The array structure resembled the letter ‘‘Y,’’
with each arm being 45.7 cm in length and separated by an
angle of 120°. The arms of the array were constructed out of
1.27-cm o.d. PVC pipe with a spherical hydrophone con-
nected to the end of each pipe and the cable running through
the center of the pipe. Another hydrophone was connected at
the geometric center of the ‘‘Y.’’The PVC pipes fit into holes
drilled into a 2.54-cm-thick delrin plate. The range of a
sound source can be determined by measuring the time of
arrival differences between the signal at the center and the
three other hydrophones. A detailed description of the array
and the location technique can be found in the appendix of
Au and Herzing ?2003?.
An underwater housing connected to the back of the
hydrophone mounting plate contained an amplifier and line-
driver for each of the hydrophones. A CCD video camera in
an underwater housing was mounted next to the center hy-
drophone. A multi-conductor cable, 7 m in length, consisting
of five coaxial lines and two dc power lines, connected the
array to an adjustable amplifier-filter box containing a power
Echolocation signals were digitized with two Gage-
1210, 12-bit dual simultaneous sampling data acquisition
boards that were connected to a ‘‘lunch box’’ computer via
two EISA slots. The data acquisition system operated at a
sample rate of 500 kHz with a pretrigger capability. When
the computer signaled the Gage-1210 to collect data, four
channels of acoustic signals were simultaneously and con-
tinuously digitized, with the results going into separate cir-
cular memories on each Gage-1210 board. When an echolo-
cation signal was detected by the center hydrophone, it
triggered the data acquisition board. Two hundred pretrig-
gered points and 200 post trigger points were collected for
each channel and downloaded into the computer. A maxi-
mum of 80 clicks could be downloaded for each episode
before the data had to be stored on the hard drive. A specially
constructed ISA board was also used to measure the time
interval between the clicks being acquired and to cause a
light emitting diode to flash indicating that clicks were being
captured. The interclick interval data were also downloaded
and stored on the hard drive. The time of capture ?to the
closest 18-ms interval of the computer timing system? of
each click was also saved and stored on the hard drive.
Acoustic measurements of echolocation signals were
conducted from an 8.8-m motorized catamaran. All the elec-
tronics including the lunch-box computer were housed on the
deck of the catamaran. The boat was driven to the vicinity of
a dolphin pod and then the engine was turned off, leaving the
boat to drift. The four-hydrophone array was deployed by
attaching a wooden rod ?2.54-cm diam? to it and the array
assembly was held over the side of the boat. The center of
the array was lowered to a depth of 1–1.5 m below the
surface. Dusky dolphins often milled about 20–30 m from
the boat, and individuals would frequently dash towards the
array, continuously echolocating as they did so.
A total of 23 files containing 1456 echolocation clicks
were collected representing 23 instances in which a dolphin
made a run at the hydrophone array. The average number of
clicks collected per file was slightly over 63. Only echoloca-
tion events in which the amplitude of the signals received by
the center hydrophone was either the highest or within 3 dB
of the highest were accepted for analysis. This criterion was
chosen to ensure that a dolphin beam was directed at the
array. The beam patterns measured for three different odon-
tocete species ?Au, 1993; Au et al., 1995? indicate that when
the major axis of the beam is directed to within ?5° of the
center hydrophone, the signal received by the center hydro-
phone will either have the highest or will be within 3 dB of
the highest amplitude for echolocation signals for ranges be-
tween approximately 5 and 10 m between the array and dol-
phins. After applying the 3-dB criterion, another level of
inspection was done. Each signal passing the 3-dB criterion
but was outside the 5–10-m range was examined visually for
any distortions. Results from the bottlenose dolphin, Tursi-
ops truncatus, the beluga whale ?Au, 1993?, and the false
killer whale ?Au et al., 1995? indicate that off-axis click of-
ten have sudden reversals in the waveform or large asymme-
try between the positive and negative excursions or longer
intervals between successive cycles in the waveform when
compared to on-axis clicks. These clicks that are off axis by
more than about ?5° will appear distorted and can be easily
distinguished from on-axis clicks. A total of 618 clicks met
the appropriate criteria.
Four spectra of echolocation click trains are presented in
a waterfall format in Fig. 1. The spectra suggest that portions
of the signals in a click train can be relatively stable in shape
but also include portions that are highly variable and com-
plex. Most of the energy in the spectra is between 30 and 130
kHz, much higher than for spectra of echolocation signals
measured for most other dolphins in the field ?Au and Herz-
ing, 2003; Rasmussen et al., 2002?. Smaller, nonwhistling
odontocetes species such as the Hector’s dolphin, Cephalo-
rhynchus hectori ?Dawson, 1988?, harbor porpoise, Phoc-
oena phocoena ?Kamminga and Wiersma, 1981?, Commer-
son’s dolphin, Cephalorhynchus commersonii ?Kamminga
and Wiersma, 1981?, finless porpoise, Neophocaena phocae-
noides ?Kamminga, 1988?, and Dall’s porpoise, Phoc-
oenoides dalli ?Hatakeyama and Soeda, 1990? emit narrow-
band echolocation signals that have slightly higher peak and
center frequencies than the dusky dolphin.
Most of the clicks emitted by dusky dolphins had bimo-
dal frequency spectra. The secondary peaks vary in shape
from a slight ‘‘bump’’ to a clearly defined peak. A secondary
peak existed if the slope of the spectrum for two consecutive
increasing frequency bins changed from positive to zero or
negative values. A bimodal spectrum is defined as one in
which the amplitude of the secondary peak is greater than
2308 J. Acoust. Soc. Am., Vol. 115, No. 5, Pt. 1, May 2004 W. W. L. Au and B. Wu ¨rsig: Dusky dolphin echolocation signals
2the amplitude of the primary peak. Au et al. ?1995? sepa-
rated echolocation signals into four types based on the num-
ber of peaks in the frequency spectra. Type 1 and 2 signals
had low-peak frequencies ?below 64 kHz?, type 1 being uni-
modal and type 2 being bimodal. Type 3 and 4 signals had
high-peak frequencies ?above 64 kHz?, type 3 being bimodal
and type 4 being unimodal.
Examples of representative signals of types 1–4 are
shown in Fig. 2 with the waveforms on the left and the
frequency spectra on the right. All the clicks were very brief,
generally less than 70 ?s in duration, with broad frequency
spectra. The percentage of the various type signals are also
shown next to the waveforms. Clicks with bimodal spectra
are obvious in the spectra plots; most of the clicks had bi-
modal spectra ?88.3%?. The type 1 signals occurred 8.9% of
the time and the type 4 signals occurred only 2.9% of the
time. Some of the bimodal spectra have relatively high-peak
frequencies ??90 kHz? whereas some have low peak fre-
quencies ??40 kHz?. The click waveforms resemble those of
other odontocetes such as bottlenose dolphins, beluga whales
?Au, 1993?, false killer whales ?Au et al., 1995?, and white-
beaked dolphins ?Rasmussen et al., 2002?.
The peak-to-peak source level as a function of range
between an echolocating dolphin and the array is shown in
Fig. 3. As the dolphin’s range to the array decreased, the
source level also decreased. The solid curve in Fig. 3 is a
regression curve represented by the equation
where SL is the source level in dB re 1 ?Pa and R is the
range in meters. The increase in source level as a function of
20LogR compensates for the one-way spherical spreading
loss as the signal propagates outward from a transmitter. The
highest amplitude echolocation signal was 210 dB emitted
by a dusky dolphin at a range of 25 m. The results also
suggest that the dolphins were echolocating on the hydro-
phone array and not on some other objects since the source
level decreased as the range to the array decreased. Also the
interclick intervals were always greater than the two-way
travel time from the animals to the array and back, which is
consistent with the notion that the dolphins were echolocat-
FIG. 1. Four waterfall spectra of
FIG. 2. Examples of some representative echolocation signal waveforms
and spectra emitted by the dusky dolphin in Kaikoura. Type 1 and 2 signals
have low peak frequencies ??64 kHz? with type 1 being unimodal and type
2 being bimodal. Types 3 and 4 have high peak frequencies ??64 kHz? with
type 3 being bimodal and type 4 being unimodal. A bimodal signal is one in
which amplitude of the secondary peak is greater than
2the amplitude at the
2309 J. Acoust. Soc. Am., Vol. 115, No. 5, Pt. 1, May 2004W. W. L. Au and B. Wu ¨rsig: Dusky dolphin echolocation signals
ing on the array. The fitted curve was constrained to vary as
20LogR; however, the ‘‘best-fit’’ logarithm’s curve would
be very similar to the 20LogR fit, with the r2values differ-
ing only in the third decimal place.
The distributions of peak and center frequencies of the
echolocation signals are shown in Fig. 4. Peak frequency is
defined as the frequency at which the frequency spectrum of
a signal has its maximum amplitude. Center frequency is
defined as that frequency which divides the energy in a fre-
quency spectrum into two equal parts ?the centroid of the
spectrum?. The peak frequency histogram has a low-
frequency peak between 50 and 60 kHz and a high-frequency
peak an octave higher between 100 and 110 kHz, reflecting
the bimodal characteristics of the dusky echolocation signals.
The mean and standard deviation of the peak frequency were
73.8?27.3 kHz (n?618).
The center frequency histogram has a well-defined peak
between 90 and 100 kHz. Ninety-two percent of the signals
had center frequencies greater than 80 kHz and 66% had
center frequencies greater than 90 kHz. The center frequency
extends to much higher frequencies than the peak frequency
and this property is indicative of signals with bimodal spec-
tra; the secondary peak causes the spectrum to be broader,
introducing a large asymmetry in the spectrum. Center fre-
quency is a more representative measure of signals with bi-
modal spectra ?Au et al., 1995?, since a slight shift in the
spectrum could move the peak frequency over an octave
away. The mean and standard deviation of the center fre-
quency were 80.5?8.7 kHz (n?618).
Histograms of the 3-dB bandwidth and the rms band-
width are shown in Fig. 5. The 3-dB bandwidth is the width
of the frequency band between the two points that are 3 dB
lower than the maximum amplitude of a spectrum. The 3-dB
points are also referred to as the half-power points since a
level 3 dB below the maximum in a continuous signal is
exactly half of the power or energy in a signal. Its distribu-
tion is rather scattered with a peak between 20 and 25 kHz,
but with distributions reaching out to 100 kHz. The mean
and standard deviation of the 3-dB bandwidth were 67.4
?27.4 kHz (n?618).
The rms bandwidth is the standard deviation of the spec-
trum about the center frequency and its distribution is clus-
tered between 40 and 50 kHz with 80% of the signals having
bandwidth in this range. The 3-dB bandwidth for bimodal
spectra can often provide a misrepresentation of the width of
the signal since the bandwidth might cover only the fre-
quency range about the peak frequency. The rms bandwidth
is probably a better measure of the width of signals with
bimodal spectra since the effects of local maxima and
minima are accounted for in the calculation. The mean and
standard deviation of the rms bandwidth were 34.0?8.7 kHz
An analysis examining the relationship between center
frequency and peak-to-peak source levels indicated that there
is not a direct relationship between center frequency and
source level. However, when examining the average source
levels for the different signal types ?Fig. 6? there seems to be
a trend showing that the higher frequency signals tend to
have higher source levels. By the manner in which the dif-
ferent types of signals were defined, the type 1 signal will
FIG. 3. Scatter plot of source level as a function of the range between an
echolocating dolphin and the hydrophone array. The solid curve represents
the one-way spherical spreading curve-fitted to the data in a least-square
FIG. 4. Histogram of peak and center frequencies of dusky dolphin echolo-
FIG. 5. Histogram of 3-dB and rms bandwidths of the dusky dolphin
FIG. 6. The mean and standard deviation of source levels for the different
2310 J. Acoust. Soc. Am., Vol. 115, No. 5, Pt. 1, May 2004W. W. L. Au and B. Wu ¨rsig: Dusky dolphin echolocation signals
have the lowest center frequency and the center frequency
will progressively increase, with type 4 having the highest
frequency. The mean source level of the type 1 signals were
significantly different (p?0.05, one-way anova? from the
that of all the other signal types ?Table I?. The mean source
level of the type 2 signals were significantly different (p
?0.05, one-way anova? from that of the type 1 and 3 signals
and the type 4 signals were significantly different (p
?0.05) from the type 1 signals.
IV. DISCUSSION AND CONCLUSIONS
The variation of source level as a function of 20LogR
range for the echolocation signals produced by dusky dol-
phins is consistent with echolocation signals of other dol-
phins that have been measured in the field with the sym-
metrical star array ?Au and Benoit-Bird, 2003?. Au and
Benoit-Bird attributed variation of the source level as a func-
tion of range to a method of obtaining a time-varying gain
function in the dolphin sonar system. The strength of echoes
from a fish school containing many individual targets will
decrease with range as a function of 20LogR ?Urick, 1983?.
Therefore, the dolphin sonar system compensates for the en-
ergy lost in the backscattered signal by increasing its source
level. This time-varying gain phenomenon is not specific to a
particular dolphin population or any geographic region but
has been confirmed by measurements in Iceland with the
white-beaked dolphins, Lagenorhynchus albirostris ?Ras-
mussen et al., 2002?, in the Bahamas with Atlantic spotted
dolphins, Stenella frontalis ?Au and Herzing, 2003?, and in
British Columbia with killer whales, Orcinus orca ?Au et al.,
The source levels utilized by the dusky dolphin are
much lower than the signals used by the white-beaked dol-
phin of the same genus. The difference in source levels can
be determined by the value of the constant in Eq. ?1?, which
is 177.8 dB for the dusky dolphin and 189.6 for the white-
beaked dolphin ?Rasmussen et al., 2002?. These values rep-
resent a difference of 11.8 dB, measured with the same array
and electronic system for both species. Also, the maximum
source level recorded for the dusky dolphin was 210 dB re 1
?Pa compared with 219 dB for white-beaked dolphin ?Ras-
mussen et al., 2002?. The difference in source levels may not
be surprising when comparing the sizes of the two species.
The white-beaked dolphin has a robust body with a short
thick rostrum. Adult white-beaked dolphins weigh between
220 and 350 kg and have lengths between 2.2 and 2.8 m
?Reeves et al., 1999? compared to approximately 100 kg and
1.75 m for the dusky dolphin. Therefore, the white-beaked
dolphin is over twice as heavy as the dusky dolphin, yet only
about 1 m longer, but with a much wider girth. Unfortu-
nately, information on the relationship of animal size and
source levels of echolocation signals does not exist. How-
ever, the Hector’s dolphin, one of the smallest dolphins, has
been reported to emit echolocation signals with typical
source levels of about 150 and 175 dB ?Dawson, 1988?. The
harbor porpoise, also one of the smallest odontocetes, was
observed to emit average source levels between 165 and 169
dB while echolocating a target at a range of 25 m ?Kastelein
et al., 1999?. These levels are much lower than those used by
the dusky dolphin.
Dusky dolphins project broadband, short duration
echolocation signals similar to those of other odontocetes
that also produce whistles. Prior to this study, the symmetri-
cal star array had been used to measure the echolocation
signals of five different species of dolphins in the field. The
majority of the echolocation signals had bimodal spectra.
The echolocation signals of the dusky dolphins in the waters
of Kaikoura also exhibited a strong bimodal tendency, with
TABLE I. Results of the statistical test of the data presented in Fig. 6.
?a? One-way ANOVA
?b? Dunnett C multiple comparison
Variable 1Variable 1
95% Confidence Interval
L. boundU. bound
Type 1 Type 2
Type 2 Type 1
Type 3Type 1
Type 4Type 1
2311J. Acoust. Soc. Am., Vol. 115, No. 5, Pt. 1, May 2004W. W. L. Au and B. Wu ¨rsig: Dusky dolphin echolocation signals
88.3% of the signals being bimodal. It is becoming apparent
that bimodality is an inherent feature in the generation
mechanism of dolphins. Type 3 signals were emitted the
most ?45.6% of the time?; however, the number of type 2
signals was close behind, occurring 42.7% of the time.
The frequency characteristics of bimodal echolocation
signals are best described by their center frequency and rms
bandwidth, rather than peak frequency and 3-dB or half-
power bandwidth. The standard deviation of the peak fre-
quency values of 27.3 kHz is three times as high as the
standard deviation of 8.7 kHz for center frequency. The dif-
ference in standard deviation is even greater when comparing
3-dB bandwidth with the rms bandwidth. The variance in the
3-dB bandwidth is relatively high with a standard deviation
of 27.4 kHz compared with a standard deviation of only 2.7
kHz for the rms bandwidth. These results suggest that more
stable measures of the spectra of echolocation signals are
center frequency and rms bandwidth.
Most of the signals had center frequencies between 90
and 110 kHz, which represent some of the highest center
frequencies for broadband signals emitted by free-ranging
delphinids. This characteristics of high center frequency may
be related to the size of the dusky dolphins. The general rule
of thumb in sound production is the smaller the animal the
higher the frequency of sounds produced. Wang et al. ?1995?
found a strong correlation between the maximum frequency
of whistles and the body lengths of seven species of dol-
The broad bandwidth of the echolocation signal provides
a good range resolution capability ?Au, 1993? that should
enable dusky dolphins, when foraging for small schooling
fish ?Wu ¨rsig and Wu ¨rsig, 1980?, to perform fine target dis-
crimination in a shallow water environment where bottom
reverberation can be troublesome. Good range or time reso-
lution would also be beneficial for dusky dolphins when for-
aging on mesopelagic prey such as myctophid fish and small
squid ?Cipriano, 1992?. Although there does not seem to be a
relationship between source level and center frequency as for
the false killer whale ?Au et al., 1995?, Au and Herzing
?2003? found that only when the source level increases be-
yond 210 dB re 1 ?Pa did a relationship between center
frequency and source level emerge. The highest level signal
for the dusky dolphin was only 210 dB. Perhaps if higher
source levels were used, a relationship between center fre-
quency and source level may emerge. However, there is a
definite trend in which the source level increased as the sig-
nal type increased from 1 to 3, which seems to indicate that
there should be some kind of relationship between center
frequency and source level. Grouping the signals into types
definitely helped in bringing out the relationship between
source levels and center frequency.
The results of this study also clearly demonstrate the
utility of using a multi-hydrophone array to measure echolo-
cation signals of dolphins in the wild. The symmetrical star
array used in this study is relatively compact and easy to
handle, and can provide information on whether a specific
received echolocation signal originated in the vicinity of the
major axis of the animal’s transmission beam. Time of ar-
rival differences between hydrophones were easily ascer-
tained because of the rapid onset of the echolocation signals.
Our results have demonstrated the value of using an array to
obtain reliable data on echolocation signals in the field.
For support in the field, we thank Ian Bradshaw, Jackie
Wadsworth, and Lynnette and Dennis Buurman of ‘‘Dolphin
Encounter’’ Kaikoura, New Zealand. The first author thanks
Kelly Benoit-Bird for her encouragement to add a ecological
perspective to this ms. He would also like to thank Kelly and
Kimberly Andrews for their help in the Anova statistics.
Thanks also to author Dr. Roland Aubauer for his suggestion
of using a symmetrical star geometry and for various discus-
sions associated with his research on detection of flying bats.
The assistance of Michiel Schotten in testing and calibrating
the array is also appreciated. For ongoing field and labora-
tory logistics, we thank the Edward Percival Field Station of
the University of Canterbury, Jack vanBerkel, station man-
ager. This work was partially funded by the Office of Naval
Research, Dr. Robert Gisiner, program manager; the Center
for Field Studies of Earthwatch; the Marlborough District
Council; the New Zealand Department of Conservation; and
a Fulbright research/teaching award to the second author.
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