Spatial orientation of different frequencies within theecholocation beam of a Tursiops truncatus and Pseudorcacrassidens

Stuart D. Ibsena)

University of California San Diego, Serf Building, Room 295 0435, 9500 Gilman Drive, La Jolla,California 92093

Paul E. NachtigallMarine Mammal Research Program, Hawaii Institute of Marine Biology, University of Hawaii,P.O. Box 1106, Kailua, Hawaii 96734

Jacqueline Krause-NehringAlfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany

Laura Kloepper, Marlee Breese, Songhai Li, and Stephanie VlachosMarine Mammal Research Program, Hawaii Institute of Marine Biology, University of Hawaii,P.O. Box 1106, Kailua, Hawaii 96734

(Received 21 November 2011; revised 13 May 2012; accepted 1 June 2012)

A two-dimensional array of 16 hydrophones was created to map the spatial distribution of different

frequencies within the echolocation beam of a Tursiops truncatus and a Pseudorca crassidens. It

was previously shown that both the Tursiops and Pseudorca only paid attention to frequencies

between 29 and 42 kHz while echolocating. Both individuals tightly focused the 30 kHz frequency

and the spatial location of the focus was consistently pointed toward the target. At 50 kHz the beam

was less focused and less precisely pointed at the target. At 100 kHz the focus was often completely

lost and was not pointed at the target. This indicates that these individuals actively focused the

beam toward the target only in the frequency range they paid attention to. Frequencies outside this

range were left unfocused and undirected. This focusing was probably achieved through sensorimo-

tor control of the melon morphology and nasal air sacs. This indicates that both morphologically

different species can control the spatial distribution of different frequency ranges within the echolo-

cation beam to create consistent ensonation of desired targets.VC 2012 Acoustical Society of America. [http://dx.doi.org/10.1121/1.4730900]

PACS number(s): 43.80.Ka, 43.80.Ev, 43.80.Gx, 43.80.Lb [JAS] Pages: 1213–1221

I. INTRODUCTION

The unique sensitivity of dolphin echolocation allows

them to differentiate targets that have small physical differ-

ences (Nachtigall, 1980; Au and Pawloski, 1992; Au, 1993;

Aubauer et al., 2000; Ibsen et al., 2007). Previous work has

been conducted to understand the frequency content in the

echolocation clicks which enables dolphins to make these

discriminations (Ibsen et al., 2007; Ibsen et al., 2009). The

Tursiops in the study described here was found to keep the

frequency content in a 10 kHz band consistent over long

periods of time, even when probing targets with considerably

different acoustic properties. This was the only band of fre-

quencies the Tursiops paid attention to while echolocating

(Ibsen et al., 2009). Outside this band the frequency content

demonstrated a significant increase in variability. In contrast

to the present study, these clicks were recorded from a single

hydrophone placed 1 m away from the Tursiops in an on-

axis orientation.

It is unknown how the Tursiops managed to tailor the

frequency content of the echolocation click to achieve con-

sistency within the particular 10 kHz band of frequencies

while allowing simultaneous variability outside this band.

The source of click production has been traced to airflow

pushing past pairs of internal phonic lip membranes causing

the membranes to slap together (Cranford, 2000). Frequency

content of the click could be manipulated by changing the

physical properties of the phonic membranes by adjusting

tension in adjoining musculature (Cranford and Amundin,

2003). However, adjusting the tension on the membranes

will change the mechanical properties of the entire mem-

brane. Studies of human vocal chords have shown that ten-

sion changes affect the fundamental frequency of sound

production (Benninger et al., 1996) making it unlikely that

membrane tension changes could be the source of such fine

frequency manipulation in small bands. The fact that there is

consistency in a single band of frequencies suggests that the

phonic membranes are producing a consistent click each

time they slap. This consistency appears to be independent

of overall click intensity as long as there is a certain thresh-

old of airflow (Rust et al., 2007; Ibsen et al., 2010). Higher

intensities result from higher airflow rates but the physical

properties of the slapping membranes remain the same.

If the frequency content of the click at its generation

is consistent over time then another possible way of

a)Author to whom correspondence should be addressed. Electronic mail:

sibsen@ucsd.edu

J. Acoust. Soc. Am. 132 (2), August 2012 VC 2012 Acoustical Society of America 12130001-4966/2012/132(2)/1213/9/$30.00

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manipulating on-axis frequency content could be through con-

trolling the spatial distribution of frequencies within the echo-

location beam itself. It has been observed that the echolocation

beam width and angle can be changed in the far field (Moore

et al., 2008) and recordings from suction cup hydrophones

attached to different parts of porpoise and dolphin melons dur-

ing echolocation has shown frequency content variation over

the entire melon (Au et al., 2006; Au et al., 2010; Madsen

et al., 2010). These observations suggest the possibility of far

field manipulation of the spatial frequency distribution through

manipulations of the melon and nasal air sacs. Although previ-

ous work has documented the echolocation beam patterns of

several cetacean species (Au and Moore, 1986; Au, 1993; Au

et al., 1995; Au et al., 1999), it is unknown how the different

frequencies are arranged relative to one another in a two-

dimensional (2D) cross section of the beam from click to click.

To understand the spatial arrangements of different frequen-

cies from within a dolphin’s echolocation beam, a 16 element

array was developed to make recordings from the beam and

map the spatial orientation of different frequencies from click

to click.

II. MATERIALS AND METHODS

A. Subjects

The clicks used in this analysis were collected during

experiments conducted at the Hawaii Institute for Marine

Biology, Marine Mammal Research Program at Kaneohe

Bay, HI. The first subject was an adult female Atlantic bot-

tlenose dolphin, Tursiops truncatus, named BJ who was 21

years of age at the time of the experiments in April of 2007.

She was fed a daily ration of approximately 6.3 kg total of

herring and capelin smelt. The second subject was an adult

female false killer whale, Pseudorca crassidens, named

Kina who was approximately 30 years of age at the time of

the experiments in October 2007. She was fed a daily ration

of 15 kg total of herring, capelin, and squid. Both were

housed in the same floating wire-net enclosure.

Both the Tursiops and Pseudorca used in this study suf-

fered high frequency hearing loss prior to the conduction of

these experiments. The Tursiops’ upper hearing limit was

about 45 kHz and the Pseudorca’s was about 40 kHz (Yuen

et al., 2005; Ibsen et al., 2011). It is unknown what the cause

of this hearing loss might have been. Despite the hearing defi-

ciency, both animals were able to consistently perform target

discrimination tasks at a rate of nearly 100% accuracy.

B. Materials

1. Experimental set-up to record clicks from thePseudorca

Clicks were recorded from the Pseudorca during train-

ing sessions for a target discrimination task. The experimen-

tal setup was the same as that described in Ibsen et al.(2011) and is briefly described here. The Pseudorca was

trained to station within a hoop to ensure that her head posi-

tion was consistent from trial to trial. The array was posi-

tioned between the subject and the target at a distance of

2.5 m from the hoop (see Fig. 1). The target was located

7.6 m from the hoop. The depth of the central hydrophone of

the array, the center of the hoop, and the target were all set

at 1 m to position the central hydrophone of the array

directly between the subject and the target.

2. Experimental set-up to record clicks from theTursiops

The clicks were collected from the Tursiops while the

animal was free swimming in a 10 � 9 m wire mesh bottom

enclosure. The array was placed inside the enclosure up next

to the wire mesh side 1 m away from the corner. This location

was chosen to minimize the risk of entanglement for her 1

month old calf, which was in the enclosure with her.

3. The array system

Hydrophone supports were constructed out of 2.54 cm

diameter PVC plastic tubing and T junctions, chosen to pro-

vide the necessary rigidity and reduce interference with the

acoustic signal as much as possible. The hydrophone wires

ran along the outside of the tubing of each arm and up along

the top supporting pipe. Each element was held in place with

plastic zip ties. The hydrophones were mounted coming out

from the central arm of the T junction pieces and oriented so

that they faced directly toward the Tursiops. This allowed

the hydrophone to extend into free space where no interfer-

ence from the PVC could occur with the hydrophone. The

central hydrophone was mounted on an extending 5 cm long

PVC pipe so it would match the other hydrophones in being

situated above the plane of the support structure and away

from the central support ring to prevent possible reflections.

The array system consisted of 16 Reson TC 4013 hydro-

phones arranged in either a 5 or 7 arm star configuration, see

Figs. 2(A) and 2(B). Star configurations were chosen to most

densely populate the center of the echolocation beam. The

frequencies and intensities of the echolocation beam directly

in line with the central hydrophone were the ones that specif-

ically reflected off the target, making this region the most

relevant to the echolocation task. The less densely populated

outer region of the array was used to record data from

regions of the echolocation beam that did not directly enso-

nify the target.

Each channel was amplified by a custom built 16 channel

amplifier. Each amplifier channel consisted of an amplifier

FIG. 1. Schematic of the experimental setup for the Pseudorca’s target dis-

crimination tasks. In this illustration the array is shown edge on in this repre-

sentation and is aligned so that the central hydrophone is directly between

the whale and the target. The dotted lines represent the echolocation beam.

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chip (Analog Devices AMP02) in line with a line driver

(Texas Instruments DRV 134) to compensate for loss over the

length of the system cables.

All signals were digitized at 1 MHz by two 8 channel

DAQ boards (National Instruments PCI-6133) that were

linked together using a RTSI Bus Cable to act as a single 16

channel digitizer. Interfaces between the DAQ boards and

the 16 channel amplifier were made through a National

Instruments BNC-2110 connector box.

A simultaneous recording of an echolocation click by

all 16 hydrophones was triggered off the central hydrophone

of the array. A rolling memory buffer was used to record the

entire click even though the loudest part of the click is what

triggered the recording. The total time of recording was long

enough to make sure that time delays between sound reach-

ing different elements did not cause temporal clipping of the

signal at any one of the hydrophones.

The entire system was run by a custom written LABVIEW

8.20 program. The system was optimized to run at a record-

ing rate of up to 20 clicks/s for bursts of up to 100 clicks. Af-

ter 100 clicks had been recorded the data had to be saved to

the hard drive of the computer and the DAQ board memory

buffers reset to record an additional 100 clicks. This data

saving process took about 5 s. It was observed that a typical

click train used by the Tursiops consisted of 80 clicks.

Two different configurations were used for the different

experimental setups. The five arm configuration was used

during the free swimming sessions to collect data over the

largest possible surface area since the animal’s distance from

the array was not controlled. The array was rearranged to

FIG. 2. Schematics showing (A) the five arm array and (B) the seven arm array designs for recording frequency content from within the echolocation beam.

The black diamonds represent the hydrophone elements and the dark black lines represent the PVC supports. The corresponding XY coordinates (in meters) for

each element are shown in the tables to the right.

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more densely populate the central region of interest with the

seven arm configuration for experiments with the Pseudorca.

Preliminary data using the five arm array showed that the

Pseudorca’s beam was focused enough that no signal was

picked up on the outermost elements of the five arm configu-

ration. The smaller cross-sectional area of the seven arm

configuration worked well for this situation because the

Pseudorca’s head position relative to the array was the same

from trial to trial due to the hoop stationing and consistent

positioning of the target directly behind the array.

4. Data analysis

Custom analysis software was developed using both LAB-

VIEW 8.20 and MATLAB 7.0.4. A fast Fourier transform of the

time waveform recorded from each element was calculated.

The intensity value for a frequency of interest was then deter-

mined and assigned its respective spatial location on a 2D

grid as shown by the element coordinates in Figs. 2(A) and

2(B). The MATLAB meshgrid and grid data functions were used

to create a three-dimensional representation of the data with

the third dimension being the intensity of the recorded fre-

quency. Contour lines of different intensities were generated

from these three-dimensional surfaces using the MATLAB con-

tour 3 function to simplify the data to a meaningful 2D repre-

sentation and allow multiple clicks to be compared at once. A

schematic of how these contours relate to the original array

design is shown in Fig. 3. Contours at 99% of the maximum

intensity were chosen to show the spatial location of the high-

est intensity portion of the beam and are represented by the

inner circle shown in Fig. 3(A) on the five arm array configu-

ration. The diameter of the 99% contour also gave an indica-

tion of how focused that particular click was. The 3 dB

contour (71% of the maximum recorded intensity) is shown

by the middle circle and the 6 dB contour (50% of the maxi-

mum recorded intensity) is shown by the outer circle. When

graphed together, these three contours at 99%, 71%, and 50%

give a 2D representation of how the beam was focused for a

single click at a single frequency of choice.

The contours from multiple clicks were overlaid to

understand the variation in beam focus from click to click

as shown in Fig. 3(B) for six clicks made by the Pseudorcaat 30 kHz.

C. Experimental procedure

1. Experimental procedure to record clicks from thePseudorca

The experimental procedures are the same as those used

previously in Ibsen et al. (2011) and are briefly described

here. The Pseudorca was trained to station with the trainer at

a paddle located at the end of the wooden pen structures and

was signaled with an underwater sound to swim into the

hoop. The subject’s visual and acoustic access to the target

pen on the opposite side of the hoop was blocked by a mova-

ble baffle. The subject was presented with an aluminum pipe

target operating under a present/absent paradigm. The pipe

was lowered into the water while the Pseudorca was sta-

tioned in the hoop. The movable baffle was lowered out of

the way and the animal was allowed to echolocate at the tar-

get location. The target present response was to back out of

the hoop and touch a paddle with the rostrum. The target

absent response was for the Pseudorca to stay in the hoop

for 6 s. If the Pseudorca was correct, a fish reward was

given. Instances of errors were not reinforced and the animal

was called back to the stationing paddle to start a new trial.

2. Experimental procedure to record clicks from theTursiops

Click train recording events occurred as the Tursiopsnaturally echolocated at the array while free swimming in

FIG. 3. (Color online) Schematic of the analysis process used to generate

intensity contour maps of individual frequencies from within the echoloca-

tion beam using the array data. (A) Each click was recorded in its entirety

by all 16 hydrophones of the array simultaneously. An amplitude spectrum

was generated from the signal recorded by each element. Each spectrum

was analyzed to find the intensity at a single frequency of interest. This in-

tensity was assigned to the XY coordinate position from the corresponding

hydrophone as shown in Fig. 2 generating a spatial intensity map. Custom

written software programs used this spatial intensity map to create contour

lines at three levels of intensity. Contours at 99% of the maximum intensity

were chosen to show the spatial location of the highest intensity portion of

the beam and are represented by the inner circle on the five arm array con-

figuration. The diameter of the 99% contour also gave an indication of how

focused that particular click was. The 3 dB contour (71% of the maximum

recorded intensity) is shown by the middle circle and the 6 dB contour (50%

of the maximum recorded intensity) is shown by the outer circle. (B) The

three contour levels are shown on an XY coordinate map illustrating real

data generated by the whale during a target discrimination task for 30 kHz.

Data from six different clicks are represented here and have been overlaid

upon one another to show the amount of variation that occurred between

these six clicks.

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the pen. These events were displayed on a computer monitor

and those events were correlated with visual observations of

the animal’s behavior. Any click trains recorded during the

session that were recorded as the calf faced the array were

discarded from the data set. The other dolphins in the facility

were housed in pens as far away as possible from the array

to prevent their clicks from being recorded. Only clicks that

showed the highest intensity on the central hydrophone of

the array or at a place which covered enough of the beam to

be able to make an analysis and determine whether the click

was on-axis were included in the data set. Clicks had to be

within the same click train. No formal target was used for

the Tursiops to echolocate at so either the array itself, or an

object behind the array, served as the target. The distance of

the Tursiops from the array was visually estimated at the

time of the recording event.

The calf’s presence in the enclosure with the Tursiopscaused her to be extra diligent while interrogating new

objects in the environment, especially at the first moments of

a recording session shortly after the array had been placed in

the water. This allowed many long click trains to be recorded

from her with her orientation directly in front of the array at

a distance of approximately 1–2 m. After the Tursiops’ initial

interest in the array she mostly ignored it and ensonified it

only briefly while passing by. Recording sessions lasted for

30 min at the end of which the array was removed from the

water.

III. RESULTS

A. Results from the Pseudorca click recordings

The results of the contour analysis from two click trains

made by the Pseudorca are shown in Fig. 4. Figures 4(A),

4(B), and 4(C) are contours at 30, 50, and 100 kHz from the

same 21 clicks collected during a single click train. Figures

4(D), 4(E), and 4(F) are contours at 30, 50, and 100 kHz from

19 clicks collected during a different click train. Figure 4(G)

shows the normalized amplitude spectra of 250 clicks col-

lected from the Pseudorca during earlier target discrimination

tasks which are fully described in Ibsen et al. (2011). These

clicks were recorded with a single hydrophone that was

located directly between the Pseudorca and the target. Figure

4(H) shows the same 250 clicks as shown in Fig. 4(G) with-

out normalization, so all their amplitudes are relative.

The contour graphs were created at 30, 50, and 100 kHz

to represent the different regions of the normalized click am-

plitude spectra shown in Fig. 4(G). The consistent region of

these normalized spectra was represented by 30 kHz, the

inconsistent region by 100 kHz, and the intermediary zone in

between by 50 kHz.

The Pseudorca was focusing the 30 kHz frequency quite

narrowly as indicated by the small diameter of the 99% con-

tours in Figs. 4(A) and 4(D). Each click was focused directly

in line with the central hydrophone and consequentially the

target. The beam was radially symmetrical with the 71% con-

tours fitting into consistent regions within the 50% contours.

Only the 99% contours come close to the central hydrophone.

The 50 kHz contours demonstrated a broadening of

focus at all levels as shown in Figs. 4(B) and 4(E) when

compared to the 30 kHz contours. In many of the clicks the

high intensity region was not always aligned with the target

and central hydrophone. The contours also showed less ra-

dial symmetry than at 30 kHz and some of the 71% contours

came close to the central hydrophone.

At 100 kHz the contours became even less radially sym-

metric as shown in Figs. 4(C) and 4(F). The 99% contours

exhibited the same extensive wandering and broadening as

seen at 50 kHz. In several cases the 71% and 50% contours

crossed the central hydrophone.

B. Results from the Tursiops click recordings

The results of the contour analysis from two click

trains made by the dolphin are shown in Fig. 5. Figures

5(A)–5(C) are contours at 30, 50, and 100 kHz from the

same 31 clicks collected during a single click train. Figures

5(D)–5(F) are contours at 30, 50, and 100 kHz from 16

clicks collected during a different click train. Figure 5(G)

shows the normalized amplitude spectra of 250 clicks col-

lected from the Tursiops during earlier target discrimination

tasks which are fully described in Ibsen et al. (2010). These

clicks were recorded with a single hydrophone that was

located directly between the animal and the target. Figure

5(H) shows the same 250 clicks as shown in Fig. 5(G) with-

out normalization, so all their amplitudes are relative.

The contours were created at three different frequencies

to represent the different regions of consistency from the

normalized click amplitude spectra shown in Fig. 5(G), as

described for the Pseudorca clicks.

The Tursiops was focusing the 30 kHz frequencies in

the same way as the Pseudorca but not as tightly. The con-

tour levels nested themselves inside one another to a lesser

degree in Fig. 5(A) than observed for the Pseudorca in Fig.

4(A). Only the 99% contours came close to the central

hydrophone. The 30 kHz contours shown in Fig. 5(D) dem-

onstrated a much tighter focus with more consistent nesting

of the different levels within one another. The contours were

elongated in overall shape, which may be ascribed to the

free swimming nature of the Tursiops experimental setup.

There was greater broadening of the 99% counters at

50 kHz than observed for the Pseudorca as seen in Figs. 5(B)

and 5(E). In Fig. 5(E) the focus moved around to four differ-

ent locations, three of which were not in line with the central

hydrophone. The 50 kHz contours show a loss of radial sym-

metry when compared to the 30 kHz contours. All three levels

of contour lines come close to the central hydrophone.

At 100 kHz, the focus of the beam was in multiple sepa-

rate locations as seen in Figs. 5(C) and 5(F), many of which

were not pointed toward the target. In Fig. 5(F), radial sym-

metry was totally lost with all three levels of contours cross-

ing the central hydrophone.

IV. DISCUSSION

Previous work demonstrated that the Tursiops could dis-

criminate between phantom targets with as little as 1 kHz of

difference in the frequency content (Ibsen et al., 2009) by

using a standardized click for every target (Ibsen et al.,2010). The functional bandwidth that the Tursiops paid

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attention to while echolocating was shown to be between 29

and 42 kHz as determined with phantom echo techniques

(Ibsen et al., 2009). The Tursiops did not pay attention to

frequencies outside this range. The various phantom targets

reflected the standardized frequency content from within this

narrow band differently creating the discrimination cues.

The key to the discrimination strategy was the consistency

of the outgoing click frequency content in this 29–42 kHz

region. The clicks made during these target discriminations

were recorded on-axis directly between the Tursiops and the

target by a single hydrophone. Figure 5(H) shows the ampli-

tude spectra of 250 consecutive clicks recorded by Ibsen

et al. (2009). This single hydrophone was located at the

same position as the central hydrophone of the array. Upon

normalization of all 250 spectra, a high degree of consis-

tency was found in the frequency intensity content within

FIG. 4. (Color online) Analysis of two different click trains generated by the Pseudorca during echolocation target discrimination tasks. Click Train 1, which

consisted of 21 clicks, is represented at three different frequencies (A) 30 kHz, (B) 50 kHz, and (C) 100 kHz. The echolocation beam at 30 kHz was tightly

focused at the central hydrophone of the array and consequentially at the target itself. The beam was radially symmetric with the 71% and 50% contours fitting

within one another. At 50 kHz the echolocation beam was less focused and was more erratically directed, sometimes away from the target. At 100 kHz the

beam became more disorganized. This same trend is demonstrated for Click Train 2 which consisted of 19 clicks and is shown in (D), (E), and (F). (G) This

graph shows the normalized amplitude spectra of 250 clicks made during target discrimination tasks of a previous study (Ibsen et al., 2011). They were col-

lected by a single hydrophone located on-axis between the Pseudorca and the target. It was observed at the time that there was a consistent region of frequency

content from 0–42 kHz that was generated from click to click with simultaneous variability in frequency content above 42 kHz. The region of consistency is

represented by the intensity contour plots at 30 kHz in (A) and (D) and demonstrated a high degree of focus. At 50 kHz there was a decrease in the focus of the

beam and a corresponding increase in frequency content variability. At 100 kHz the high degree of viability in frequency content was represented by a signifi-

cant loss of beam focus and directional control. (H) Here the amplitude spectra of the same 250 clicks shown in (G) are displayed without any normalization

so their individual intensity levels are all relative.

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the 0–42 kHz band, but consistency was lost above 42 kHz

[Fig. 5(G)]. A more detailed description of the normalization

analysis is described in Ibsen et al. (2010).

The three frequencies chosen for the contour plots

were 30, 50, and 100 kHz to represent the consistent, transi-

tional, and inconsistent regions of the click spectra as

shown by the black lines in Fig. 5(G). The beam contours

in Figs. 5(A)–5(F) illustrate that the 30 kHz frequency was

focused and aimed directly at the central hydrophone. The

focus and aim of the 30 kHz frequency was more consistent

than the 50 kHz frequency and obviously more consistent

than the 100 kHz frequency. For some clicks, the central

focus of the 50 and 100 kHz frequencies passed through the

central hydrophone resulting in a high amplitude for those

FIG. 5. (Color online) Analysis of two different click trains generated by the Tursiops during free swimming recording sessions. Click Train 1, which con-

sisted of 31 clicks, is represented at three different frequencies (A) 30 kHz, (B) 50 kHz, and (C) 100 kHz. The echolocation beam at 30 kHz was focused at the

central hydrophone of the array. The beam was radially symmetrical with the 71% and 50% contours fitting within one another. These contours show that the

beam was less focused than that of the Pseudorca, but the Tursiops was in a free swimming situation with no real consistent target positioned behind the array.

At 50 kHz the echolocation beam was less focused and was more erratically directed, sometimes away from the target. At 100 kHz the beam became more dis-

organized. This same trend is demonstrated for Click Train 2 which consisted of 16 clicks and is shown in (D), (E), and (F). (G) This graph shows the normal-

ized amplitude spectra of 250 clicks made during target discrimination tasks of a previous study (Ibsen et al., 2010). They were collected by a single

hydrophone located on-axis between the dolphin and the target. Just like for the Pseudorca, it was observed that there was a consistent region of frequency

content from 0–42 kHz that was generated consistently from click to click with simultaneous variability in frequency content above 42 kHz. The region of con-

sistency represented by the intensity contour plots at 30 kHz in (A) and (D) demonstrated a higher degree of focus than at 50 kHz where there was a corre-

sponding increase in frequency content variability. At 100 kHz the high degree of viability in frequency content was represented by a significant loss of beam

focus and directional control. (H) Here the amplitude spectra of the same 250 clicks shown in (G) are displayed without any normalization so their individual

intensity levels are all relative.

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clicks. For other clicks the high intensity region was

directed away from the central hydrophone causing it to be

ensonified with lower amplitudes. The accumulated varia-

tions in the spatial location of the high intensity region for

the 50 and 100 kHz frequencies resulted in the high degree

of variation above 42 kHz seen in Fig. 5(G).

This same pattern was also observed with the Pseudorca(Fig. 4). This animal’s 30 kHz contours were extremely sym-

metrical and consistent in spatial location from click to click.

The Pseudorca’s alignment and spatial distance to the target

was kept consistent throughout each trial due to the hoop sta-

tioning. The Pseudorca’s data set shows the characteristics of

spatial frequency distributions in a controlled on-axis experi-

mental setup. Even though the Tursiops data used in the anal-

ysis was collected with visual observation of the Tursiopsfacing the array, the experimental setup was free swimming

without the same level of control as for the Pseudorca. The

similarity between the Pseudorca and the Tursiops data shows

that the spatial frequency distributions from the Tursiops were

not an artifact of the Tursiops’ free swimming experimental

setup. It appears that both the Tursiops and the Pseudorcawere using the same echolocation strategy while free swim-

ming and while performing target discrimination tasks.

It is likely that both the Tursiops and the Pseudorca gen-

erated clicks with consistent frequency content and then

used manipulations of the melon and nasal air sacs to steer

the frequencies they were paying attention to toward the tar-

get positioned directly in front of them. The shape of the

melon and nasal air sacs might only be morphologically

manipulated to focus a certain range of acoustic frequencies

in a similar way that optical lenses can only effectively focus

certain wavelengths of light. The shape of the melon is under

muscular control (Harper et al., 2008). Sometimes extreme

shape variations can be observed, especially with beluga

whales. In the case of this Tursiops and Pseudorca, since

they both did not pay attention to frequencies above 45 kHz

they did not get any feedback about the focus of the higher

frequencies. A range of melon and nasal air sac morpholo-

gies may optimize the focusing of frequencies between 0

and 42 kHz while simultaneously affecting the higher fre-

quencies differently, resulting in the observed variation in

focus.

Using the array to analyze clicks of additional species

with different melon morphologies and echolocation fre-

quency ranges, such as the Risso’s dolphin with its bifur-

cated melon (Nachtigall et al., 1990), or the beluga with its

ability to create dramatically different melon shapes will

shed more light on the role of the melon in beam formation

and the spatial distribution of frequencies within that beam.

V. CONCLUSION

Understanding the spatial distribution of frequencies

within the echolocation beam of cetaceans is critical to

understanding click formation as a significant part of a larger

echolocation strategy. The 16 hydrophone star array and

analysis developed for this study offers a unique look at the

spatial distribution of frequencies from a 2D cross-section of

the echolocation beam. This allowed intensity contour maps

to be created for individual frequencies from a series of

clicks collected during the same click train. Both the Tur-siops truncatus and Pseudorca crassidens considered in this

study were shown to use the same echolocation strategy.

Both created clicks that were consistent in frequency content

between 0 and 42 kHz with simultaneous inconsistency

above that range. This corresponded to observed consistency

and precision of spatial focusing of the echolocation beam at

30 kHz with simultaneous loss of focus at 50 and 100 kHz. It

appears that these two dolphins were capable of tightly

focusing and directing the frequencies they were paying

attention to while performing discrimination tasks. The fre-

quencies outside this range were not paid attention to and

were not focused or controlled to nearly the same degree.

The focusing could be caused by muscular manipulations of

the melon and nasal air sacs where an optimum range of

morphologies exist to acoustically focus a certain frequency

range while leaving other ranges unfocused.

ACKNOWLEDGMENTS

The authors are grateful for the support of Dera Look,

Andrew Box, Nick Kent, Vincent De Paolo, Aude Pacini,

and the Marine Mammal Research Program of the Hawaii

Institute of Marine Biology at the University of Hawaii. The

authors also want to thank John Hildebrand for support in

fabricating the array, Jules Jaffe for supporting the design

and construction of the 16 channel amplifier, and Robert

Glatts for his advice in the electronics design. This research

project was funded by the Office of Naval Research (Grant

No. 0014-08-1-1160 to P.E.N.) for which the authors thank

Bob Gisiner, Jim Eckman, and Neil Abercrombie. Work was

completed under NOAA NMFS Marine Mammal Permit No.

978-1857-00 issued to P.E.N.

Au, W. W. L. (1993). The Sonar of Dolphins (Springer-Verlag, New York),

pp. 104–108, 177–215.

Au, W. W. L., Houser, D. S., Finneran, J. J., Lee, W.-J., Talmadge, L. A.,

and Moore, P. W. (2010). “The acoustic field on the forehead of echolo-

cating Atlantic bottlenose dolphins (Tursiops truncatus),” J. Acoust.

Soc. Am. 128, 1426–1434.

Au, W. W. L., Kastelein, R. A., Benoit-Bird, K. J., Cranford, T. W., and

McKenna, M. F. (2006). “Acoustic radiation from the head of echolocat-

ing harbor porpoises (Phocoena phocoena),” J. Exp. Biol. 209,

2726–2733.

Au, W. W. L., Kastelein, R. A., Rippe, T., and Schooneman, N. M. (1999).

“Transmission beam pattern and echolocation signals of a harbor por-

poise (Phocoena phocoena),” J. Acoust. Soc. Am. 106, 3699–3705.

Au, W. W. L., and Moore, P. W. B. (1986). “Echolocation transmitting

beam of the Atlantic bottlenose dolphin,” J. Acoust. Soc. Am. 80,

688–691.

Au, W. W. L., and Pawloski, D. (1992). “Cylinder wall thickness difference

discrimination by an echolocating Atlantic bottlenose dolphin,” J.

Comp. Physiol., A 172, 41–47.

Au, W. W., Pawloski, J. L., Nachtigall, P. E., Blonz, M., and Gisner, R. C.

(1995). “Echolocation signals and transmission beam pattern of a false

killer whale (Pseudorca crassidens),” J. Acoust. Soc. Am. 98, 51–59.

Aubauer, R., Au, W. W. L., Nachtigall, P. E., Pawloski, D. A., and DeLong,

C. M. (2000). “Classification of electronically generated phantom targets

by an Atlantic bottlenose dolphin (Tursiops truncatus),” J. Acoust. Soc.

Am. 107, 2750–2754.

Benninger, M. S., Alessi, D., Archer, S., Bastian, R., Ford, C., Koufman, J.,

Sataloff, R. T., Spiegel, J. R., and Woo, P. (1996). “Vocal fold scarring:

Current concepts and management,” Otolaryngol.-Head Neck Surg. 115,

474–482.

1220 J. Acoust. Soc. Am., Vol. 132, No. 2, August 2012 Ibsen et al.: Spatial structure of echolocation frequency

Downloaded 26 Sep 2012 to 137.132.3.10. Redistribution subject to ASA license or copyright; see http://asadl.org/terms

Cranford, T. (2000). “In search of impulse sound sources in odontocetes,” in

Hearing by Whales and Dolphins, edited by W. W. L. Au, A. N. Popper,

and R. J. Fay (Springer-Verlag, New York), pp. 132–135.

Cranford, T. W., and Amundin, M. (2003). “Biosonar pulse production in

odontocetes: The state of our knowledge,” in Echolocation in Bats andDolphins, edited by J. A. Thomas, C. F. Moss, and M. Vater (The Uni-

versity of Chicago Press, Chicago), pp. 27–35.

Harper, C. J., McLellan, W. A., Rommel, S. A., Gay, D. M., Dillaman,

R. M., and Pabst, D. A. (2008). “Morphology of the melon and its tendi-

nous connections to the facial muscles in bottlenose dolphins (Tursiopstruncatus),” J. Morphol. 269, 820–839.

Ibsen, S., Au, W., Nachtigall, P., and Breese, M. (2009). “Functional band-

width of an echolocating Atlantic bottlenose dolphin (Tursiopstruncatus),” J. Acoust. Soc. Am. 125, 1214–1221.

Ibsen, S., Au, W., Nachtigall, P., Delong, C., and Breese, M. (2007).

“Changes in signal parameters over time for an echolocating Atlantic

bottlenose dolphin performing the same target discrimination task,” J.

Acoust. Soc. Am. 122, 2446–2450.

Ibsen, S. D., Au, W. W. L., Nachtigall, P. E., DeLong, C. M., and Breese,

M. (2010). “Changes in consistency patterns of click frequency content

over time of an echolocating Atlantic bottlenose dolphin,” J. Acoust.

Soc. Am. 127, 3821–3829.

Ibsen, S. D., Krause-Nehring, J., Nachtigall, P. E., Au, W. W. L., and

Breese, M. (2011). “Similarities in echolocation strategy and click char-

acteristics between a Pseudorca crassidens and a Tursiops truncatus,” J.

Acoust. Soc. Am. 130, 3085–3089.

Madsen, P. T., Wisniewska, D., and Beedholm, K. (2010). “Single source

sound production and dynamic beam formation in echolocating harbour

porpoises (Phocoena phocoena),” J. Exp. Biol. 213, 3105–3110.

Moore, P. W., Dankiewicz, L. A., and Houser, D. S. (2008). “Beamwidth

control and angular target detection in an echolocating bottlenose dol-

phin (Tursiops truncatus),” J. Acoust. Soc. Am. 124, 3324–3332.

Nachtigall, P. E. (1980). “Odontocete echolocation performance on object

size, shape, and material,” in Animal Sonar Systems, edited by R. G. B.

J. F. Fish (Plenum Press, New York), pp. 71–95.

Nachtigall, P. E., Pawloski, J. L., Schroeder, J. P., and Sinclair, S. (1990).

“Successful maintenance and research with a formerly stranded Risso’s

dolphin (Grampus griseus),” Aquat. Mamm. 16, 8–13.

Rust, A. C., Mandre, S., and Balmforth, N. J. (2007). “The feasibility of gen-

erating low-frequency volcano seismicity by flow through a deformable

channel,” Geological Society of London Special Publications 307, 45–56.

Yuen, M., Nachtigall, P., Breese, M., and Supin, A. (2005). “Behavioral and

auditory evoked potential audiograms of a false killer whale (Pseudorcacrassidens),” J. Acoust. Soc. Am. 118(4), 2688–2695.

J. Acoust. Soc. Am., Vol. 132, No. 2, August 2012 Ibsen et al.: Spatial structure of echolocation frequency 1221

Downloaded 26 Sep 2012 to 137.132.3.10. Redistribution subject to ASA license or copyright; see http://asadl.org/terms