939
The Journal of Experimental Biology 198, 939–951 (1995)
Printed in Great Britain © The Company of Biologists Limited 1995
ATMOSPHERIC CONTROLS ON ELEPHANT COMMUNICATION
MICHAEL GARSTANG1, DAVID LAROM1, RICHARD RASPET2 AND MALAN LINDEQUE3
1Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903, USA, 2National Center
for Physical Acoustics, University of Mississippi, University, MS 38677, USA and 3Etosha Ecological Institute,
Etosha National Park, PO Box Okaukuejo via Outjo, Namibia
Accepted 14 November 1994
Summary
Atmospheric conditions conducive to long-range
transmission of low-frequency sound as used by elephants
are found to exist in the Etosha National Park in Namibia
during the late dry season. Meteorological measurements
show that strong temperature inversions form at the
surface before sunset and decay with sunrise, often
accompanied by calm wind conditions during the early
evening. These observations are used in an acoustic model
to determine the sensitivity of infrasound to the effects of
(a) the strength, thickness and elevation of temperature
inversions, and (b) the growth and decay of an inversion
typical of dry, elevated African savannas. The results
suggest that the range over which elephants communicate
more than doubles at night. Optimum conditions occur
1–2 h after sunset on clear, relatively cold, calm nights. At
these times, ranges of over 10 km are likely, with the
greatest amplification occurring at the lowest frequency
tested. This strong diurnal cycle in communication range
may be reflected in longer-lasting changes in weather and
may exert a significant influence on elephant behaviour on
time scales from days to many years.
Key words: elephant, communication, infrasound, Loxodonta
africana, low-frequency sound.
Introduction
The systematic study of the use of sound by animals
accelerated greatly as a result of advances made in underwater
acoustics during the Second World War. Watkins and Wartzok
(1985) review marine mammal sensory and communications
systems and provide a list of some 67 species of marine
animals whose sounds have been recorded. Spiesberger and
Fristrup (1990), in a paper dealing with bird calls, review the
transmission of sound in the atmosphere and the ocean and
describe processes in each fluid that influence the transmission
of sound. They draw upon the analogy between light and sound
and treat high-frequency sound transmission in the atmosphere
in terms of rays.
Payne et al. (1986) were the first to document the use of lowfrequency sound or infrasound by elephants. Initial studies of
elephants begun in zoos (Payne et al. 1986; Langbauer et al.
1989) were extended to the wild (Poole et al. 1988; Langbauer
et al. 1991). These studies show that elephants use infrasound
with fundamental frequencies from 14 to 35 Hz for longdistance communication at ranges up to 4 km. The apparent
selection of low- over high-frequency sound by these animals
may have been because the transmission of sound of higher
frequency is limited in range by atmospheric attenuation.
Evolutionary selection for long-range low-frequency
communication would be further enhanced by the existence of
a low-frequency sound channel. The sound channel would
have to be relatively stable and contain relatively still air (low
wind speeds), otherwise wind noise would prevent
communication.
In this paper, evidence is presented that a high savanna
environment that is typical for Loxodonta africana has
atmospheric conditions conducive to low-frequency sound
transmission. The physical and mathematical conditions that
must be met in order to describe low-frequency sound
transmission are shown to be compatible with atmospheric
conditions typical of the African savannas. Temperature
profiles measured above such a savanna show that strong
nocturnal inversions replace daytime lapse conditions.
Nighttime cooling stratifies the air near the ground, often
resulting in calm conditions and reduced wind noise at the
surface. An acoustic model predicts that, under these
conditions, low-frequency sound propagation is enhanced and
ranges of communication are maximized soon after sunset. The
findings contribute to the interpretation of elephant behaviour
and provide clear-cut hypotheses which can be tested in the
field.
Low-frequency sound transmission in the atmosphere
Theoretical considerations
Sound propagation in a stratified atmosphere is
approximated by the Helmholtz form of the acoustic wave
equation:
940
M. GARSTANG AND OTHERS
=2p + k2p = 24pd(x,y,z) ,
(1)
and by the effective speed of sound, ceff:
ceff = c + w ,
(2)
where c = gRT = the speed of sound = v/k, v is angular
frequency, k = 2pf/ceff = wave number, f is the frequency, w
is the horizontal component of wind in the direction of
propagation, g = Cp/Cv = 1.4, which is the ratio of specific
heats at constant pressure, P, and constant volume, V, R is the
gas constant for air (287 J kg21 K21) , T is atmospheric
temperature (K), p is the pressure amplitude of the sound wave,
= is a del operator, see equation 3 below, d is a unit impulse
function, and x, y and z are spatial variables in a Cartesian
coordinate framework.
The wave equation provides a description of the
transmission of sound in a fluid medium such as the
atmosphere or the ocean (Rayleigh, 1878; Pierce, 1981). A
complete solution of the wave equation in the atmosphere is
both mathematically and computationally impractical because
the speed of sound and the fluid velocity vary unpredictably in
space and time.
Solutions for sound of audible or higher frequencies may be
obtained by the ray approximation, in which the acoustic-wave
equation is simplified to treat sound in terms of rays in the form
of the Eikonal equation (Pierce, 1981). In this form, sound
travels along ray paths that are determined by solving Snell’s
Law governing refraction through the medium. The intensity of
the sound at the receiver is then the sum of the intensities of all
the rays reaching the receiver by all possible paths (Fig. 1). The
ray equation is also referred to as the high-frequency solution
because it is valid when the acoustic wavelength is small,
compared with the length scale over which significant change
occurs in the atmospheric refractivity.
The ray approximation does not hold at low frequencies in
the atmosphere. At low frequencies, a large portion of the
wavefront, rather than the few locations assumed in the ray
approximation, contributes to the received signal. Analogy to slit
diffraction illustrates these concepts (Fig. 2). As parallel waves
approach a wall at normal incidence, their behaviour on the other
side of a slit in the wall depends on their frequency. Highfrequency waves (Fig. 2A) look like rays, but low-frequency
waves (Fig. 2B) are strongly diffracted and spread much more
widely. Only the ray can be said to have directionality.
For low-frequency (15–30 Hz) atmospheric sound
transmission, which is the focus of this paper, a more complete
solution of the wave equation must be used. The atmosphere is
assumed to consist of n homogeneous layers (Fig. 3) with layer
n next to the ground and layer 1 the highest layer that has any
significant effect on the sound levels at the height of interest.
For each layer, the Helmholtz equation (equation 1) operates
for a harmonic point source of angular frequency, v, and wave
number, k, and is solved in cylindrical coordinates where:
=2 =
­2
1 ­  ­  1 ­2
+
r  +
,
r ­r  ­r r2 ­f2 ­z2
(3)
where r is the horizontal distance from the source and f is the
azimuth angle. If cross-wind effects are negligible, azimuthal
symmetry may be assumed and equation 3 reduces to:
­2p
­r2
+
1 ­p
r ­r
+
­2p
­z2
+ k2p = 0 .
(4)
This is the two-dimensional cylindrical form of the Helmholtz
equation, which can now be reduced to a one-dimensional
equation by a Hankel transform:
⌠∞
p^ =  p(r,z)J0(Kr)rdr ,
⌡0
(5)
where p̂ is the pressure transform, r is the horizontal radial
distance from the source, z is altitude, J0 is the 0th-order Bessel
function of the first kind, and K is the transform variable
corresponding to r.
A
B
A
B
Source
Receiver
C
Fig. 1. Ray approximation in which rays (A) are bent through a
stratified atmosphere, (B) received directly from the source and (C)
reflected off the ground.
Fig. 2. (A) High-frequency transmission through a slit approximated
by a ray on the other side of the wall. (B) Low-frequency transmission
through a slit spreading by diffraction on the other side of the wall.
z, altitude.
Atmospheric controls on elephant communication
This yields a form of Bessel’s equation in the transformed
domain:
d2p^
——
+ [k2(z) + K2] = 22d(z 2 zS) ,
(6)
dz2
where zS is the source height.
Discrete samples of the wave number space can now be
obtained, and equation 5 can be approximated by a Fast Fourier
Transform. The pressure variable p(r,z) at a range of points is
yielded by a single transform. This method, because of its
computational speed, is called the Fast Field Program or FFP
(Lee et al. 1986). Equation 6 is then solved with an impedance
boundary condition at the ground and a radiation boundary
condition at the top of the atmosphere. Equation 6 is a onedimensional Helmholtz equation in a layered medium and can
be solved by a variety of techniques. In the particular FFP used
for calculation in this paper, the equation is solved using a
transmission line analogy for wave propagation in layered
media giving p(K,z). The inverse Hankel transform:
⌠∞
^
p(r,z) =  p(K,z)J
0(Kr)KdK ,
⌡0
(7)
is then computed, yielding p(r,z), the acoustic pressure field.
In the sections that follow, pressure fields will be discussed
in terms of the sound pressure level (SPL). The SPL, given in
decibels (dB), is defined by:
SPL = 10log(P2/Pr2) ,
where
P2
(8)
2
and Pr are, respectively, the mean squared pressure
Top layer
z1
Layer 1
z2
z3
Detector
zD
zD+1
Source
zS
zS+1
zn
Layer n
Ground surface
Fig. 3. Layered atmosphere bounded by the ground and a top layer,
in which the top layer is the highest layer of interest at the ground.
941
at the point in question and at a reference distance (Pierce,
1981; p. 61). In this paper, the SPL is taken relative to the
sound pressure at 1 m from the source. The FFP computations
in this study were performed on an RS6000 computer at the
University of Virginia, USA.
Physical factors
Practical application of the above theory still requires that a
number of assumptions be made as well as consideration of
other physical effects, not dealt with above, which may
influence low-frequency sound propagation in any given
situation.
Simplifications include the assumption that the emitting
source is stationary; that the sound emitted is a simple sine
wave of angular frequency, v, radiating equally in all
directions; that the acoustic response is linear, i.e. twice the
source pressure will result in twice the pressure at the receiver;
and that there is no seismic contribution, i.e. there is negligible
transmission in the earth’s surface.
Physical effects that may influence low-frequency sound
propagation in the atmosphere include ground attenuation, the
effects of topography, scattering by vegetation, atmospheric
absorption, attenuation by turbulence and wind shear, and the
effects of temperature gradients.
Over a perfectly hard, flat surface, sound levels will be
doubled as a result of reflection. Real surfaces have a complex
impedance, which Attenborough (1985) has modelled as a
function of flow resistivity, porosity and pore and grain shape
factors. The Attenborough four-parameter model is
incorporated into the FFP. Vegetation can reduce impedance
by loosening the soil. Reduced impedance causes a loss of
strength and a phase change in the reflected signal. The phase
change causes a shadow effect, which reduces sound levels at
long distances. Impedance is frequency-dependent: surfaces
are more reflective of lower frequencies, and low-frequency
sound produces ground and surface waves which penetrate the
shadow zone. Only very porous or nonresistive soils, soft sand
and thick forest humus (Price et al. 1988) have a sufficiently
low impedance to attenuate sound of a frequency of 30 Hz or
less by more than 6 dB over 10 km. At 15 Hz, all but the softest
surfaces are almost perfect acoustic reflectors; for example,
even snow has little effect below 100 Hz (Nicolas et al. 1985).
Topography with slopes of less than 1 ˚ can noticeably
reduce shadow zones (Piercy et al. 1977). Canard-Carauna et
al. (1990) provide corroboration that a mild upward slope can
increase enhancement. Robertson et al. (1989) performed a
numerical study of the effects of a triangular ridge 100 m high
on low-frequency sound propagation. Peak topographic
enhancement occurs at or in front of the ridgetop on the slope
facing the source, and peak attenuation occurs at the base of
the ridge on the far side. The shadow zone is extended when
the ridge is closer to the source. At 10 Hz, this can cause a 5 dB
enhancement at the ridgetop and a 5 dB attenuation at the base
of the ridge on the far side. The enhancement and attenuation
increase to 10 dB at 20 Hz. If the ridge is downwind from the
source, a strong acoustic shadow can develop behind the ridge.
942
M. GARSTANG AND OTHERS
For upwind propagation, sound levels behind a ridge are
enhanced relative to those measured over flat ground. Similar
effects to those noted above have been modelled for higherfrequency sound using ray-tracing (Lamancusa and Doroux,
1993).
Vegetation, depending on geometry, can increase or
decrease sound levels. Canard-Carauna et al. (1990) found that
the narrowing of a forest gap in the direction of propagation
enhanced 63 Hz sound levels by 3 dB. Scattering from
vegetation, particularly large tree trunks, can significantly
attenuate sound. However, theory suggests that scattering is
only significant when the size of the scatterer is of the same
magnitude or larger than the wavelength of the sound. The
largest trees have diameters of perhaps 3 m, while 30 Hz sound
has a wavelength of more than 10 m and 15 Hz sound has a
wavelength of more than 20 m in the atmosphere. Thus,
infrasonic communication should not be strongly affected by
scattering from vegetation. In an experimental study of sound
propagation in a forest, Price et al. (1988) found that from a
peak at about 250 Hz attenuation fell sharply as the frequency
decreased to 100 Hz, the lowest frequency tested.
Atmospheric absorption of sound is significant at audible
frequencies, exceeding 40 dB per 100 m at the upper range of
human hearing. In the infrasonic range, absorption is
essentially nonexistent under normal conditions, never
exceeding 1 dB per 10 km for frequencies below 30 Hz and
relative humidity above 20 % (Bass et al. 1990; Zuckerwar and
Meredith, 1984). Under extremely dry conditions (relative
humidity well under 5 %), absorption may have some
importance, since an attenuation of up to 1 dB per 1000 m is
possible for completely dry air.
Wind is highly detrimental to low-frequency sound
communication. Wind noise from intrinsic turbulence is
proportional to rUu9, where r is the air density, U is the wind
speed (typically the average speed over a 5–15 min period) and
u9 is the average instantaneous fluctuation speed. Wind noise
grows rapidly with wind speed and fluctuation speed,
increasing, for example, by approximately 20 dB as wind speed
increases from 3 to 10 m s21. Hot-wire anemometer studies
(Morgan and Raspet, 1992) show that wind noise at 20 Hz is
approximately 10 dB greater than that at 200 Hz. This is
because the majority of the turbulent kinetic energy in the
atmosphere lies below 40 Hz. These turbulent pressure
fluctuations, although not sound by its strict definition as a
propagating pressure wave, are nonetheless indistinguishable
from sound when measured by instruments or by the ear and
form an important source of random noise. Induced turbulence,
which is highly dependent on receiver geometry, also adds to
wind noise and makes the effects of wind noise difficult to
quantify (Schomer et al. 1990). However, it is generally true
that communication range will be greatly degraded under
windy conditions.
The effect of turbulent scattering on low-frequency sound
transmission is small (Daigle, 1979). This is because the
scattering strength of atmospheric turbulence for infrasound is
proportional to the product of the square of the average index
of refraction (m2) and to the square of the the wave number
(k2), both of which are small for the atmospheric propagation
of infrasound.
Changes in atmospheric temperature with height above the
surface have a marked effect on sound transmission. In general,
lapse conditions, in which the atmospheric temperature
decreases with height, are upward-refracting and tend to
decrease sound levels near the surface. Shadow zones may be
formed, beyond which, according to ray theory, sound
propagation is forbidden. Inversions, in which temperature
increases with height, are downward-refracting and will
increase sound levels. Canard-Carauna et al. (1990) observed
an enhanced acoustic signal around sunset and sunrise
compared with the middle of the day. These correspond to the
times of formation and decay of temperature inversions.
Similarly, wind shear tends to enhance propagation downwind
and to attenuate it upwind. This makes propagation directional
and effectively degrades two-way communication. A
downward-refracting atmosphere increases acoustic energy
levels near the surface, but at the same time causes multiple
ground reflections. Since acoustic energy is lost with each
bounce, ground attenuation can be greater with distance under
extreme temperature gradients or wind shear than under
moderate gradients or shear (Raspet et al. 1992).
The impact of each of the above factors upon the
transmission of low-frequency sound as estimated by the FFP
depends upon the surface and atmospheric conditions
encountered in the field. These are addressed in the section
below.
Surface and atmospheric conditions in the field
Okaukuejo (latitude 19 ˚S, longitude 16 ˚E), in the southcentral part of the Etosha National Park of Namibia, was
chosen as the field site. Field measurements were taken from
1 September to 15 October 1992 at the end of the dry season.
The location, at 1100 m, is typical of the elevated dry African
savannas. The topography is largely flat. The savanna
vegetation in Etosha often thins to open grassland and the
surface is stony, with extensive areas of calcrete.
Continuous measurements of temperature, humidity,
pressure and wind velocity were made at Okaukuejo at two
levels (5 and 10 m) above ground. Global radiation (solar direct
+ solar diffuse) and rainfall were also measured. Specialized
low-level soundings measuring temperature, humidity,
pressure and wind speed and direction were made from the
surface to about 1.5 km using a tethered balloon system. The
tethered balloon measurements of air temperature shown in
Fig. 4 provide high vertical (<10 m) and time (every hour)
resolution of the temperature structure in the lowest 1300 m of
the atmosphere. Soundings by the tethered balloon were
concentrated around sunset and sunrise, which are times of
rapid change in the thermal structure of the atmosphere near
the surface. Measurements during the rest of the day and night,
when little change was taking place, were made less frequently.
Routine upper air soundings, which penetrate to much greater
Atmospheric controls on elephant communication
1400
diurnal cycle in wind speed and direction (Fig. 5). Daytime
wind speeds may be either strong or weak depending on
synoptic weather conditions. However, because of the
decoupling of the surface layer mentioned above, nocturnal
speeds are usually weak, dropping to calm or low speeds after
sunset, as shown in Fig. 5. Roughly one-third of all days in the
field had an early evening wind minimum similar to that shown
in Fig. 5. Topographically induced slope winds are observed
at Etosha after 20:00 h LST (Fig. 5) (Preston-Whyte et al.
1994). The topographic winds weaken the nocturnal inversion
through mixing. Thus, although the low-level inversion may
persist through the night, it frequently reaches its maximum in
the early evening. In contrast to the nighttime conditions,
daytime surface heating under clear skies is intense. Superadiabatic conditions, in which temperature drops rapidly with
height above the hot surface, can prevail from mid-morning to
mid-afternoon. Days with significant surface winds (greater
than 7 m s21) result in moderate turbulent mixing and adiabatic
lapse rates.
The surface and atmospheric conditions as encountered in
the field, together with the known effects on low-frequency
sounds, eliminate a number of effects from consideration while
identifying others as crucial to the propagation of lowfrequency sound. Effects which are essentially eliminated from
consideration are scattering by vegetation and turbulence,
upslope enhancement, downslope degradation and wind shear.
In each case, the above effects are less pronounced for low than
for high frequencies, but all are at a minimum or negligible in
the Etosha location. This is due to the intrinsic nature of the
N
A
Wind direction
heights in the atmosphere than the tethered balloon
measurements, were made twice a day at 12:00 h and 24:00 h
Universal Time (UT). These soundings are not capable of
providing either the vertical space resolution or the necessary
time resolution to capture the thermal structure delineated by
the tethered balloon measurements. Measurements of soil and
air temperatures from 10 cm below the surface to 2 m above
the surface were also made.
The period of measurement, at the end of the dry season
prior to the onset of the summer rains, was characterized by
dry, cloudless conditions. The larger-scale weather pattern was
typically dominated by the south Atlantic subtropical
anticyclone, with high-pressure ridging extending over the
subcontinent (Garstang et al. 1994). It rained on two of the 45
days in the field, giving a total of 4 mm of recorded
precipitation. Twenty-three of the field experiment days were
essentially cloud-free. Cloud cover on 5 days reduced the
measured solar insolation to less than 50 % of the integrated
clear day value.
Under these conditions, outgoing long-wave radiation from
the surface exceeds incoming solar radiation as early as 16:00 h
Local Solar Time (LST). Local Solar Time (UT+1 h) is used
because of the importance of the radiative balance (solar minus
terrestrial radiation) on surface temperatures. Sunrise and
sunset are at approximately 06:00 h and 18:00 h LST. From this
point until after sunrise the next day, the surface cools in
response to long-wave radiational losses. The most rapid
cooling takes place during the late afternoon and soon after
sunset, creating a strong low-level inversion, as illustrated in
Fig. 4. With the formation of this nocturnal inversion, stability
increases and the surface layers of the atmosphere are
thermally decoupled from the deeper atmosphere. The
inversion may weaken later during the night in response to
mixing (wind), but will remain in place until after sunrise.
The above conditions are typically accompanied by a strong
E
S
W
N
1200
00:00
04:00
08:00
12:00
16:00
Time (h LST)
20:00
04:00
08:00
12:00
16:00
Time (h LST)
20:00
1000
800
Wind speed (m s−1)
Height (m)
943
600
400
200
0
22
24
26
28
30
Temperature (°C)
32
34
Fig. 4. Tethered balloon soundings of temperature at Okaukuejo at
17:00 h LST (solid line), 18:00 h sunset (long-dashed line), 19:00 h
(short-dashed line) and 20:00 h (dotted line) on a clear, calm evening
(18 September 1992).
B
20
10
0
00:00
Fig. 5. Wind direction (A) and wind speed (B) at 10 m above the
ground at Okaukuejo over 24 h (18 September 1992). Time is in Local
Solar Time (LST) with sunrise near 06:00 h and sunset near 18:00 h.
944
M. GARSTANG AND OTHERS
site: hard, flat ground, with sparse vegetation and small slopes.
It may also be due to the physical nature of the system, where
wind speeds at certain times may drop to low or calm values.
Effects encountered in the field that remain crucial to
frequencies used in elephant communication (14–35 Hz) are
limited to the role of vertical gradients of temperature, wind
and wind noise. Wind noise grows rapidly both with average
wind speed and with the speed of turbulent fluctuations. Both
these wind noise effects, however, are at a minimum at certain
times of day. Thus, low-level vertical gradients of air
temperature emerge as the single most important physical
factor controlling the transmission of low-frequency sound
under the conditions described in this paper.
Inversion sensitivity studies
In this section, the FFP model is used to determine the
sensitivity of low-frequency sound propagation to the vertical
gradients of air temperature immediately above the surface.
Three characteristics of temperature inversions are examined:
inversion strength, inversion thickness and the height of the
inversion above the ground. These three characteristics are
then combined in a simulation of inversion growth and decay.
The vertical lapse of temperature is considered between the
surface and 200 m above the surface. Atmospheric effects on
the propagation of low-frequency sound generated at the
surface are not considered above a height of 200 m. The
propagation of two sound frequencies, 15 and 30 Hz, is
considered in this 200 m thick surface layer. The two
frequencies chosen represent the highest and lowest
frequencies used by elephants for long-range communication
(Payne et al. 1986; Langbauer et al. 1989). The source of the
sound at these frequencies is considered to be a point source
at 2.5 m and the receptor to be at 3 m above the surface. The
sensitivity of the calculations to source and receptor heights
was tested and found to be insignificant for heights between 1
and 10 m. The heights chosen in the range 1–10 m are thus not
critical, but are close to the heights of the possible sound
generators and receptors of an adult elephant.
Fig. 6A provides a reference adiabatic lapse rate. Fig. 6B
shows a typical super-adiabatic lapse rate near the surface
changing to an adiabatic lapse rate above 50 m, which is usual
on a hot, clear day in Etosha National Park. The composite
daytime sounding of temperature (Fig. 6B) consists of three
sections. In the first 10 m above the ground, the temperature
decreases logarithmically from 40 to 30 ˚C. Between 10 m and
50 m, the temperature decreases linearly by 1 ˚C over 40 m.
Above 50 m, the temperature decreases at the dry adiabatic rate
of 0.98 ˚C per 100 m.
Fig. 6C–E isolates three factors that may influence lowfrequency sound transmission: (1) the strength of the inversion;
(2) the thickness of the inversion; and (3) the elevation of the
inversion. Fig. 6F,G combines these factors to show the growth
and decay of an inversion considered typical of African
savannas. In each instance, we have constrained the limits of
change to be within or close to values observed in the field and
detailed by Garstang et al. (1994). For example, the most
extreme inversion modelled (Fig. 6C) is a lapse of 10 ˚C per
50 m. The most extreme ground-level inversion observed at
Etosha National Park during the experiment was 9 ˚C over
50 m.
The calculations for each of the above three inversion cases
must assume surface (ground) characteristics. Hard ground
(flow resistivity = 5003103 kg m22 s21), including the real and
imaginary parts of the impedance, has been taken as
representative of surface conditions in the Etosha National
Park. Such an assumption produces an enhancement of lowfrequency sound transmission under non-refracting conditions
of about 6 dB near the source, decreasing to 3 dB at 10 km from
the source. This surface effect is included in all the cases that
follow. Temporal changes, however, are the subject of this
paper, and the ground impedance will not change significantly
in the dry season from day to day or over the whole season.
In a homogeneous atmosphere and far from the ground,
sound travels in a spherical wave and the sound pressure level
(SPL) drops by 6 dB for each doubling of distance from the
source (see, for example, Pierce, 1981, p. 43). The acoustic
enhancement, defined by removing the spherical wave
component from the calculated SPL, is the increase in SPL due
to ground effects and atmospheric inhomogeneities. The
acoustic enhancement is the negative of the more commonly
used excess attenuation, when the latter is used correctly. This
has been a source of confusion throughout the literature. The
graphs that follow depict the acoustic enhancement, rather than
the SPL, in order to emphasize the effects of inhomogeneities
in temperature with height.
Inversion strength
Fig. 7A, for a frequency of 15 Hz, and Fig. 7B, for a
frequency of 30 Hz, show the effects of inversion strength on
sound propagation. The temperature fields used in the FFP
model are those of Fig. 6C, with an increase in temperature
between the surface (0 m) and 50 m above the surface, and
adiabatic conditions prevailing above 50 m. Effects under
adiabatic and super-adiabatic lapse conditions (Fig. 6A,B) are
shown for comparison, as are results for isothermal conditions
between the surface and 50 m (Fig. 6C).
Enhancement increases with inversion strength for both
frequencies. For 15 Hz (Fig. 7A), a 5 ˚C inversion gives an
enhancement of 12–15 dB for all ranges exceeding 0.8 km.
This may be thought of as a duct in which sound pressures in
all directions are at least four times greater than those of a
reference adiabatic atmosphere at all distances from 1.6 km up
to 10 km.
The ducting effect is not as strong for 30 Hz (Fig. 7B); the
enhancement diminishes more with distance but, compared
with propagation in the adiabatic atmosphere, the pressure is
tripled beyond 2.5 km. The inversion profiles for both
frequencies contrast even more strongly with the superadiabatic case, in which the enhancement lessens greatly with
distance, becoming negative beyond 1–2 km.
The oscillations in the 10 ˚C case are due to mode
Atmospheric controls on elephant communication
200
200
A
z (m)
180
160
160
140
140
120
120
100
100
80
80
60
60
40
40
20
20
0
0
24 25 26 27 28 29 30 31 32
C
180
140
160
120
z (m)
100
100
Homogeneous
80
60
Strength (°C)
Thickness
= 50 m
10 5
40
20
60
3 2 1 0
50 m
40 m
30 m
20 m
10 m
20
0
21 22 23 24 25 26 27 28 29 30 31
24 25 26 27 28 29 30 31 32
200
E
180
160
Elevation (m)
150
180
F
160
140
140
120
z (m)
100 m
80
40
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22 23 24 25 26 27 28 29 30 31
60
60
40
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Temperature (°C)
G
160
140
120
100
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40
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0
200 m
150 m
140
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180
Adiabatic
above
50 m
160
z (m)
22 24 26 28 30 32 34 36 38 40
200
200
0
B
180
1 2 3 4 5
22 23 24 25 26 27 28 29 30 31
Temperature (°C)
Fig. 6. Reference and case study temperature lapse rates based upon
Okaukuejo soundings between the surface (0 m) and 200 m. (A) Reference
(theoretical) sounding: dry adiabatic lapse rate. (B) Reference sounding: superadiabatic lapse rate immediately (0–50 m) above the surface typical of hot
surface daytime conditions. (C) Case study sounding: strength of the inversion
ranging through six strength levels between the surface and 50 m above the
surface, 0, 1, 2, 3, 5 and 10 ˚C. (D) Case study sounding: a 5 ˚C inversion, 10,
20, 30, 40, 50, 100, 150 and 200 m thick. (E) Case study sounding: elevation
of the inversion where a 5 ˚C inversion 10 m thick is elevated from 0 to 150 m
above the surface. (F) Case study sounding: growth of the inversion from 1 ˚C
strength and 10 m height to 5 ˚C strength and 50 m height. (G) Case study
sounding: decay of the inversion where a 5 ˚C, 50 m inversion decays in stages
1–5 from 50 m thick to an elevated 10 m thick inversion.
945
946
M. GARSTANG AND OTHERS
18
20
10
15
A
5
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Enhancement (dB)
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0
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Range (m)
Fig. 7. Effects of inversion strength (in ˚C) on sound transmissions at
frequencies of (A) 15 Hz and (B) 30 Hz for conditions depicted in
Fig. 6C.
Fig. 8. Effects of inversion thickness (in m) on sound transmissions
at frequencies of (A) 15 Hz and (B) 30 Hz for conditions depicted in
Fig. 6D.
interference, an effect beyond the scope of this paper (for
details, see Raspet et al. 1992). Sound levels received at long
ranges depend on the energy trapped in the surface duct and
on ground attenuation. In Fig. 7B, the 10 ˚C gradient gives less
enhancement than the weaker inversions. Although it traps
more energy, the extreme gradient results in multiple ground
reflections and enhanced losses with distance. This is more
pronounced at 30 Hz, because the ground impedance effects
are greater for this frequency than for 15 Hz. This same effect
would be observed in Fig. 7A if the predictions were extended
to even longer ranges.
The salient point is that even a shallow inversion (20–30 m)
may strongly enhance sound propagation. Since inversions
tend to grow rapidly in thickness as sunset approaches and
radiative cooling sets in, a sharp increase in low-frequency
sound propagation can be expected before and immediately
after sunset. This suggests that the enhancement of lowfrequency sound propagation approaches a maximum within
1–2 h after sunset.
Mode interference effects (oscillations) are quite
pronounced for inversion thicknesses greater than 100 m.
Oscillations are a function of stability, so signal fluctuations
are likely as inversion thickness increases. A steady signal is
therefore more likely during the early evening, when the
inversion is still shallow.
Inversion thickness
Fig. 8A,B shows the effects of inversion thickness on
acoustic enhancement for inversions of strength 5 ˚C and 0 m
elevation (Fig. 6D). For 15 Hz, the increase in enhancement is
large for thicknesses between 10 and 30 m. Additional
thickness, greater than 30 m, has little further effect on
enhancement. For 30 Hz, there is little change in enhancement
once thicknesses equal or exceed 20 m.
Elevation of the inversion
Fig. 9A,B shows the effects of changing the height above
the surface of a 5 ˚C inversion with a thickness of 10 m. As
shown in the section above, changing the thickness of the
inversion to more than 20 m has little further effect on the
Atmospheric controls on elephant communication
20
A
150
50
20 40
15
30
10
100
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Surfa
ce
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Enhancement (dB)
0
Adi
aba
tic
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4000
6000
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20
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15
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iab
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−15
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4000
6000
Range (m)
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10000
Fig. 9. Effects of inversion elevation (in m) on sound transmissions
at frequencies of (A) 15 Hz and (B) 30 Hz for conditions depicted in
Fig. 6E.
transmission of 15 or 30 Hz sound. This observation guided our
choice of the thickness (10 m) of the inversion to be tested for
changes in elevation.
A large increase in enhancement of more than 10 dB at 10 km
occurs when the height of the base of the inversion changes
from 0 m elevation to a height of 10 m. Changes in elevation
between 10 m and 50 m above the surface have little further
effect at 15 Hz. At 100 and 150 m above the surface for 15 Hz,
and at 50m and above for 30 Hz, mode interference (oscillation)
occurs. This may be the result of transferring a strong inversion
(5 ˚C over 10 m) to elevations as high as 50–150 m above the
surface. The large increase in transmission between the surface
and 10m demonstrates the effect of ground attenuation on longrange sound propagation in an inversion. Ground interaction is
reduced as the inversion lifts off the surface.
Inversion growth and decay
The studies isolating the strength and thickness effects
discussed above both show that enhancement in sound
propagation occurs early in the development of the inversion,
rapidly reaching a maximum. Beyond 3 ˚C strength or 20–30 m
947
thickness, the increase in enhancement is much less. Actual
inversions (Fig. 4) show a rapid and simultaneous growth in
both strength and thickness, exceeding 5 ˚C and 50 m within
2 h of sunset. The changes in low-frequency transmissions due
to inversion growth and decay are examined in this section.
In the model of evening inversion formation (Fig. 6F), the
strength and thickness intensify from 1 ˚C and 10 m to 5 ˚C and
50 m. The model of morning inversion decay (Fig. 6G) shows
how surface heating simultaneously decreases the strength of
the inversion, elevates it and decreases its thickness, giving rise
to an adiabatic layer from ground level to the base of the
inversion. Figs 10 and 11 show the effects of evening growth
and morning decay of the inversion on sound propagation. The
temporal progression of acoustic enhancement follows the
numbered sequence of growth and decay used in Fig. 6F,G.
Fig. 10A shows that the change in sound amplification is
greatest between stages 1 and 3. By stage 4, the enhancement
for 15 Hz has reached 14.3 dB at 4750 m from the source. The
sense of change at 30 Hz (Fig. 10B) is the same as that at
15 Hz, but with a maximum enhancement of 13 dB observed
much closer to the source (1450 m).
Morning decay of the inversion (Figs 6G, 11A,B) shows
transmission levels similar to the maximum values reached in
the evening, but the decay is much more rapid (large changes
between stages 3 and 5). The effects of decreasing strength are
initially offset by increasing elevation.
The effects of evening development and morning decay of
the inversion on the propagation of sound suggest that acoustic
enhancement should reach a maximum early in the nocturnal
cooling cycle and decay rapidly with daytime heating. During
the night, development of local circulation fields responding to
surface cooling will weaken the inversion and hence reduce the
enhancement of low-frequency sound transmission.
Standardized evening inversion
A standard evening inversion (SEI) representative of the
optimum evening conditions at Etosha National Park can be
used to illustrate the combined effect of all the inversion
characteristics considered upon low-frequency sound
transmission. An SEI with a strength of 5 ˚C, a thickness of
50 m and with the base of the inversion at the surface (profile
5, Fig. 6F) is assumed. Fig. 12 contrasts the acoustic
enhancement in an SEI with the enhancement during adiabatic
and super-adiabatic conditions. Fig. 12A, for 15 Hz, shows that
there is more than 12 dB enhancement at 4 km, with enhanced
values of just under 10 dB persisting beyond 10 km. Fig. 12B,
for 30 Hz, shows a similar enhancement within 1 km of the
source, after which the signal attenuates to zero enhancement
at 10 km. The results for both frequencies contrast strongly
with daytime super-adiabatic conditions, which show a rapidly
falling enhancement reaching 220 dB by 4 km. The 12–50 dB
difference in enhancement between evening and daytime
conditions for distances beyond 2 km represents a four- to 300fold increase in sound pressure. Comparison of the SEI
calculations with results based on an adiabatic atmosphere
show that, for SEI conditions, sound levels increase by
948
M. GARSTANG AND OTHERS
15
5
16
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iab
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−20 B
0
4
10
5
−5
Enhancement (dB)
Enhancement (dB)
−5
0
2000
6000
4000
Range (m)
8000
10000
−2
0
2000
4000
6000
Range (m)
8000
10000
Fig. 10. Effects of the growth of an evening inversion on sound
transmissions at frequencies of (A) 15 Hz and (B) 30 Hz for conditions
depicted in Fig. 6F. Numbers 1–5 refer to stages of growth, see text.
Fig. 11. Effects of the decay of a morning inversion on sound
transmissions at frequencies of (A) 15 Hz and (B) 30 Hz for conditions
depicted in Fig. 6G. Numbers 1–5 refer to stages of decay, see text.
9–20 dB, a three- to tenfold increase in sound pressure beyond
2 km. The transition from adiabatic to SEI conditions takes
place rapidly before and immediately following sunset.
categories. Martin (1978) suggests that elephant groups in
Zimbabwe communicate over distances of up to 5 km.
These workers have suggested that low-frequency calls are
vital to elephant reproduction because females are typically in
oestrus for only 2–4 days every 4 years and reproductive males
may not be in close proximity to these females. Although
reproduction is not confined to males in musth, dominant males
in musth are the preferred partners, adding yet another variable
to the reproduction cycle. Musth and oestrus both being
limiting factors, the importance of long-distance
communication is increased. Males in musth use lowfrequency sounds to avoid encounters with each other.
Matriarchs in herds and females in groups with calves use a
range of sounds to remain in contact and to signal various
events, including the approach of other elephants (Poole et al.
1988). Martin’s (1978) work suggests that elephant groups use
low-frequency sound to maintain range separation and to
minimize competition for resources. This suggestion has
recently been strengthened by the results of a season in which
W. R. Langbauer, R. A. Charif, R. B. Martin and K. B. Payne
Discussion
Payne et al. (1986), Langbauer et al. (1989, 1991) and Poole
et al. (1988) leave little doubt that elephants respond to lowfrequency sounds in the range 14–35 Hz. They also
demonstrate that elephants can generate sounds in these
frequencies up to levels of 103±3 dB (mean ± S.D.) at 5 m from
the source. Langbauer et al. (1991) show, from the studies
carried out in the Etosha National Park, that under the
conditions of that experiment, elephants responded to lowfrequency sounds over distances of at least 2 km. They
postulated from the Etosha study that the range of lowfrequency sound communication was likely to be at least
double the observed value, i.e. at least 4 km, since their
broadcasts were made at half the sound pressure levels they
had recorded from elephants making sounds of the same
Atmospheric controls on elephant communication
15
A
10
SEI
Adiab
atic
5
0
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pe
r-a
dia
ba
tic
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−15
Enhancement (dB)
−20
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0
2000
4000
6000
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10000
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B
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SEI
Adiab
atic
Su
pe
r-a
dia
ba
tic
0
−10
−20
−30
−40
−50
−60
0
2000
Fig. 12. Response of sound transmission to a standardized evening
inversion (SEI) for frequencies of (A) 15 Hz and (B) 30 Hz, where an
SEI consists of a 5 ˚C inversion at the surface over a depth of 50 m.
The effects of an SEI are compared with the effects of an atmosphere
with super-adiabatic and adiabatic lapse rates on sound transmission.
radio-collared 16 elephants from separated family groups. The
collars were capable of transmitting the vocalizations of the
animals as well as their locations (K. B. Payne, personal
communication). It is of interest to note from our results that
the distance over which such communications could be sent
would increase as a function of drought. Clear, dry
atmospheres with a low moisture content, typical of drought
conditions, would produce the most pronounced and frequent
(consecutive evenings) nocturnal inversions. This would
maximize the distance between groups and minimize wasteful
expenditure of energy in competing for severely depleted
resources.
The results of our study suggest that the efficiency of
elephant communication is a function of weather conditions.
Communication is optimized during the dry season and on dry
days in the wet season. A pronounced maximum in the
frequency and effectiveness of the communications should be
observed starting an hour or so before sunset and reaching a
949
peak 1–2 h after sunset. This would happen with the greatest
probability on clear, dry, calm and (relatively) cold evenings.
Communication using low frequencies should work best at
night. Nocturnal winds, which are often produced by
topographic features, including modest and distant sloping land
surfaces, increase surface wind speeds and turbulence,
weakening or even breaking down the surface nocturnal
inversion. Thus, optimum conditions might not prevail long
into the evening or night. Nocturnal conditions, however, are
predictably better than daytime conditions. Communication
using low frequencies will not be as effective in daytime
compared with nighttime conditions, particularly where the
surface has a low heat capacity and warms rapidly to high
temperatures. Under such conditions, super-adiabatic lapse
rates prevail just above the surface and sound transmission is
severely attenuated owing to the absence of inversion ducting
and enhancement of turbulent fluctuations in temperature and
velocity. J. H. Poole (personal communication) has noted an
early evening peak in elephant social activity with associated
levels of vocalization.
Under optimum inversion conditions, there is no reason why
elephants cannot communicate over distances of 10 km.
Inversion effects amplified the lower more than the higher of
the two frequencies tested. Under typical inversion conditions,
sound levels at 15 Hz are greater than those at 30 Hz for all
distances greater than 1 km. For long-range communication,
elephants might favour the lower over the higher end of the
range of frequencies they have been observed to use
(14–35 Hz).
The distance over which communication can be effected will
be optimized in terms of time (year, season, day, time of day),
place (terrain, vegetation, soils) and the relationship between
the place and its weather. If behaviour is limited by the range
of communication elephants can achieve, changes in weather
on time scales greater than the diurnal should produce a change
in behaviour that is commensurate with the effects of these
weather changes on low-frequency sound propagation. Day-today storm or synoptic-scale changes as well as monthly to
seasonal (wet and dry) differences in communication range
induced by weather should be discernible in behaviour. Long
period (decadal) and large spatial variations (regional to
subcontinental) in climate patterns which produce alternating
dry and wet conditions should result in changes in patterns of
communication. The larger spacing of elephant groups under
dry conditions is not only dictated by resources but is made
possible by the meteorological conditions governing
communication. The competition for localized resources, such
as water, may also be influenced by enhanced long-range
communication. Elephants in dry conditions might converge to
drink most frequently in the early evening, as noted by K. B.
Payne (personal communication) and many others. Kinship
groups often assemble en route to waterholes or synchronize
their arrivals at waterholes. The occurrence of this behaviour
in the early evening or at night may be a response to the fact
that low-frequency sounds propagate best under dry conditions
at these times. Laws et al. (1975, p. 164) report observations
950
M. GARSTANG AND OTHERS
from the Galana Ranching Scheme and the Tsavo National
Park in Kenya, suggesting that elephants adopt a routine of
drinking at night under dry conditions. One of the authors
(M. Lindeque) has noted that, at the beginning of the rainy
season, elephant herds leave the southern reaches of the Etosha
National Park and migrate northeastwards up to 2 weeks before
other herd animals initiate a similar migration. This
observation raises the possibility that elephants are reacting to
low-frequency sound generated by thunderstorms moving into
northeastern Namibia at the end of the dry season.
A knowledge of the atmospheric controls on acoustic
enhancement can be used in understanding, controlling and
preserving elephants. It is possible that elephant range is in part
related to the factors that control distance of communication,
thus influencing the amount of territory occupied by the
animals. Given the enhanced acoustic transmission conditions
of early evening, the identification and tracking of individual
animals may be possible. Certainly, the tracking of entire herds
by infrasound should be feasible. A knowledge of the nocturnal
enhancement of low-frequency sound transmission may help
in attempts to develop acoustic fencing or warning systems.
Similarly, a knowledge of infrasound could be used to
minimize stress transmitted to neighbouring groups during
culling operations.
Our findings suggest that the use of low frequencies as a
means of communication by terrestrial animals should be
treated in a broad biophysical context. The observation,
interpretation and prediction of terrestrial animal
communications can be greatly enhanced by the use of
numerical models based on physical and biological field
measurements.
The field work for this study was carried out within the
framework of the Southern African Fire–Atmosphere Research
Initiative (SAFARI), a multinational programme under the
auspices of the International Geosphere–Biosphere Program.
The field work was supported by Grant ATM-92-07924,
awarded by the National Science Foundation to the University
of Virginia, directed at the characterization of aerosols over
southern Africa. Support of students in the field was also
generously provided by the Eugene P. and William E. Odum
Foundation. SAFARI was carried out with the approval and
kind cooperation of the Namibian government. Field
operations at Okaukuejo in the Etosha National Park were
made possible by the generous support and cooperation of the
Director and staff of Etosha Ecological Institute and the Chief
Ranger and his staff. We draw, in an interdisciplinary study
such as this, upon the talents and contributions of many groups
and individuals. In particular, we wish to recognize the
University of the Witwatersrand for their overall
administration of SAFARI, the University of Natal for
operating the surface meteorological tower and upper air
sounding system, the Council for Industrial and Scientific
Research of South Africa for the tethered balloon
measurements and the South African and Namibian Weather
Bureaux for overall support of weather observations. Not only
were these contributions essential but they were also provided
with the utmost enthusiasm. We are most grateful for this help.
This paper draws upon a dissertation to be submitted by D.L.
as part of the requirements of the PhD degree at the University
of Virginia. Ideas central to this paper were initially discussed
with Mr Richard Garstang.
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