Ecological Constraints for Sound Communication: From Grasshoppers to Elephants 69
suggest that microclimatic conditions very close to the ground could have an
enhancing effect for sound propagation.
Layers of air in close contact with the ground are largely determined by sun
radiation, with highest temperatures occurring where radiation is absorbed. In a
meadow where the grass was about 30-40 em high, we found a temperature profile
as shown in Fig. 4 (Seither and Romer, unpubl.). Firstly, the data indicate an
inverse relationship between temperature and humidity of the vegetation: in the
early morning the meadow is wet with dew, and after sunrise temperature does not
increase before most of the humidity has evaporated. Only then can temperature
increase substantially. A very steep gradient of up to I 0 °C within a height of only
30 em developed, with the maximum occurring at the top of the vegetation. By
comparison, the mean gradient in the free atmosphere in Middle Europe is 0.6 oc
over I 00 m (Geiger et a!. 199 5).
40 ,---· ··---· ··---·· -···-- ········--· -···-· ······--· -··---1----· ----· ·----· ---· ··-·-··· -··-···- ·----··- --- 100
90
so
70 ~
2......
60 >.
.....
:.a
50 "§
:s
40 ;:c:
30 Qj
~
20
10
r...-;:
10
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
"' "' "'
lt)
"' "' "'
If)
o.n
If)
If)
If)
If)
If)
If)
If)
0
~ ;! "' ~ ~ gj 0 c'i ~ "'
«i
0
c'i
~
"'
~
~
~
Time of Day
Fig. 4. Microclimatic conditions within and above a meadow on a summer day (12 and 13
August 1997), measured with three thermosensors and one sensor for humidity. Note that
max. temperatures occur at the top of the grass, and that the gradient at about 16.00 h is I 0
°C over only 30 em. (Seither and Romer, unpubl.)
We therefore compared the transmission characteristics of such a meadow under
a variety of meteorological conditions, for frequencies between I 00 Hz and
15 kHz. Particular attention was given to the temperature gradient.
Figure 5 shows the result of a comparison between sound transmission on a
sunny day with a steep temperature gradient, and transmission on a cloudy day
suggest that microclimatic conditions very close to the ground could have an
enhancing effect for sound propagation.
Layers of air in close contact with the ground are largely determined by sun
radiation, with highest temperatures occurring where radiation is absorbed. In a
meadow where the grass was about 30-40 em high, we found a temperature profile
as shown in Fig. 4 (Seither and Romer, unpubl.). Firstly, the data indicate an
inverse relationship between temperature and humidity of the vegetation: in the
early morning the meadow is wet with dew, and after sunrise temperature does not
increase before most of the humidity has evaporated. Only then can temperature
increase substantially. A very steep gradient of up to I 0 °C within a height of only
30 em developed, with the maximum occurring at the top of the vegetation. By
comparison, the mean gradient in the free atmosphere in Middle Europe is 0.6 oc
over I 00 m (Geiger et a!. 199 5).
40 ,---· ··---· ··---·· -···-- ········--· -···-· ······--· -··---1----· ----· ·----· ---· ··-·-··· -··-···- ·----··- --- 100
90
so
70 ~
2......
60 >.
.....
:.a
50 "§
:s
40 ;:c:
30 Qj
~
20
10
r...-;:
10
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
"' "' "'
lt)
"' "' "'
If)
o.n
If)
If)
If)
If)
If)
If)
If)
0
~ ;! "' ~ ~ gj 0 c'i ~ "'
«i
0
c'i
~
"'
~
~
~
Time of Day
Fig. 4. Microclimatic conditions within and above a meadow on a summer day (12 and 13
August 1997), measured with three thermosensors and one sensor for humidity. Note that
max. temperatures occur at the top of the grass, and that the gradient at about 16.00 h is I 0
°C over only 30 em. (Seither and Romer, unpubl.)
We therefore compared the transmission characteristics of such a meadow under
a variety of meteorological conditions, for frequencies between I 00 Hz and
15 kHz. Particular attention was given to the temperature gradient.
Figure 5 shows the result of a comparison between sound transmission on a
sunny day with a steep temperature gradient, and transmission on a cloudy day