Biology Reference
In-Depth Information
intrinsic temporal variability of amplitude, whereas some types of geophonies
(e.g., wind, flowing waters, rain) and anthrophonies (e.g., car passing or airplane
transit) present more constant amplitude values.
When the acoustic variations of amplitude are converted from a time domain
into a frequency domain by a Fourier transform, such variations appear along every
frequency category, resembling an indented border. More different are the values of
amplitude along a temporal step, and more information is contained in the spectral
representation. The ACI index measures indirectly such information.
The ACI calculates the absolute difference (
Δ
k
) between two adjacent values of
spectral amplitude (
I
k
and
I
(
k
+1)
) in a single frequency bin (
Δ
f
l
):
I
k
Δ
k
¼
I
kþ
1
Þ
j
(9.1)
ð
k
encompassed in a temporal step
J
of the recording
(e.g.,
J
may be 1 s, 5 s, 30 s, 60 s, etc.):
Δ
and then adds together all the
X
X
n
1
Δ
n
1
Δ
D
¼
k
for
j
¼
t
k
;
n
¼
number of
Δ
t
k
in
j
(9.2)
where
D
is the sum of all the
k
contained in
j
. To obtain the relative intensity and
to reduce the effect of the distance of the animals from the recording microphone,
this result is then divided by the total sum of the amplitude values registered during
the time interval
j
:
Δ
D
X
n
1
I
k
ACI
¼
(9.3)
where the ACI is calculated in a single temporal interval (
j
) and in a single
frequency bin (
fl
)
.
Thereafter, the ACI, which was worked out on all the temporal
steps encompassed in the recording, is calculated:
Δ
X
m
ACI
ðÞ¼
Δ
fl
ACI
(9.4)
1
for
m
¼
number of
j
intervals in the entire recording, where the ACI(
Δ
fl
)
corresponds to the ACI of an entire frequency bin.
Finally, the total ACI for all the frequency bins is calculated:
X
q
X
q
1
Δ
ACI tot
ðÞ¼
ACI
ðÞ
Δ
fl
for
: Δ
f
¼
fl
q
¼
number of
Δ
fl
(9.5)
1
where the ACI(tot) is the total value of the index for the entire recording (for more
details, see Pieretti et al.
2011
).
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