Biomedical Engineering Reference
In-Depth Information
(a)
1.0
1.0
F
-1.0
0.0
.665
time (s)
.677
0.0
frequency (Hz)
1000
(b)
1.0
1.0
F-f
F+f
-1.0
0.0
.665
time (s)
.677
0.0
frequency (Hz)
1000
Fig. 6.8.
(
a
) Harmonic signal. Its spectrum is composed of a single frequency
F
= 1000 Hz. (
b
) The same signal as in (
a
), but with its amplitude modulated at
f
= 100 Hz. The signature of the coupling between the two oscillations
F
and
f
(amplitude modulation, in this case) is the appearance of new frequencies: the sum
and difference frequencies
be the case for the magpie tanager (
Cissopis leveriana
) [Straneck 1990b], one
of its songs being displayed in Fig. 6.9. Notice that the traces in the sonogram,
to the left of the dotted line, denote the existence of two independent sources:
the upper and lower traces are related neither temporally nor harmonically
10000
two
independent
sou
rces
two
coupled
sources
0.0
0.0
time (s)
0.7
Fig. 6.9.
In this syllable of a magpie tanager's song (
Cissopis leveriana
), we see
at the begining the typical signature of two independent sound sources (on the two
sides of the syrinx). As the frequencies become closer, a series of subharmonics can
be observed. It is likely that coupling between the two sources is acting here
