5.15 Biological Generation of Sounds
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Fig. 5.8 Spectrogram (and oscillogram) of a European cicada. Vertical axis: Frequency, Horizontal axis: Time (s) (From www.cicadasong.eu)
Bats use the sound they generate for echolocation, which is the active use of
sound for navigation and ranging (SONAR). Most bats produce sound with their
larynx, but some click their tongue. The sound is emitted from the mouth, or, in
the case of horseshoe bats, from specially shaped nostrils. The sounds range in
frequency from 20 kHz to 200 kHz (a range above human hearing) (Fig. 5.9). This
makes sound of wavelength in air in the range of 1.7 cm to 1.7 mm. Thus bats
can resolve, with sound, objects in the millimeter size range. Their calls vary in
frequency and pulse length, and in intensity (near mouth) from 50 dB to as high
as 120 dB. The bat brain can detect Doppler shifts as small as 0.1 Hz, giving them
information about the speed of their insect prey. Before emitting a call, bats protect
their own ears as we do by contracting the muscles of the middle ear which decouple
the three middle ear bones from the inner ear window. This action occurs about 6 ms
before the bat emits a sound pulse of about 1 ms, and then 2 ms after, the muscle
relaxes to allow the returning echoes to be transmitted to the inner ear. This means
that bats can sense echoes from objects no less than 34 cm away. As the bat reaches
it prey, the pulse rate increases to just under 200 a second. With a flight speed up
to 2682 cm/s, the bat would reach its prey in just 13 ms after getting the last echo.
To distinguish a call from those of other bats, the call pitch and its harmonics varies
from bat to bat. Note that the bat ears also play a strong role in collecting sound
from a particular direction.
Dolphins and some whales also use echolocation (Fig. 5.10). Because sound
enters and leaves the prey’s body due to close impedance matching, a dolphin can
‘see’ if a fish’s heart is beating and other physiological properties related to the
prey’s health.
145
20k
16k
12k
8k
4k
0
0
2
4
6
8
Fig. 5.8 Spectrogram (and oscillogram) of a European cicada. Vertical axis: Frequency, Horizontal axis: Time (s) (From www.cicadasong.eu)
Bats use the sound they generate for echolocation, which is the active use of
sound for navigation and ranging (SONAR). Most bats produce sound with their
larynx, but some click their tongue. The sound is emitted from the mouth, or, in
the case of horseshoe bats, from specially shaped nostrils. The sounds range in
frequency from 20 kHz to 200 kHz (a range above human hearing) (Fig. 5.9). This
makes sound of wavelength in air in the range of 1.7 cm to 1.7 mm. Thus bats
can resolve, with sound, objects in the millimeter size range. Their calls vary in
frequency and pulse length, and in intensity (near mouth) from 50 dB to as high
as 120 dB. The bat brain can detect Doppler shifts as small as 0.1 Hz, giving them
information about the speed of their insect prey. Before emitting a call, bats protect
their own ears as we do by contracting the muscles of the middle ear which decouple
the three middle ear bones from the inner ear window. This action occurs about 6 ms
before the bat emits a sound pulse of about 1 ms, and then 2 ms after, the muscle
relaxes to allow the returning echoes to be transmitted to the inner ear. This means
that bats can sense echoes from objects no less than 34 cm away. As the bat reaches
it prey, the pulse rate increases to just under 200 a second. With a flight speed up
to 2682 cm/s, the bat would reach its prey in just 13 ms after getting the last echo.
To distinguish a call from those of other bats, the call pitch and its harmonics varies
from bat to bat. Note that the bat ears also play a strong role in collecting sound
from a particular direction.
Dolphins and some whales also use echolocation (Fig. 5.10). Because sound
enters and leaves the prey’s body due to close impedance matching, a dolphin can
‘see’ if a fish’s heart is beating and other physiological properties related to the
prey’s health.