64
Heiner Romer
grasshopper can be found either on gravel or sparse vegetation, but rarely in dense
vegetation (for review see Michelsen 1998).
3 Shadow Zones and Enhanced Transmission Due to
Environmental Gradients
Another means to increase the active space of a signal is to use spatial sound
windows and to adapt to atmospheric conditions influencing long-distance animal
vocalizations. Atmospheric conditions have rarely been considered here,
particularly with respect to insects. In the following this singular aspect on sound
communication outdoors will be considered. This will be achieved by comparing
animals of very different sizes, from small insects to elephants and finback whales,
communicating in different environments and microhabitats.
The wave-like nature of the propagation of sound results in a deflection when
sound passes through environments in which the speed of sound varies. When
sound crosses a boundary in air or water with different speed of propagation, part
of the wave is reflected back into the first environment (like light being reflected
from the surface of water). Vertical gradients of temperature (and of wind speed)
develop during the day as a result of sun radiation. On a sunny day the air is
warmer close to the ground (Fig. 2A), and this increases the sound speed as the
ground is approached. Under these super-adiabatic conditions, in which
temperature drops rapidly with height above ground, the gradient in the
propagation speed of sound results in a bending of the sound rays toward the
direction of lower speed, i.e., upwards (Fig. 2B).
This, in tum creates a shadow zone at some distance from the source, where no
sound can penetrate directly. After sunset, the opposite occurs: the ground cools
faster than the atmosphere; somewhere above the ground it is warmer compared to
the surface, before at higher altitudes temperature decreases again (Fig. 2A). In
such a temperature inversion the sound rays curve downwards during propagation
into the layer of air with lower temperature. As a result, a sound channel can be
created by repeated refraction towards the ground and reflection from the ground
into the atmosphere. This channeling is even more effective if the ground is
relatively smooth with little vegetation, so that sound is not scattered too much.
Humans experience such a sound channel due to temperature inversion after sunset
and at some distance from a highway, when we hear traffic noise some kilometers
away. During the day this is not the case, because of the sound shadow zone. Are
these conditions of any relevance for animal acoustic communication?
3.1 Elephants in the Savanna
Garstang et al. (1995) presented evidence that African elephants make use of
atmospheric conditions conductive to low-frequency sound transmission.
Heiner Romer
grasshopper can be found either on gravel or sparse vegetation, but rarely in dense
vegetation (for review see Michelsen 1998).
3 Shadow Zones and Enhanced Transmission Due to
Environmental Gradients
Another means to increase the active space of a signal is to use spatial sound
windows and to adapt to atmospheric conditions influencing long-distance animal
vocalizations. Atmospheric conditions have rarely been considered here,
particularly with respect to insects. In the following this singular aspect on sound
communication outdoors will be considered. This will be achieved by comparing
animals of very different sizes, from small insects to elephants and finback whales,
communicating in different environments and microhabitats.
The wave-like nature of the propagation of sound results in a deflection when
sound passes through environments in which the speed of sound varies. When
sound crosses a boundary in air or water with different speed of propagation, part
of the wave is reflected back into the first environment (like light being reflected
from the surface of water). Vertical gradients of temperature (and of wind speed)
develop during the day as a result of sun radiation. On a sunny day the air is
warmer close to the ground (Fig. 2A), and this increases the sound speed as the
ground is approached. Under these super-adiabatic conditions, in which
temperature drops rapidly with height above ground, the gradient in the
propagation speed of sound results in a bending of the sound rays toward the
direction of lower speed, i.e., upwards (Fig. 2B).
This, in tum creates a shadow zone at some distance from the source, where no
sound can penetrate directly. After sunset, the opposite occurs: the ground cools
faster than the atmosphere; somewhere above the ground it is warmer compared to
the surface, before at higher altitudes temperature decreases again (Fig. 2A). In
such a temperature inversion the sound rays curve downwards during propagation
into the layer of air with lower temperature. As a result, a sound channel can be
created by repeated refraction towards the ground and reflection from the ground
into the atmosphere. This channeling is even more effective if the ground is
relatively smooth with little vegetation, so that sound is not scattered too much.
Humans experience such a sound channel due to temperature inversion after sunset
and at some distance from a highway, when we hear traffic noise some kilometers
away. During the day this is not the case, because of the sound shadow zone. Are
these conditions of any relevance for animal acoustic communication?
3.1 Elephants in the Savanna
Garstang et al. (1995) presented evidence that African elephants make use of
atmospheric conditions conductive to low-frequency sound transmission.