Environmental Degradation

Signals transmitted between animals usually have to travel some distance, and are often affected by their environmental. The first kind of environmental effect is attenuation, which is simply defined as an increase in the faintness of a sound with increasing distance. Sounds propagate spherically and base attenuation predicts a 6 dB decrease in sound intensity for each doubling of distance. However, sounds rarely conform to this model and experience excess attenuation due to the environment. Excess attenuation is affected by foliage density, temperature, humidity, and many other environmental conditions. Generally speaking, high frequency sounds will attenuate faster because they tend be more absorbed by the atmosphere than low frequency sounds. However, low frequency sounds close to the ground tend to suffer from interference due to sound waves reflecting off of the ground. The second kind of environmental effect on signals is degradation. Sound reflecting off of the environment will cause successive elements to become difficult to distinguish and will blur element form, effects collectively known as reverberation. This effect is especially noticeable in high frequency, rapidly modulated sounds, which suffer from greater scattering and element blurring. Scattering occurs more in high frequency sounds as they tend to bounce off of objects instead of wrapping around them like low frequency sounds. In forests, there are many objects that will cause reverberation, causing element form to break down, especially at high frequency. Rapidly modulated sounds, frequency or amplitude modulated, will degrade much more quickly in high object density environments, due to scattering and interference. Songs with elements in quick succession, such as rapid trills, will become more blurred the higher the object density in the environment. Whistles however will retain their frequency and won’t be blurred when they reflect off of the environment. As such, it is often the case that birds will communicate using lower frequencies and simpler songs in forests as opposed to open environments.