Sound transmission and its significance for animal vocalization
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Attenuation of white noise and pure tones from 350 Hz to 10 kHz was measured at three secondary forest sites in Panama at different stages of maturity. Graphs of excess attenuation (E.A.) versus frequency were obtained near ground level, and at heights of 1, 2, 5, 10, and 12 m in each habitat.
The pattern of E.A. vs. frequency was similar for all habitats. For all heights other than ground level and 1 m the lower the frequency the better the sound carried. Sounds below 2 kHz were attenuated by a ground effect if the source was 1 m or less from the ground. Consequently there was a minimum of E.A. in all three habitats between 500 Hz and 2 kHz at ground level and 1 m.
Relevance of the data to assessment of the role of environmental variables in the natural selection of vocalization for long-distance transmission is discussed. Two facts mitigating against Morton's sound “window” as an explanation for lower frequencies in songs of forest as opposed to open country birds are presence of similar “windows” in both habitats and restriction of windows to a zone close to the ground in most habitats.
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