Sound transmission and its significance for animal vocalization
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Sound transmission was measured in open fields, mixed decidous forest with and without leaves and coniferous forest in Dutchess County, New York. Attenuation of white noise and pure tones was measured between one microphone close to a loudspeaker and another microphone 100 m away, at the same height. Graphs of excess attenuation (E.A. in dB/100 m) against frequency were obtained at 0.15, 1, 2, 5, and 10 m above the ground. An analysis of variance was conducted to estimate effects of height, frequency and habitat.
Height and frequency affect sound transmission more than habitat. With a sound source close to the ground (15 cm and 1 m) all frequencies were more attenuated than at greater heights. The patterns of E.A. as a function of sound frequency were basically similar in all habitats. At all source heights the lower the frequency the better the sound carried, with the exception that close to the ground, sounds below 2 kHz were excessively attenuated. Comparing open field and forest, trees improved transmission of frequencies below 3 kHz, especially close to the ground.
Some general trends can be predicted for maximization of transmission distances of animal sounds in these habitats. For an animal vocalizing higher than 1 m above the ground, the lower the frequency the further the sound travels. Close to the ground, low frequencies are again preferred for maximization of transmission distances, but the frequencies must be pitched above a range of attenuated, low-pitched sounds, the limits of which vary to some extent with habitat, creating the ‘sound window’ of Morton. This ‘window’ of least-attenuated frequencies, only occurring close to the ground, tends to be pitched somewhat lower in forest than in open habitats. However, for an animal producing sounds in the habitats tested, perch height and sound frequency are more important than the habitat in determining how far the sound will carry.
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