Slabbekoorn, H., Yeh, P., & Hunt, K. (2007). Sound transmission and song divergence: A comparison of urban and forest acoustics. The Condor, 109, 67-78.

In this investigation, sound transmission properties from an urban environment were compared to those in a forest environment, and these differences were applied to Dark-eyed Junco songs in those environments. While natural habitats are more well-studied, urban environments consist of concrete buildings at various angles with little vegetation, which can lead to highly unpredictable sound transmission patterns. The authors conducted transmission experiments using artificial tone playbacks. All tones were fixed at 2.5 kHz (in the range of Junco songs) and projected from a speaker at a height of 3 m. All sounds were recorded, also at a height of 3 m, after transmission over 25 m. Approximations of a “slow” Junco song trill rate and a “fast” Junco song trill rate were projected by altering the durations of the tones. Signal transmission was recorded in two environments: the forests of the Laguna Mountain Recreational Area, California and the urban environment of the University of California San Diego campus. Real Junco songs were also recorded from both locations.

There were several differences in sound transmission properties between the forest and urban habitats. On the UCSD campus, there were discrete echoes following all of the tones, whereas in most forest recordings, there were no echoes. In the forest, reverberations caused a gradual tail of reflected sounds that faded over time, instead of multiple discrete echoes. In an urban environment, these echoes could mask biologically important sounds in a very unpredictable way and change the perception of the song. Fast trills were affected by the echoes much more than the slow trills. From these experiments, the authors expected urban Junco songs to favor high frequencies, slower trill rates, and shorter durations. In reality, the urban Juncos did have a higher minimum frequency than forest Juncos, but there was no indication of slower trill rates or shorter songs.

This paper is relevant to our lecture on Fourier transformations. The tones were digitized at a sampling rate of 25 kHz, and spectrograms were produced using Hanning windows with a 512-point FFT. The Junco song recordings were digitized with a sampling rate of 44.1 kHz.

The majority of the paper applies to our lecture on sound propagation.

In their methods, the authors picked 2.5 kHz for their playback tone because lower frequencies do not attenuate as rapidly as high frequencies, and this value was within the range of a Junco song. This paper discusses signal degradation in detail and how attenuation, scattering and reverberations change in different habitats. In forests, vegetation causes both diffractive and simple scattering. But the concrete, angular walls of an urban environment can cause multiple, discrete echoes which can mask subsequent sounds in an unpredictable way.

Our discussion of signal design also applies to this paper. The authors predict that urban juncos will have higher frequency songs than forest juncos because while lower frequencies can travel farther than high, echoes will be louder for lower frequencies. To avoid unpredictable echoes, juncos may shift their songs up in frequency. Because reverberations can have a greater affect on songs with a fast trill rate, urban juncos may slow their trill rate down.