Audio signal strength versus distance.
A. Wave (upper row) and spectral intensity (bottom row) from one voice recorder as seen in Audacity, the software we used for annotation. Shown is a recording of a single trill call played on a speaker at various distances. There is a steep fall off between 0.05 m (the approximate distance between the animals’ mouth and the recorder) and 1 m. Beyond 1 m, fall off is less steep. Besides lower intensity, the spectrogram also shows less detail, such as the sinusoidal frequency modulation that is typical of trill calls. B. Animals were video-recorded from overhead. A video frame taken at the onset time of a trill call shows two animals wearing voice recorders marked pink and green. The inter-animal distance is 549 mm. C. Spectrograms of the call from the two voice recorders show a difference in intensity that can be used to infer which animal called. This call is attributed to animal 1 (green). D. A video frame with inter-animal distance 76 mm. E. Spectrograms show higher intensity for animal 2 (pink), to whom the call is attributed.