Swarm of Sound-to-Light Conversion Devices to Monitor Acoustic Communication Among Small Nocturnal Animals.

While many robots have been developed to monitor environments, most studies are dedicated to navigation and locomotion and use off-the-shelf sensors. We focus on a novel acoustic device and its processing software, which is designed for a swarm of environmental monitoring robots equipped with the device. This paper demonstrates that a swarm of monitoring devices is useful for biological field studies, i. e., understanding the spatio-temporal structure of acoustic communication among animals in their natural habitat. The following processes are required in monitoring acoustic communication to analyze the natural behavior in the field: (1) working in their habitat, (2) automatically detecting multiple and simultaneous calls, (3) minimizing the effect on the animals and their habitat, and (4) working with various distributions of animals. We present a sound-imaging system using sound-to-light conversion devices called "Fireflies"and their data analysis method that satisfies the requirements. We can easily collect data by placing a swarm (dozens) of Fireflies and record their light intensities using an offthe-shelf video camera. Because each Firefly converts sound in its vicinity into light, we can easily obtain when, how long, and where animals call using temporal analysis of the Firefly light intensities. The device is evaluated in terms of three aspects: volume to light-intensitycharacteristics, battery life through indoor experiments, and water resistance via field experiments. We also present the visualization of a chorus of Japanese tree frogs (Hyla japonica) recorded in their habitat, that is, paddy fields.