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Determining a smartphone's placement by material detection using harmonics produced in sound echoes

In this study, we propose a system to determine the placement of a smartphone using acoustic properties of the surface materials nearby. Detecting the surrounding materials allows the smartphone to change notification method automatically based on situational factors. Researchers have studied how to recognize the position in which smartphones are worn while walking, using accelerometer data; however it is difficult to identify a smartphone's position while stationary, because the accelerometer value does not change significantly when the smartphone is put down. In this study, we developed a method to recognize surface materials close to a smartphone using echoes, based on an assumption that echoes of a selected frequency will differ in their properties, depending on the smartphone's placement and the surface materials nearby. As a result of our experiment, our proposed method could classify six types of placement with 89.2% accuracy. We also revealed the limited influence of other environmental factors and the orientation of the smartphone, suggesting a rather robust result.