Human bodies become an emerging type of human-computer interfaces recently. Not only because our skin is a surface that is always available and highly accessible, the sense of how our body is configured in space also allows us to accurately interact with our bodies in an eye-free manner. Hence, this method is suitable to be applied on extending the interaction space of mobile devices or providing more degrees-of-freedom for enhancing gaming experiences1. Nevertheless, the additional gesture detector may be obtrusive or hard to be portable for users in daily life.

Based on this insight, we’re motivated to design SonarWatch, a wristwatch equipped with an ultrasonic rangefinder and a capacitive touch sensor, to appropriate an user’s forearm as a readily available input interface. By wearing the SonarWatch, a user can slide on an arm or tap on the watch for performing corresponding operations with passive haptic sensation on the skin. Three applications, photo browsing, music playlist controlling and gaming, are then developed to demonstrate the effectiveness of using SonarWatch in our daily life. We believe this more implicit design can facilitate body- sensing based interaction techniques.