Facial expressions are important indicators of user needs that can be used in many interactive computing applications to adapt the system behaviors and settings. Current computing approaches to recognizing human facial expressions, however, either rely on con-tinuous camera recordings that are energy consuming, or require custom sensing hardware that are expensive and difficult to use on commodity systems. In this paper, we present FaceListener, a new sensing system that recognizes human facial expressions by only using commodity headphones. The basic idea of FaceListener is to transform the commodity headphone into an acoustic sensing device, which captures the face skin deformations caused by fa-cial muscle movements with different facial expressions. To ensure the recognition accuracy, FaceListener leverages the knowledge distillation technique to learn the subtle correlation between face skin deformation and the acoustic signal changes. Experiment re-sults over multiple human beings demonstrate that FaceListener can accurately recognize more than 80% of different facial expressions. FaceListener is highly energy efficient, and can well adapt to different headphone models, host systems and user activities.

Respiratory diseases have been a significant public health challenge. Efficient disease evaluation and monitoring call for daily spirometry tests, as an effective way of pulmonary function testing, out of clinic. This requirement, however, is hard to be satisfied due to the large size and high costs of current spirometry equipments. In this paper, we present SpiroSonic, a new system design that uses commodity smartphones to support complete, accurate yet reliable spirometry tests in regular home settings with various environmental and human factors. SpiroSonic measures the humans' chest wall motion via acoustic sensing and interprets such motion into lung function indices, based on the clinically validated correlation between them. We implemented SpiroSonic as a smartphone app, and verified SpiroSonic's monitoring error over healthy humans as <3%. Clinical studies further show that SpiroSonic reaches 5%-10% monitoring error among 83 pediatric patients. Given that the error of in-clinic spirometry is usually around 5%, SpiroSonic can be reliably used for disease tracking and evaluation out of clinic.

Stylus pens have been widely used with today's mobile devices to provide a convenient handwriting input method, but also bring a unique security vulnerability that may unveil the user's handwriting contents to a nearby eavesdropper. In this paper, we present MagHacker, a new sensing system that realizes such eavesdropping attack over commodity mobile devices, which monitor and analyze the magnetic field being produced by the stylus pen's internal magnet. MagHacker divides the continuous magnetometer readings into small segments that represent individual letters, and then translates these readings into writing trajectories for letter recognition. Experiment results over realistic handwritings from multiple human beings demonstrate that MagHacker can accurately eavesdrop more than 80% of handwriting with stylus pens, from a distance of 10cm. Only slight degradation in such accuracy is produced when the eavesdropping distance or the handwriting speed increases. MagHacker is highly energy efficient, and can well adapt to different stylus pen models and environmental contexts.

Device-free acoustic motion tracking allows a commodity mobile device to precisely track the human user's motion, without applying any extra hardware tracker on the human body. Most of current device-free acoustic motion tracking systems, however, are limited to tracking the motion of small parts of the human body with negligible sizes, such as human fingers. Their accuracy of motion tracking will significantly degrade when being applied to targets with large sizes, such as humans' hands, arms or body trunk. We envision the key reason to such degradation as the target size's significant impact on the pattern of the reflected acoustic signal, and develop analytical modeling of such reflected acoustic signal from large targets. Based on such modeling, we present a new system called Acoustic Tracking over targets with LArge Sizes (ATLAS), which ensures precise motion tracking over large targets by correctly interpreting the reflected acoustic signal and extracting the phase from the signal. Experiment results over commodity Android smartphones show that ATLAS can reduce the error of motion tracking by more than 75%, when being applied to targets with heterogeneous sizes in practice.