Physiological Acoustic Sensing Based on Accelerometers: A Survey for Mobile Healthcare YATING HU, 1 ERIC GUORUI KIM, 2 GANG CAO, 3 SHENG LIU, 4 and YONG XU 5 1 Engineering Technology, Middle Tennessee State University, Murfreesboro, TN 37132, USA; 2 Electrical Engineering, Wayne State University, Detroit 48202, MI, USA; 3 Mechanical Science & Engineering, Huazhong University of Science & Technology, Wuhan 430074, China; 4 Cross-Disciplinary Institute of Engineering Sciences, School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China; and 5 Electrical Engineering, Wayne State University, Detroit 48202, MI, USA (Received 28 February 2014; accepted 5 September 2014; published online 19 September 2014) Associate Editor Tingrui Pan oversaw the review of this article. Abstract—This paper reviews the applications of accelerom- eters on the detection of physiological acoustic signals such as heart sounds, respiratory sounds, and gastrointestinal sounds. These acoustic signals contain a rich reservoir of vital physiological and pathological information. Acceler- ometer-based systems enable continuous, mobile, low-cost, and unobtrusive monitoring of physiological acoustic signals and thus can play significant roles in the emerging mobile healthcare. In this review, we first briefly explain the operation principle of accelerometers and specifications that are important for mobile healthcare. Applications of accel- erometer-based monitoring systems are then presented. Next, we review a variety of accelerometers which have been reported in literatures for physiological acoustic sensing, including both commercial products and research proto- types. Finally, we discuss some challenges and our vision for future development. Keywords—Physiological acoustic sensing, Accelerometers, Heart sounds, Lung sounds, Phonocardiography, Ballisto- cardiography, Seismocardiography. INTRODUCTION In mobile healthcare, accelerometers have been widely used for activity monitoring, posture detection, and fall detection for elderly people. 75 In this review, we focus on the sensing of physiological acoustic signals from the human body for mobile healthcare using accelerometers. For this application, there are more demanding requirements on the sensitivity/noise per- formance of accelerometers. Physiological acoustic signals include, but are not limited to, heart sounds, respiratory sounds and gastrointestinal sounds, each of which are rich reservoirs of vital physiological and pathological information. Further, this review discusses the measurement of low-frequency (e.g., <20 Hz) vibrations caused by heart beats, such as in ballisto- cardiography (BCG). Continuous or mobile monitor- ing of these physiological sounds is expected to play important role in the emerging mobile healthcare field. The traditional tool for measuring these sounds is the stethoscope in which the primary component is a bell-shaped air chamber which picks up and amplifies sound signals. As shown in Fig. 1, the sound generates a vibration on the chest skin with an amplitude of x 1 . Due to the bell-shape of the air chamber, the vibration amplitude x 2 at the narrow end of the chamber is much larger. For modern electronic stethoscopes, a micro- phone is placed at the narrow end to pick up the signal. Although widely used, the stethoscope has a number of limitations for continuous monitoring. For instance, its bulky air chamber prevents it from being body- worn or applied to obstructed auscultation sites. Fur- thermore, in order to pick up signal effectively, the stethoscope needs to stationary relative to the skin surface, and so held against the skin during measure- ment. Because of these limitations, the stethoscope is mainly used for intermittent auscultation rather than long-term measurement. The vibration of the skin due to sounds generated inside human body can also be detected by an accel- erometer, which is more appropriate for wearable applications. 40,60,82,116 Compared with the stethoscope, the accelerometer can be in direct contact with the skin and does not need an air chamber to couple the acoustic signal, sometimes giving them the name con- tact-type microphones. Accelerometer-based systems Address correspondence to Yong Xu, Electrical Engineering, Wayne State University, Detroit 48202, MI, USA. Electronic mail: yxu@indigo.eng.wayne.edu Annals of Biomedical Engineering, Vol. 42, No. 11, November 2014 (Ó 2014) pp. 2264–2277 DOI: 10.1007/s10439-014-1111-8 0090-6964/14/1100-2264/0 Ó 2014 Biomedical Engineering Society 2264