  This paper presents a comprehensive review of cardiorespiratory auscultation
sensing devices which is useful for understanding the theoretical aspects of
sensing devices, as well as practical notes to design novel sensing devices.
One of the methods to design a stethoscope is using electret condenser
microphones (ECM). In this paper, we first introduce the acoustic properties of
the heart and lungs, as well as a brief history of stethoscope evolution. Then,
we discuss the basic concept of ECM sensors and a recent stethoscope based on
this technology. In response to the limitations of ECM-based systems, we
explore the potential of microelectromechanical systems (MEMS), particularly
focusing on piezoelectric transducer (PZT) sensors. This paper comprehensively
reviews sensing technologies, emphasizing innovative MEMS-based designs for
wearable cardiopulmonary auscultation in the past decade. To our knowledge,
this is the first paper to summarize ECM and MEMS applications for heart and
lung sound analysis. Keywords: Micro-electro-mechanical Systems (MEMS);
Electret Condenser Microphone (ECM); Wearable Sensing Devices;
Cardiorespiratory Auscultation; Phonocardiography (PCG); Heart Sound; Lung
Sound
