An innovative physiological monitoring device allows passive acoustic detection of heartbeats, breaths, and other body sounds. Since the human body is mostly water, a liquid-filled bladder placed in contact with the torso acts as a fluid extension of the human body to form an acoustical conduit to a sensitive hydrophone within the pad that detects body sounds. Careful selection of interface material and internal fluid can help match acoustic impedance and transmission characteristics to that of a human body so that sounds travel easily through the coupling surface and fluid to the hydrophone. Acoustic analysis of the sensor-pad output can provide amplitude, phase, frequency, duration, rate, and correlative information that may be useful for medical diagnosis, patient care, and research. Joint time-frequency Fourier analysis of collected data shows that human cardiopulmonary function contains intricate spectral details that include infrasonic signals. Spectrogram representations of missed heartbeats, murmurs, held breaths, and fetal motion have demonstrated high sensitivity and SNR can be achieved through this coupling mechanism. The acoustic sensor pad can be useful for military and civilian casualty care, transport, performance evaluation, and continuous health monitoring of infants and the elderly.