Acoustic calibrator for plethysmographic surface pressure sensors

Plethysmographic surface pressure sensors measure heart sounds, breathing sounds, blood pressure, blood flow sounds, fetal movement, and other sounds indicative of physiological conditions. An acoustic calibrator for such sensors should provide a specific acoustic impedance and permit a mounting arrangement resembling in‐service conditions. The calibrator described here consists of a water‐filled balloon, on which the test sensor is mounted at the equator. A hydrophone positioned in the center of the balloon serves as a nearly isotropic source at low frequencies. Since the excited sound field is not free but reverberant, a free‐field calibration of the hydrophone as projector cannot be used. Rather, a rigidly supported reference sensor of known calibration is tightly coupled to the balloon surface to calibrate the amplitude of the local surface pressure (which is sensitive to mounting conditions) as a function of frequency. The calibration of an acoustically based fetal heart tone monitor will be presented as an example.