Measurement and modeling of the acoustic impedance of the maternal abdomen

Transducers to detect fetal heart, breathing, and movement vibrations with increased sensitivity and bandwidth are under development. To understand the inherent acoustical properties of the maternal abdomen and its interaction with these sensors, the acoustic impedance Z(j omega ) was measured in nine women during their last trimester of pregnancy. A shaker with a contact area of 2.85 cm² produced abdominal vibrations between 10 and 500 Hz, and the resulting Z(j omega ) was estimated using spectral techniques from movements performed using an impedance head. The reactance depicted a dominant compliance effect at low frequencies, a resonance at a mean frequency of 17 Hz, and a dominant mass effect at higher frequencies. A series resistance-mass-compliance (R-M-C) circuit modeled these characteristics of well, especially when R was allowed to mimic the frequency-dependence that was observed in the real part of Z(j omega ). Estimated element values were of the order of 27 Ns/m for R, 7 * 10^-3 kg for M, and 5 * 10^-3 m/N for C. Values for R and M are comparable to those determined for other body tissues, while the relatively large C of the abdomen results in a lower observed resonance frequency. This information provides a framework to accurately detect and interpret low-level sonic vibrations on the surface of the maternal abdomen.