Christopher L. Morfey
Roger J. Pinnington
Inst. of Sound and Vib. Res., Univ. of Southampton, Southampton SO17 1BJ, United Kingdom
A common noninvasive test for fetal well-being involves transmitting an audio-frequency acoustic signal into the amniotic fluid by means of a mechanical vibrator, which is applied to the mother's abdomen close to the fetal head. This test has been simulated in the laboratory, using a silicone rubber model uterus of spherical shape filled with water. The outside of the model was excited by an impedance head and a load-distributing contact disk. Mechanical impedance measurements were compared with similar measurements on volunteer subjects at about 30 weeks' pregnancy, and showed similar trends as a function of frequency. The model was then used to establish a transfer function between the exciting force and the acoustic pressure in the water-filled cavity. A simple theoretical model is presented which accounts for the main features observed experimentally. Results indicate that commercial fetal stimulators can produce intrauterine sound pressures as high as 30 Pa rms (180 Pa peak pressure) close to the point of excitation. The actual level depends on the thickness of the subcutaneous fat layer which couples the vibrator to the uterus.