BASIC SCIENCE SECTION Effects of vibration frequency and tissue thickness on intrauterine sound levels in sheep Douglas S. Richards, MD: Robert M. Abrams, PhD: Kenneth J. Gerhardt, PhD: and Mary E. McCann, MAb Gainesville, Florida Pure-tone sinusoidal mechanical vibratory stimuli ranging .in frequency from 4 to 4000 Hz were applied to the ventral abdominal wall of pregnant ewes, and intrauterine sound pressure levels were recorded with a hydrophone 5, 10, 15, and 20 cm from the surface. There were significant decreases in sound pressure levels with increasing frequencies (p < 0.0001) and increasing depth (p = 0.01). There was no significant interaction between these two variables. An electronic artificial larynx was also activated on the abdominal wall, resulting in quite high sound pressure levels (mean, 119 dB) measured 5 cm from the surface, with a significant decrease in sound pressure levels as the thickness of the intervening maternal tissues increased (p = 0.005). These results suggest that the frequency of vibratory stimulus used and the thickness of the maternal abdominal wall could influence fetal response to the vibroacoustic stimulation test by affecting intrauterine sound pressure levels. (AM J OBSTET GYNECOL 1991 ;165:438-42.)

Key words: Fetal sound, antepartum testing, acoustic stimulation, sheep The vibroacoustic stimulation test has become popular as a test of fetal well-being. The factors that affect intrauterine sound pressure produced by the vibratory devices used for this test have not been adequately addressed. An understanding of these factors is important to develop testing procedures in which fetuses receive reproducible stimuli of appropriate intensity. In an earlier study, Gerhardt et al. 1 reported high sound pressure levels produced by an electronic artificiallarynx applied to the abdom