Distance from an intrauterine hydrophone as a factor affecting intrauterine sound pressure levels produced by the vibroacoustic stimulation test

OBJECTIVE: The purpose of this study was to determine whether intrauterine sound pressure levels produced by vibroacoustic stimulation were associated with distance from an intrauterine hydrophone in human parturients and to evaluate the effects of distance on the spectrum of the stimulus. STUDY DESIGN: Measurements of intrauterine sound were taken in eight volunteer parturients in normal active-phase labor by use of an intrauterine hydrophone. Vibroacoustic stimulation was performed on the maternal abdomen directly overlying the hydrophone; at distances of 5, 10, 15, 20, and 25 cm from the hydrophone; and at the maternal sternum. Intrauterine sound pressure levels were tape-recorded for later analysis. Fetal heart rate and fetal movement were assessed with each vibroacoustic stimulation. Spectral anlyses were performed by taking the fast Fourier transform of the tape-recorded stimulation at each position. RESULTS: Analysis of variance with repeated measures indicated at a statiscally significant decrease (F = 4.1, P = 0.004) in the sound pressure levels as distance increased. Spectral analysis indicated large variability between and within subjects. CONCLUSION: Sound exposure of the fetal ear is on average decreased as the distance between the ear and the vibroacoustic stimulation is increased. The spectrum of the stimulus produced with vibroacoustic stimulation is highly variable.