Sound Pressure Gradients Across the Head of Fetal Sheep in Utero During Vibroacoustic Stimulation

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Abstract
This study was undertaken to determine if sound pressures reaching the right and left ears of the sheep fetus are equal during vibroacoustic stimulation using an electronic artificial larynx (EAL). Three ewes (one carrying twins) were sacrificed as part of other experiments. Two miniature hydrophones were fixed to skin in the left and right temporal areas of the exteriorized fetal heads. The fetal head was replaced in the uterus and aligned such that the two hydrophones were in line with the axis of stimulation and at right angles to the abdominal surface. Intrauterine sound pressure levels (SPL) from both hydrophones were compared during extraabdominal stimulation with an electronic artificial larynx. SPL at the proximal hydrophone ranged from 138-145 dB (re: 20 μPa), and were on average 17 dB greater than SPL from the distal hydrophone. One-third octave band analyses of the two hydrophone outputs revealed marked differences in frequency content. For example, differences in SPL were 19 dB at 100 Hz. 25 dB at 2,000 Hz, 34 dB at 5,000 Hz and 15 dB at 20,000 Hz. During vibroacoustic stimulation using an EAL, a significant differential in sound pressure level and frequency content reaching the right and left fetal ears may occur.

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