Fetal vibroacoustic stimulation test: vibrator response characteristics in pregnant sheep postmortem.

OBJECTIVE

To determine the effect of small decreases in battery voltage on the sound pressure level and spectral features of sound within the uterus during vibroacoustic stimulation with a battery-powered electronic artificial larynx.

METHODS

In eight pregnant, recently sacrificed sheep, the fluid-borne, intra-amniotic sound pressures resulting from vibration of the abdominal wall with an electronic artificial larynx were compared with airborne sound pressures produced by an artificial larynx and measured at 50 cm.

RESULTS

At all frequencies, the intrauterine sound pressure was higher than the sound pressure in air. Decreasing battery voltage from 9 to 7 V resulted in a 5-18-dB decrease in intrauterine sound pressure at the electronic artificial larynx fundamental frequency and most overtones, but only a 4-8-dB decrease in the airborne signal. The results indicate that for frequencies relevant to fetal hearing, the reduction of sound pressure level caused by inadequate battery voltage may result in a difference in fetal response.

CONCLUSION

Routine verification of the function of a vibroacoustic device by means of a battery voltage check is recommended.