In-utero sound levels when vibroacoustic stimulation is applied to the maternal abdomen: an assessment of the possibility of cochlea damage in the fetus.

OBJECTIVE

To measure sound pressure level in utero while a vibro acoustic stimulator is applied to maternal abdomen and to calculate whether the estimated effect on fetal cochlea cilia vibration would be hazardous.

DESIGN

Prospective descriptive study.

SETTING

Labour ward, National University Hospital, Singapore.

SUBJECTS: Eight women undergoing induction of labour.

INTERVENTION: A hydrophone was introduced via the cervix into the uterus and placed under ultrasound guidance near the fetal ear. Sound pressure was recorded when a vibroacoustic stimulator was applied directly to the maternal abdomen and also when separated by 2 cm of air.

MAIN OUTCOME MEASURES: Sound pressure levels in utero.

RESULTS

The sound level recorded from the vibrator diaphragm in air was 107 dB at 2 cm and 74 dB at 1 m. The mean sound pressure level in utero was 90.7 dB (range 75-96 dB) when the vibrator was in contact with the abdominal wall and 80.1 dB (range 70-88 dB) when separated by 2 cm of air.

CONCLUSIONS

Analysis of factors affecting displacement of cochlear sensing cilia in utero show that, for equal sound pressures, sound intensity and sound vibration are about 4000 times less in amniotic fluid, compared to that produced in air. Further protection is provided by viscous and hydrodynamic features of the ear. The estimated effect on cilia vibration by the mean sound pressure registered in utero, about 90 dB, corresponds to that produced postnatally by an airborne sound registering about 40 dB, which would not be hazardous.