Abstract:
The importance of understanding noise exposure effects on pregnant women and domestic animals was made apparent following findings that environmental sounds readily penetrate the abdominal wall, uterus, and fetal fluids. Relatively low-frequency sounds (<0.5 kHz) are transmitted, with sound pressure levels (dB re: 20 (mu)Pa) often higher in utero than SPLs in the air surrounding the abdomen. Overall shapes of intra-abdominal isosound pressure contours reveal important differences in the manner in which airborne and vibroacoustic sounds pass through the abdomen. Measurements of fetal cochlear microphonics show attenuation to be 10--15 dB for sounds 0.5 kHz and 40--45 dB for sounds between 0.5--2.0 kHz. Sound transmission is predominantly through bone conduction rather than through the fetal external and middle ear systems. Elevated thresholds for hearing can result from high-intensity noise exposure in fetal sheep. Nonauditory effects in humans include increases in heart rate and heart rate variability and body movements. Changes in behavioral state in the sheep fetus are easily induced with amplitude and frequency-modulated signals produced at the abdominal surface of the ewe. Stimulation in both REM and non-REM sleep led to a change in electrocortical activity, implying central arousal. [Work supported by NIH and March of Dimes Birth Defects Foundation.]