Abstract:

Sounds which originate outside the abdomen of pregnant women reach the fetal inner ears after being filtered by the materials surrounding the fetal head and by its skull. In sheep, exogeneous sounds are low-pass filtered from 0.25 to 4.0 kHz at a rate of 6 dB/octave before reaching the head. There is little difference in sound pressures for low-frequency signals when recorded inside and outside the abdomen. The transmission route for fetal stimulation is via bone conduction with low-frequency sounds (<0.25 kHz) being attenuated by about 10 dB and higher-frequency sounds (0.5--2.0 kHz) attenuated by up to 45 dB. The amplitude and waveform of an impulse is influenced markedly by the location of the recording hydrophone within the uterus. Close to the intra-abdominal wall, the impulse is best characterized as a simple Friedlander wave (duration <1.0 ms) with a peak sound pressure level (pSPL) that is approximately 5 dB less than that recorded in air. With the hydrophone located deep inside the uterus, the pSPL is reduced by over 15 dB and the waveform resembles that of an impact with a decay time of greater than 12 ms. Fetal cochlear microphonic resembles the waveform of the impulse when it is recorded with a hydrophone located by the fetal head.