Intrauterine noise levels produced in pregnant ewes by sound applied to the abdomen.

Sound pressure in amniotic fluid was created by application of either an electronic artificial larynx or a standard audiometric earphone to the abdominal surface of pregnant ewes. Sound transmission was assessed with a hydrophone placed near the ear of the fetus within the intact amnion. Sound pressures produced by the electronic artificial larynx located directly over the hydrophone averaged 134.9 dB, whereas the highest sound pressure produced by a similarly placed earphone was 95.2 dB. Sound transmission decreased dramatically with increasing distance between the sound source and the hydrophone; this may account for some of the variability in fetal response when the sound test is used clinically. Caution is recommended in administering the sound test until the effects of high noise levels on the fetus are better understood.