Transmission of Airborne Sound from 50-20,000 Hz into the Abdomen of Sheep

The transmission of audible sounds from the environment of the pregnant woman to the foetus is of growing interest to obstetricians who utilize foetal vibracoustic stimulation in their examinations, and to occupational health professionals who believe that high-intensity sound in the workplace is potentially damaging to the foetus. Earlier reports on transmission of sound into the abdomen and uterus of sheep revealed a significant amount of sound attenuation at frequencies above 2,000 Hz. and some enhancement at frequencies below 250 Hz. However, frequencies above 10,000 Hz, and stimulus levels as possible variables, were not studied. In this report, the effects of frequency from 50-20,000 Hz. and stimulus levels (90 to 110 dB sound pressure level), were studied in five sheep. Sound attenuation varied as a function of frequency (p<0.001). Sound attenuation varied inversely as a function of stimulus level for low frequencies (50-125 Hz) and for high frequencies (7,000–20,000 Hz) (p<0.001). In the mid frequency range (200-4,000 Hz), no effect of stimulus level (p=0.96) was found. Additionally, in the 800-2,000 Hz range there was enhancement of sound pressure of up to 10 dB.