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

Abstract

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.

What sounds reach fetuses: Biological and nonbiological modeling of the transmission of pure tones

“…Concerning airborne stimuli -emitted via an aircoupled or distant source -human and sheep studies have demonstrated that their transmission depends on: (a) the frequency composition of the signal, and (b) the in utero location of the acoustic transducer (Peters, Abrams, Gerhardt, & Griffiths, 1993a). Frequencies below around 400 Hz suffer almost no pressure attenuation and may even be enhanced, relative to the external level (Peters, Gerhardt, Abrams, & Longmate, 1993b).…”

“…Studies performed by Vince and colleagues (1982;Vince et al, 1985), Gerhardt, Abrams, and Oliver (1990), and Richards et al (1992) suggested that in utero sound transmission is likely to occur according to a nonlinear pattern. More precisely, a recent study analyzing transmission of airborne sounds from 50 -20,000 Hz conducted by Peters et al (1993a) in sheep demonstrated, among other frequency-related effects, that: "...from 12,500 to 20,000 Hz there was a reversal of the attenuation pattern showing that the effectiveness of transmission gradually improved again..." (p. 22). Concerning the location of the transducer, isosonic curves defined by Peters et al (1993b) in sheep demonstrated that the in utero pressure level of an airborne source decreased when the distance to the source of the hydrophone placed in the cavity was increased.…”

Intelligibility of Sentences Recorded from the Uterus of a Pregnant Ewe and from the Fetal Inner Ear

“…The transmission of sound into the fetal environment in humans [Bench, 1968;Querleu et al, 1988] and sheep [Armitage et al, 1980;Peters et al, 1993] has been well documented. The fluids and tissues surrounding the fetus act as a low-pass filter.…”

“…More specifically, Bench reported 19 dB of attenuation at 0.2 kHz and up to 78 dB at 8.0 kHz. Peters et al [1993] examined the transmission of externally generated white noise at different levels from air into the uterus of a nonpregnant ewe. Negative attenuation, at 0.05, 0.063, 0.4, 1.0, and 1.25 kHz was noted.…”

“…The abdominal wall contents act as a low-pass filter and may even enhance low-frequency sounds [Peters et al, 1993]. The human and sheep abdomens act as low-pass filters with an 8 dB/octave roll-off from 1.25 to 10.0 kHz [Abrams et al, 1995;Armitage et al, 1980;Bench, 1968;Peters et al, 1993;Querleu et al, 1989].…”