Aemil J.M. Peters

Cochlear microphonics recorded from fetal and newborn sheep

PURPOSE Sounds present within the uterus stimulate the fetal inner ear and central auditory pathway. This study was undertaken to determine the efficiency of transmission of exogenous airborne stimuli to the fetal inner ear. In this way, we may quantify the extent to which the fetal auditory system is isolated from sounds produced outside the mother. MATERIALS AND METHODS Cochlear microphonics were recorded from fetal and newborn sheep to evaluate the extent to which the fetus is isolated from sounds exogenous to the ewe. Electrodes were surgically placed in contact with the round window membrane in nine near-term fetal sheep. Cochlear microphonics were recorded in response to 1/3 octave-band noises (0.125 to 2.0 kHz) delivered through a loudspeaker 1.8 m from one side of the pregnant ewe. Sound pressure levels generated by the noises were simultaneously recorded ex utero with a microphone and in utero with a hydrophone previously sutured to the fetal neck. After cochlear microphonic amplitudes were recorded, the fetus was delivered through an abdominal incision. Recordings were repeated from the newborn lamb. Fetal sound isolation was calculated as the difference between the sound pressure levels that were necessary to evoke equal cochlear microphonic amplitudes from the fetus and from the newborn lamb. RESULTS The sound attenuation observed was variable for all frequencies. The fetus was isolated from external sounds by 11.1 dB for 0.125 kHz, 19.8 dB for 0.25 kHz, 35.3 dB for 0.5 kHz, 38.2 dB for 1.0 kHz, and 45.0 dB for 2.0 kHz. CONCLUSIONS Other investigators have demonstrated that the immature auditory system is more susceptible to damage produced by noise exposure than is the mature auditory system. Low-frequency noise produces damaged cells that later in life code higher frequencies. A possibility of fetal hearing loss produced by intense noise exposure needs more careful evaluation.

Three-dimensional intraabdominal sound pressures in sheep produced by airborne stimuli.

OBJECTIVE Our aims were to investigate how airborne sound was distributed within the abdominal cavity of sheep as function of frequency. STUDY DESIGN Airborne broad-band noise was measured with a hydrophone at 45 locations within the abdomen of five nonpregnant sheep post mortem and with a microphone extraabdominally. Sound pressure attenuation provided by the abdomen and its contents was determined for frequencies between 50 and 5000 Hz. An analysis of variance was applied to assess the effects of frequency, hydrophone location, and animal on intraabdominal distribution of sound pressures. RESULTS Below 250 Hz sound pressure was higher inside the animal than outside. Little attenuation (< 10 dB) was found for sounds > 3000 Hz. Attenuation was greatest in the center of the abdomen and least along the inner margin of the abdomen. Intraabdominal sound pressure level varied with frequency (p < 0.0001) and with position of the hydrophone in the cross-sectional plane (p < 0.005) but not in the sagittal plane (p = 0.51). There was no animal effect (p = 0.18). CONCLUSION During maternal exposure to airborne, broadband noise the fetus could be subjected to intense sound pressures at low frequencies regardless of position within the uterus and at high frequencies when positioned near the abdominal surface.

Noise induced hearing loss in fetal sheep

The auditory brainstem response (ABR) was recorded in utero from chronically instrumented fetal sheep prior to and following exposure of pregnant ewes to intense broadband noise (120 dB SPL for 16 h). ABRs were elicited by clicks and tone bursts (0.5, 1, 2, and 4 kHz) delivered through a bone oscillator secured to the fetal skull. Latency-intensity functions for most of the four vertex-positive waves (labelled I-IV) were prolonged and ABR thresholds were temporarily elevated by an average of 8 dB following the noise exposure. Results show that exogenous sounds can penetrate the uterus and result in alterations of the fetal ABR.

Intraabdominal sound levels during vibroacoustic stimulation.

Vibroacoustic stimulation is widely used in the United States as a test for fetal well-being, yet little information is available on the adequacy of the electronic artificial larynx that is commonly used as the stimulator. To determine whether the intraabdominal sound pressure level was dependent on the static force applied to the electronic artificial larynx, we measured the sound pressure level at a position 20 cm from the surface of the anterior abdominal wall in nonpregnant ewes. The electronic artificial larynx was pressed against the surface with three levels of static force: mild, moderate, and strong. Between mild and strong static forces there was a trend toward a reduction in sound pressure level at the fundamental frequency (85 Hz) and the overtones (170 to 1600 Hz) (p less than 0.08). To further examine the relationship between the static force of sound source against the abdomen and the intraabdominal sound pressure level, sine wave oscillations between 20 Hz and 4.0 kHz were produced with an industrial shaker. With a constant dynamic force (0.16 N) applied to the shaker across frequencies, the sound pressure level was greatest at lower frequencies (100 to 110 dB) and less at higher frequencies, above 1.0 kHz (60 to 80 dB). Consistent with the electronic artificial larynx experiments, strong application of the shaker resulted in greater transmission of sound than did mild application (p less than 0.02).

Three dimensional sound and vibration frequency responses of the sheep abdomen

There is growing awareness and concern about reproductive hazards in the workplace. Sound and vibration have long been recognized as occupational hazards, but there are no guidelines or consensus standards for foetal protection. A three-dimensional analysis of sound pressure level (SPL) within the abdomen was used to show how sound and vibration of various frequencies are transmitted throughout the abdominal segment of sheep. Sinusoidal stimulation was provided with a mechanical vibrator placed on the anterior abdominal wall just posterior to the umbilicus. A hydrophone was placed at 45 intraabdominal locations within a space measuring 20×20×25 cm centered over the mechanical oscillator. Pure tones of 10, 100, 1000 and 2000 hertz were used to stimulate the abdominal wall. Intraabdominal SPL was negatively correlated with distance between the hydrophone and the vibrator (p<0.0001) and was negatively correlated with frequency of vibration (p<0.0001). The rate at which the system attenuated high frequencies was greater than for lower frequencies and it attenuated higher frequencies more uniformly throughout the entire abdomen (p<0.0001).

Vibration of the Abdomen in Non-Pregnant Sheep: Effect of Dynamic Force and Surface Area of Vibrator

Vibration exposure is a growing concern for increasing numbers of working pregnant women. The coupling between vibrating machines and the body determines to a great extent the degree of exposure and thus any maternal or foetal morbidity. In the present experiments we determined the effect of dynamic force and surface area of a vibrator on intraabdominal sound pressure level (SPL). Intraabdominal SPL was measured with a miniature hydrophone during mechanical vibration between 1–5000 Hz. Three different dynamic forces: 0.07, 0.14 and 0.42 newtons (N), and actuator head sizes: 2, 8 and 32 cm 2 were used with a constant static force of 9 N. Increasing the applied dynamic force increased the generated SPL (p<0.0001). The different actuator head sizes did not meaningfully affect sound transmissibility across frequencies.

Vibroacoustic Stimulation with a Complex Signal: Effect on Behavioral State in Fetal Sheep

An ideal vibroacoustic stimulus for testing fetal reactivity has yet to be developed. In the present study in fetal sheep we tested the effect on behavioral state of an amplitude and frequency-modulated signal produced at the abdominal surface of the ewe. The stimulus was presented during periods of fetal non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. Evaluation of behavioral state was accomplished by visual observation of strip-chart recordings. Assessed in this manner, vibroacoustic stimulation during NREM sleep consistently resulted in a change to an indeterminate state. However, stimulation during REM sleep failed to have an effect. Additional evaluation applying spectral analysis to the fetal electrocorticogram during stimulation in NREM sleep revealed a marked decrease in delta band power from 100 to 27 +/- 5% and in theta band power from 100 to 40 +/- 4% resulting in a decrease in total power from 100 to 35 +/- 4% (p < 0.05). Stimulation during REM sleep revealed a significant increase in beta band power from 100 to 123 +/- 14%. Vibroacoustic stimulation in both NREM and REM sleep led to an increase in spectral edge frequency, implying central arousal.

Maternal sound level device and method for protecting fetal hearing

The present invention is a maternal sound level device that functions to protect fetal hearing by alerting the mother whenever her fetus is exposed to sound pressure levels (SPL) which exceed predetermined levels. The device is comprised of a microphone (14) for SPL detection, preamplifier (2), weighting network (4), level detector (6), and alarm mechanism (8). This device is battery operated and can be easily clipped on a belt so as to be worn by pregnant women at the abdominal level to monitor SPL at the abdomen. SPLs are measured in the range of greater than 80 dB and less than 100 dB over a frequency range of greater than 30 Hz and less than 10,000 Hz. The mother is alerted to SPLs that are exceeding a predetermined level to protect the hearing of the mothers developing fetus by a visual and/or a mechanical alarm. A distinguishing feature of the invention is the weighting network which enhances low frequency SPLs (greater than 30 Hz and less than 1000 Hz) by greater than 5 dB and less than 15 dB and attenuates high frequency SPLs (greater than 1000 Hz). In a preferred embodiment, an alarm signal will be presented to the user of the device when sound pressure levels in the vicinity of the user are greater than 80 dB at frequencies between greater than 30 Hz and less than 1000 Hz, with gradually increasing SPLs required with higher frequency such that 100 dB SPL is required to produce an alarm at greater than 4000 Hz and less than 5000 Hz.

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.

Resonance of the Pregnant Sheep Uterus

Low frequency mechanical vibrations introduced on the skin are known to pass through tissues with relative ease. How these vibrations affect the foetus in utero is a question of current concern since more and more women of reproductive age are in the workplace. In the present study we mechanically vibrated the abdomen of four term pregnant sheep, antepartum and immediately postpartum, at frequencies between 2–1000 Hz and at a constant dynamic force of 0.5 newtons. Sound pressure levels (SPL) were measured intraabdominally with a miniature hydrophone fixed loosely to one side of the foetal head. Thereafter, ewes and foetuses were sacrificed, foetuses were delivered and the hydrophone replaced in the abdominal cavity in its former position. SPL showed a peak amplitude between 6 and 18 Hz in both the antepartum and postpartum conditions (p <0.01) suggesting a resonance phenomenon of the abdomen in this frequency range.