Sound environment of the fetal sheep Kenneth J. Gerhardt, PhD: Robert M. Abrams, PhD," and Calvin C. Oliver, PhDc Gainesville, Florida The internal sound pressure levels within the intact amnion of pregnant ewes surgically implanted with a hydrophone was determined during conditions of quiet and during sound field exposures to broadband and octave-band noise. Measurements were made of sound pressures outside and inside the ewe, and sound attenuation through maternal tissues and fluids was calculated. Sound pressures generated by low frequencies «0.25 kHz) were 2 to 5 dB greater inside than outside the ewe. Above 0.25 kHz, sound attenuation increased at a rate of 6 dB per octave. For 4.0 kHz, sound attenuation averaged 20 dB. The sound pressure recorded at different locations within the amnion with respect to the sound source varied by up to 6 dB. The internal noise floor in the absence of externally generated sounds was as low as 50 dB (spectrum level) above 0.2 kHz. Thus the fetus is developing in an environment that is rich with internal and external sounds. (AM J OesTET GVNECOL 1990;162:282-7.)
Key words: Fetus, sheep, sound, sound attenuation The sound environment of the fetus has been the subject of several recent studies. l . s There is general agreement that sounds within the mother, particularly those associated with respiration, gastrointestinal activity, and circulation, provide an intrauterine background above which maternal vocalizations and some external sounds are present. However, the degree of contribution of external sounds to the fetal auditory experience has been debated. A marked attenuation of external sounds and a significant masking by internal noise levels were inferred from results of experiments in humans. Thus in one study calculations of sound attenuation were made in a woman at 37 weeks' gestation by using externally delivered pure tones from 200 Hz through 4000 Hz.7 An air-coupled loudspeaker was held in a foam-rubber annulus one-half inch from the abdomen and directly over the fetal head. A crystal microphone encased in a rubber covering was placed near the cervix. Attenuation of sound pressure was 19 dB at 200 Hz and increased to ~48 dB at 4000 Hz. Grimwade et al. 8 made similar measurements in nine pregnant women by using a small condenser microphone covered by a rubber sleeve. The microphone From the Department of CommUnicatIOn Processes and DISorders. College of Ltberal Arts and Saences, and the Institute for Advanced Study of the CommUnication Processes," the Department of Obstetncs and Gynecology, College of Medicme,' and the Department of Mechanical Engmeermg, College of Engmeering,' UnIvemty of Flonda. This study was aided by ReproductIVe Hazards in the Workplace Research Grant No. 15076 from the March of Dimes Btrth Defects Foundation and by National Institutes of Health Grant No. 20084. ReceIVed for pubhcatlOn December 21, 1988; revised July 7. 1989; accepted August 4, 1989. Repnnt requests: Kenneth J. Gerhardt, PhD, Department of CommunicatIOn Processes and Disorders, Dauer 335. Univemty of Flonda, Gamesville, FL 32611. 611115878
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was passed through the cervical canal after membrane rupture and was placed at the level of the fetal head. Sound attenuation ranged from 39 dB at 500 Hz to 85 dB at 5000 Hz. In a later article the same authors provided correction factors for their data to compensate for the impedance mismatch of the fluid, rubber, and microphone diaphragm and to compensate for the normal barometric pressure behind the diaphragm, which was sealed by a rubber membrane. 9 The corrected values still indicated attenuation as high as 70 dB for frequencies above 2000 Hz. A theorylO developed by Westervelt, which permitted prediction of the sound attenuation in utero, was consistent with the empiric data provided by the authors cited above. Westervelt concluded that when fraction q of the effective area of the human body is insonified, the sound attenuation for any frequency below the resonant frequency of existing gases in the body is 20 log q decibels. At frequencies above the resonant frequency of existing gases, the sound attenuation was predicted to increase at a rate of 12 dB per octave. Serious doubts of the accuracy of these intrauterine sound pressure readings and of the applicability of the Westervelt model were raised after the publication of further studies in the human' and sheep, 1. 2. 5 in which hydrophones were used to measure intrauterine sound pressure levels. Querleu et al. 3 concluded from their work with pregnant women that sound from outside the mother was rarely attenuated by more than 30 dB. In sheep the attenuation in the higher frequencies (>4.0 kHz) was 20 to 30 dB, but in the lower frequencies «0.2 kHz) the sound level was enhanced by a few dB." 11 Several other attributes of the fetal sound environment were described by these authors. Growing interest in the use of acoustic stimulation as an adjunct to the antepartum fetal nonstress test l2 . 16 prompted us to measure the intrauterine sound pres-
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Key words: Fetus, sheep, sound, sound attenuation The sound environment of the fetus has been the subject of several recent studies. l . s There is general agreement that sounds within the mother, particularly those associated with respiration, gastrointestinal activity, and circulation, provide an intrauterine background above which maternal vocalizations and some external sounds are present. However, the degree of contribution of external sounds to the fetal auditory experience has been debated. A marked attenuation of external sounds and a significant masking by internal noise levels were inferred from results of experiments in humans. Thus in one study calculations of sound attenuation were made in a woman at 37 weeks' gestation by using externally delivered pure tones from 200 Hz through 4000 Hz.7 An air-coupled loudspeaker was held in a foam-rubber annulus one-half inch from the abdomen and directly over the fetal head. A crystal microphone encased in a rubber covering was placed near the cervix. Attenuation of sound pressure was 19 dB at 200 Hz and increased to ~48 dB at 4000 Hz. Grimwade et al. 8 made similar measurements in nine pregnant women by using a small condenser microphone covered by a rubber sleeve. The microphone From the Department of CommUnicatIOn Processes and DISorders. College of Ltberal Arts and Saences, and the Institute for Advanced Study of the CommUnication Processes," the Department of Obstetncs and Gynecology, College of Medicme,' and the Department of Mechanical Engmeermg, College of Engmeering,' UnIvemty of Flonda. This study was aided by ReproductIVe Hazards in the Workplace Research Grant No. 15076 from the March of Dimes Btrth Defects Foundation and by National Institutes of Health Grant No. 20084. ReceIVed for pubhcatlOn December 21, 1988; revised July 7. 1989; accepted August 4, 1989. Repnnt requests: Kenneth J. Gerhardt, PhD, Department of CommunicatIOn Processes and Disorders, Dauer 335. Univemty of Flonda, Gamesville, FL 32611. 611115878
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was passed through the cervical canal after membrane rupture and was placed at the level of the fetal head. Sound attenuation ranged from 39 dB at 500 Hz to 85 dB at 5000 Hz. In a later article the same authors provided correction factors for their data to compensate for the impedance mismatch of the fluid, rubber, and microphone diaphragm and to compensate for the normal barometric pressure behind the diaphragm, which was sealed by a rubber membrane. 9 The corrected values still indicated attenuation as high as 70 dB for frequencies above 2000 Hz. A theorylO developed by Westervelt, which permitted prediction of the sound attenuation in utero, was consistent with the empiric data provided by the authors cited above. Westervelt concluded that when fraction q of the effective area of the human body is insonified, the sound attenuation for any frequency below the resonant frequency of existing gases in the body is 20 log q decibels. At frequencies above the resonant frequency of existing gases, the sound attenuation was predicted to increase at a rate of 12 dB per octave. Serious doubts of the accuracy of these intrauterine sound pressure readings and of the applicability of the Westervelt model were raised after the publication of further studies in the human' and sheep, 1. 2. 5 in which hydrophones were used to measure intrauterine sound pressure levels. Querleu et al. 3 concluded from their work with pregnant women that sound from outside the mother was rarely attenuated by more than 30 dB. In sheep the attenuation in the higher frequencies (>4.0 kHz) was 20 to 30 dB, but in the lower frequencies «0.2 kHz) the sound level was enhanced by a few dB." 11 Several other attributes of the fetal sound environment were described by these authors. Growing interest in the use of acoustic stimulation as an adjunct to the antepartum fetal nonstress test l2 . 16 prompted us to measure the intrauterine sound pres-
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