What fetuses and babies hear

Robert Traynor
March 25, 2014

iunBack in the late 1970s, I remember being a research assistant on a project that investigated the use of recorded intrauterine noise  to provide comfort to infants.  It was theorized that the infant had been listening to the internal sounds of their mother for a number of months in the womb and that these sounds would be comforting to the baby. This research was ongoing in many centers around the world and was one of the hot audiology research topics of the time.

Fast forward to 20014, and the actual sound environment of the iun2womb is very rich. There are various calculations as to the noise level, which seems to range somewhere between 30 and 96 dB.  Some researchers believe that the ambient sound a baby hears in the womb  mainly consists of blood running through the blood vessels and the movement of the stomach and intestines and actually reaches the level of about 90 dB (about the level of background noise in an apartment next to an elevated train). While the child’s developing ears can take those internal noise levels, exposure to very loud external noises can endanger an unborn baby’s hearing.

Deliege & Sloboda, 1996 report that the acoustic environment in which the fetus resides is composed of continuous cardiovascular, respiratory, and intestinal sounds that are punctuated by isolated, shorter bursts during maternal body movements and vocalizations.  While vibrations on the external surface of the mother’s abdomen can induce sounds inside the uterus, the distribution iun3of sounds seems to be confined to frequencies below 300 Hz.  The half-round soundiun4 pressure contours in the abdomen during vibroacoustic stimulation differ from the circular distribution of contours resulting from airborne sound pressure exposure. Deliege & Sloboda say that the static and dynamic forces of the vibrator and the vibrator distance from the target are also factors in sound transmission. Responses to sound are best described in animals and include changes in behavioral state, brain bloodflow, auditory brainstem response, and local cerebral glucose utilization along the central auditory pathway.

Research with hydrophones has revealed that the womb is a “relatively quiet place,” somewhat comparable to what we experience in our environment between 50 and 60 dB. Uterine sounds form a “sound carpet” over which the mother’s voice, in particular, appears very distinct and which the prenate gives special attention to because it is so different from its own amniotic environment.  To listen to these sounds click on the red picture of the womb.

These sounds are of major importance for baby and mother because they establish the first patterns of communication and bonding. Some researchers have discovered that newborns become calmer and more self-regulated when exposed to intrauteiun1rine sound (Murooka et. al 1976; DeCasper 1983; Rosner 1979). The soothing sounds of the ocean and water are probably reminiscent of the fluid environment in which we began life. Tomatis ( Click on the picture at right) suggests that the maternal heart beat, respiration, and intestinal gurgling all form the source for our collective attraction to the sound of surf and may have to iun6do with our inborn sense of rhythm. Tomatis and others feel that Prenatal sounds form an important developmental component in prenatal life because they provide a foundation for later learning and behavior. With fetal sound stimulation the brain functions at a higher level of organization. Since the ear first appears in the 3rd week of gestation and becomes functional by the 16th week, the fetus begins active listening by the 24th week. Research from ultrasound observations has shown that the fetus hears and responds to a sound pulse starting about 16 weeks of age (Shahidullah & Hepper, 1992); this is even before the development of the ear is complete. The cochlear structures of the ear appear to function by the 20th week and mature synapses have been found between the 24th and 28th weeks (Pujol et al. 1991). For this reason most formal programs of prenatal stimulation are usually designed to begin during the third trimester. The sense of hearing is probably the most developed of all the senses before birth. Four-month-old fetuses can respond in very specific ways to sound; if exposed to loud music, their heart beat will accelerate.

A Canadian Study on Sleep Machines

New research on infant sleep machines shows that when played at maximum levels, some of these devices — which play soothing sounds to help babies sleep — could exceed the noise limit recommended for infants at hospital nurseries.  Researchers tested 14 unnamed infant sleep machines that are commonly sold in the U.S. and Canada. They played 65 sounds at the maximum volume and tested them from three different distances: 30 centimeters (which is about the distance from the infant to the crib rail), 100 centimeters (placement near the crib), and 200 centimeters (across the room, or about 6.5 feet).iun5

All of the machines exceeded the 50-dB (dBA) noise limit that hospitals adhere to at the 30- and 100-centimeter distances. Three devices reached levels greater than 85 dBA from the 30-centimeter placement. If played consistently for eight hours, those sounds are higher than the noise limits set by the National Institute for Occupational Safety and Health for adult workers.

“Unless parents are adequately warned of the danger, or the design of the machines by manufacturers is changed to be safer, then the potential for harm exists, and parents need to know about it,” Dr. Gordon B. Hughes, the program director of clinical trials for the National Institute on Deafness and Other Communication Disorders who was not involved in the study, told the New York Times.

Senior study author Dr. Blake Papsin, otolaryngologist-in-chief at the Hospital for Sick Children in Toronto, told USA Today that he was also concerned that white noise may harm a developing brain, since infants need to hear all sounds in order to help their brains mature.  “Completely removing all informational content at a loud, potentially damaging level is the worst,” he said. The study authors urged parents to lower the volume and move the machines more than 200 centimeters away from the infant to prevent hearing loss.


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