This was sent to me by a midwife friend (initially a speech therapist)who maintains that the increased numbers of babies with hearing loss might have a whole lot to do with the introduction of ultrasound.
Abrazos, Joni


ULTRASOUND SCANS ARE AUDIBLE TO A FETUS,
researchers reported at this week's meeting of the Acoustical Society of
America in Fort Lauderdale, Florida. Ultrasound by definition is sound
that lies beyond the range of human hearing. So how can a fetus hear an
ultrasound scan? As explained by the researchers (Mostafa Fatemi, Mayo
Foundation, Minnesota, fatemi.mostafa@mayo.edu), traditional imaging
systems produce ultrasound as sequences of short-duration, high-energy
bursts, called "pulse trains." When the pulses enter the body, they tap
internal organs at a regular rate. When the ultrasound points at the
head of the fetus, its sensitive hearing structure gets vibrated at a
rate equal to the number of pulses per second. (Typically, several
thousand pulses are transmitted per second in a pulse train, a rate
equal to several thousand Hertz.) The fetus senses these vibrations as
tones, equivalent to the high notes of a piano. The sound can get
loud--about the equivalent of 100-120 decibels of airborne sound, or the
level of sound of an approaching subway train. Rather than being akin to
a sound from the outside world, though, the sensation is more like what
you hear when your finger taps a spot close to an ear-which is why it's
inaudible to others, including the mother. What's more, the sound is focused on a tiny, square-millimeter spot, and the sound diminishes rapidly from that spot, so that the fetus could quickly adjust its position to avoid the loudness. Fatemi stresses that their findings do not suggest that this sound is harmful to a fetus. These studies can help explain physicians' observations that a fetus moves vigorously when ultrasound is directed at its head. They eliminate the notion that ultrasound is a passive observation technique, but they may also inspire new ultrasound exams for testing normal fetal function. (Paper 1pBB6 at meeting; abstract at http://asa.aip.org/asasearch.html)