I'm still a little puzzled.
I'll guess that the midwife used a Medsonics or Medasonics FP3A Doppler
Fetoscope (Medsonics changed their name to Medasonics about that time for
legal reasons that I never understood). This device was an obstetrical
version of the "pocket rocket" that was commonly used for fetal
monitoring. I'm confident that this was a CW Doppler operating at 3 MHz
(I'm guessing from the 3 in the model number). Because it was CW, there
is no Pulse Repetition Frequency to bang on the ear.
I searched on the WEB for "dog whistles" and found "The Acme Silent Dog
Whistle produces ultra high frequencies in the range of 5800Hz. to
12400Hz." with an audible example that you could play through your
computer speakers. Another site said "Typically, a dog whistle is within
the range of 16000 Hz to 22000 Hz with only the frequencies below 20000 Hz
audible to the human ear." Note that your computer sound card probably
samples at 44,000 Hz so the Nyquest frequency is 22,000 Hz, the highest
frequency that cna be correctly recorded and played. The computer card
was probably designed for dog whistles.
Here is the only explanation that I can think of. Both you and the
midwife had fetal Dopplers, hers (or his, can a man be a midwife? Is my
assumption sexist?) and yours. Hers was transmitting at 3.000 MHz and
yours was transmitting at 3.001 MHz, a 1 KHz difference. Because of the
nonlinear sound propigation in the tissue and amniotic fluid, two
additional frequencies were generated, 6.001 MHz, the sum frequency and
0.001 MHz = 1 KHz) the difference frequency. The fetus was hearing the
difference frequency, when the two transmitted ultrasound sound beams
So how did this fetal experience affect your son's taste in music?
On Tue, 4 Apr 2006, Johnson, Bill wrote:
> Bill Johnson, Yakima WA
> I wish I had your elegant explanation of how fetuses "hear" ultrasound
> 22 years ago. Our midwife had a new hand held Doppler, and got
> frustrated with our son since every time she used it he would turn in
> the womb. It was completely obvious to my wife that he "heard" the
> ultrasound, since he would be lying peacefully until the midwife touched
> her with the Doppler. I told her he "sensed" it but I doubted he could
> hear it. "It uses energy," I said, and mumbled something the amniotic
> fluid might enhance the effect, but the frequency was above human range.
> In spite of my explanation (or because of it) she figured we could get a
> dog whistle to call him home when he was old enough to run around.
> The midwife said he was the only one of the eight or so babies she was
> preparing to birth that reacted that way, a positive aversion to the
> monitor. When she asked what I did for a living, she was quite taken
> aback. "Don't expect him to follow in your footsteps!"
> Never tried the dog whistle.
> On Sat, 1 Apr 2006, Kirk Beach wrote:
> "As an expert in obstetrics, you know that a fetus will "hear" an
> ultrasound examination, not because the fetal ear can hear 3 MHz
> ultrasound (they don't "hear" CW Doppler fetal monitors) but because the
> radiation force of the pulsed ultrasound bursts, at a 5 KHz PRF for
> Doppler or at a 30 Hz frame rate for 2-D B-mode imaging, knocks on the
> oval window of the coclea (and probably even on the hair cells in the
> coclea) to allow the fetus the "hear" the ultrasound PRF, which is
> their hearing range."
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