Today's Science Lesson: Doppler

Here is an everyday Joe low-down on Doppler and how you can hear baby's heartbeat (because you asked!)

All waves act the same way.  Light from the Sun.   Water in the ocean.  Sound from a car driving by.

Have you been to the racetrack?  When the cars are coming towards you they have a high pitched sound. Then when they go past you it is a lower pitched sound.  To me it sounds like " Vrrrrr-o-o-o-o-o-o-m".  

Hmm...why is that?  Well, the sound waves from the car's engine coming towards you compress (squish together) as they reach your eardrum.  Then when they go past, they spread back out (see cool diagram).  The difference in "squish-factor" (frequency) creates the difference in pitch. And it is related to how fast the car is going! Frequency is how Nerds measure how far apart the crests of waves are.  

Well, that is the same idea behind the doppler device at the doctor's office (say that three times fast).  The little hand-held device puts out sound at a one specific frequency.  As blood flows by, the sound waves bounce off blood cells back towards the device. Just like the race-car.  Blood cells coming TOWARDS the device bounce back at a different frequency than those going AWAY from the device.  A computer inside the device can use the difference between TOWARDS and AWAY to calculate how fast the blood is pulsing and you can hear the 150 beats per minute for Shrimpy.  That's how they know baby's heartbeat. 

PS: This is how the cops get you too!

You can find more via the link to the right. 

Sound waves are also how they make the cool ultrasound photos.  But sonar is a lesson for another day.

Thanks to: www.howstuffworks.com  for good examples, and the cool picture.