CHAPTER 9. DOPPLEREFFECT 9.2
Ultrasound and the Doppler Effect ESCEJ
Ultrasonic waves (ultrasound) are sound waves with a frequency greater than 20 000 Hz
(the upper limit of human hearing). These wavescan be used in medicineto determine
the direction of blood flow. The device, called aDoppler flow meter, sends out sound
waves. The sound waves can travel through skin and tissue and will be reflected by
moving objects in the body (like blood). The reflected waves return to the flow meter
where its frequency (received frequency) is compared to the transmitted frequency.
Because of the Dopplereffect, blood that is moving towards the flow meter will change
the sound to a higher frequency and blood that is moving away fromthe flow meter
will cause a lower frequency.
SkinBloodTissuered blood cellsdirection of flowtransmitter receiverUltrasound can be usedto determine whether blood is flowing in the right direction
in the circulation systemof unborn babies, or identify areas in the body where blood
flow is restricted due tonarrow veins. The use of ultrasound equipmentin medicine is
called sonography or ultrasonography.
Exercise 9 - 1
- Suppose a train is approaching you as you stand on the platform atthe station.
As the train approachesthe station, it slows down. All the while, the engineer is
sounding the hooter ata constant frequency of400 Hz. Describe the pitch and
the changes in pitch that you hear. - Passengers on a trainhear its whistle at a frequency of 740 Hz. Anjais standing
next to the train tracks. What frequency does Anjahear as the train moves directly
toward her at a speed of25 m·s−^1?