Transcript Overview of Ch.12.7: Doppler Effect

Music, Math, and Motion
Physics of Sound Ch.12 § 7-9
Dr. E.J. Zita
The Evergreen St. College
Winter week 8
Friday 27 Feb. 2009
Ch.12c: Doppler effect, Shock
waves, and Applicaations
• Overview of 12b: § 7, 8, 9
• Discuss sections
• Practice problems together
• Choose homework together?
• Looking ahead
Overview of Ch.12.7: Doppler Effect
Both observers hear the same siren tone
When the source is “fixed”, or not moving
Who hears a higher frequency when the
truck is moving?
How much higher?
Frequency of the source (firetruck siren) f never changes, but the perceived
frequency f’ depends on what?
• Whether the truck is approaching you or moving away
• How fast
• Whether you are approaching a stationary truck? Would this be the same as the truck
approaching you?
Overview of Ch.12.8: Shock waves and
the Sonic boom
Overview of Ch 12.9 Applications:
Sonar, Ultrasound, and Medical imaging
Speed of sound in human tissue v ~ 1540 m/s (depends on what kind of tissue!), close to vwater
Jigsaw learning-through-discussion
1. Count off by 7, 8, 9
• Each team discuss your section
• Identify key points
• Zita will circulate, answering questions
2. Mix teams so there is one expert for each chapter
on each team
• Discuss all sections – share your key points
3. Whole group gather: resolve questions, share
insights and examples, and choose homework
together
More on Ch.12.7: Doppler Effect
More on Ch.12.7: Doppler Effect
Truck moving away from stationary observer:
More on Ch.12.7: Doppler Effect
Observer moving toward stationary source:
More on Ch.12.7: Doppler Effect
for light is “Redshift” (or “blue shift”)
http://snap.lbl.gov/science/darkenergy.php
More on Ch.12.7: Doppler Effect for light is
“Redshift” (or “blue shift”)
Planets are discovered by observing red and blue
shifts in the light of the star they orbit. Therefore,
most of the planets discovered are very large and close to their
parent star – the small or far ones have little Doppler effect.
Only the far right model fits observed evidence.
http://universe-review.ca/I07-20-detection.jpg
http://snap.lbl.gov/science/darkenergy.php
More on Ch.12.8: Shock waves and the
Sonic boom
Cerenkov radiation and the Solar Neutrino
Problem: one way to count the number of electron
neutrinos (n) coming from the Sun is to circulate
cleaning fluid in a huge vat, counting how many
Chlorines convert to Argon:
p+n n+e
37Cl
+ n  36Ar + e
When electrons travel faster than light in cleaning
fluid, they cause shock waves, emitting blue light.
• The orientation of the shock cone tells us the
direction the e came from, and the
• the angle of the shock cone tells us the e speed
http://www.sns.ias.edu/~jnb/Papers/Popular/Scientificamerican69
/scientificamerican69.html
http://abyss.uoregon.edu/~js/hc209/lectures/lec10.html
Why is Cerenkov light is blue? There are basically two reasons.
In water, the blue light comes from excited atoms that emit blue light.
The atoms in the water become excited by the Cerenkov shock wave
and then de-excite, emitting blue light.
In addition, the number of photons emitted by a Cerenkov electron is
inversely proportional to wavelength (or proportional to the energy).
Since these are high energy particles, this means that more photons are
emitted with shorter wavelengths, thereby tilting the spectrum to the blue side.
Amended from
http://www.physlink.com/Education/AskExperts/ae219.cfm
More on Ch 12.9 Applications: Sonar,
Ultrasound, and Medical imaging
X-ray: echoing hi-energy (high-frequency, short-wavelength)
electromagnetic waves – the first internal imaging technique,
discovered accidentally by Roentgen in late 1800s.
Ultrasound: high frequency (short-wavelength, therefore highresolution) sound waves are bounced off hard and soft tissue,
revealing internal details (such as fetal presence or deformity)
CAT-scan (Computer-Aided Tomography) formerly known as
EMI scan, developed by the music and recording business EMI.
Provides detailed, cross-sectional views.
PET scan (Positron Emission Tomography) measures radiation
(positrons, gamma rays, or other particles) given off by the
body after ingestion or injection of radioactive tracers. This
enables dianosis of flows and dynamics in body, e.g. detection
of cancer spread.
MRI scan (Magnetic Resonance Imaging – formerly known as
Nuclear magnetic resonance) In the presence of a strong
background field, a second field of varying radio
http://www.radiologyinfo.orgfrequency is rapidly oscillated. The
different oscillations are absorbed preferentially by different
tissues, revealing, for example, damage to bone, cartilage, soft
tissue, or slipped disks.
http://www.radiologyinfo.org
Let’s try some HW together – you choose which
Problems from Ch.12-8
General Problems Ch.12.7-9
Doppler Effect:
Shock Waves:
Looking ahead