Check for understanding
A band is playing, producing an intensity of 250 W/m2 measured directly in front of the tower of speakers. What is the corresponding decibel reading?
This concert goer usually listens to the band's music on their iPod with an intensity level of 3 X 10­4W/m2. What is the decibel difference between the two intensity levels?
HW
P. 470 QQ 14.2 C
P. 471 Q 14.4 Yes
EX 14.4 448 Hz
P. 492
13. intensity2 = 3 X 10­8 W/m2
14. a) P = 5 X 10­17W
b) P = 5 X 10­5 W
Sound Barrier and Shock waves
Pressure waves are formed at the front and back of the object like bow and stern waves on a boat. These pressure waves travel at the speed of sound.
BUT... as the object goes faster than the speed of sound, the waves end up compressing together and make a HUGE pressure wave that travels at the speed of sound.
Mach 1 = 761 mph
http://www.youtube.com/watch?v=gWGLAAYdbbc
Sonic Boom http://vimeo.com/9508176
meets Sun Dog http://vimeo.com/9410195
20 Hz Video
http://vimeo.com/30668685
Decibels measure Intensity Levels
Threshold of hearing
Doppler Shift
The apparent shift in frequency when there is relative motion between a source and an observer
Doppler shift for light
www.srh.noaa.gov/lzk/?n=svr1009dyr.htm
Ted Fujita
Doppler Shift Math
Doppler Review
1. What is the frequency heard by a person driving at 15 m/sec toward a blowing factory whistle ﴾f = 800 hz﴿ if the speed of sound is 340.6 m/sec?
2. What frequency would he hear after passing the factory if he continues at the same speed?
3. While standing near a railroad crossing, a person hears a distant train horn. According to the train's engineer, the frequency emitted by the horn is 440 hz. The train is traveling at 20.0 m/sec and the speed of sound is 346 m/sec.
a.﴿ What would be the wavelength of the train's horn if the train were at rest?
b.﴿ What is the adjusted wavelength that reaches the bystander as the train approaches the crossing?
c.﴿ By how much is the horn's wavelength changed as a result of the train's motion?
d.﴿ Find the two frequencies heard by the bystander at the train crossing ﴾approaching and receding﴿.
4. You are in an ambulance driving towards an accident at 42 m/sec. You hear the horns of the cars in the accident sounding at a frequency of 642 Hz, what is the actual frequency of the car horns?
You are driving at 20m/s in your car as an ambulance approaches you driving the opposite direction at 25m/s. The ambulance siren emits sound at a frequency of 720Hz. What two frequencies are observed by you as the ambulance approaches and then passes by you?