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# PH 223 20:6-7

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## Richard Datwyler

on 14 September 2016

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#### Transcript of PH 223 20:6-7

Traveling waves part 3
Power, intensity, Decibels
Doppler effect

Example 35

A loudspeaker on a tall pole emits sound in all directions, What is the speaker's power output if the sound intensity level is 90dB at a distance of 20 m?
Your turn, how far would you need to be away to have an intensity of 70 dB?
A physics professor demonstrates the doppler effect by tying a 600 Hz sound generator to a 1.0 m long rope and spins it over her head at a speed of 100 rpm. What are the highest and lowest frequency heard by a student in the room?
"The equations in 20.41 come from Einsteins theory of relativity and are used to determine the frequency of light waves from a moving source. It is said that this is "stated without proof". Are there other theories out there or is it something we know is correct and is impossible to prove? Also how accurately can we measure red and blue shifting."
"Is a jet breaking the sound barrier basically breaking through the build up of waves caused by the doppler effect?"
"I don't think I understand the doppler effect. I kind get it, if I'm standing on the sidewalk and an ambulance passes with it's sirens on, the moment it passes me it doesn't seem as loud, and that's because the sound waves are compressed to a higher frequency? "
"I'm still not sure why we distinguish between intensity and decibels or what the distinction is."
"Do waves affect each other while they travel? Mechanical waves travel according to the characteristics of their medium, but do those waves affect the medium so as to change the way that the next wave travels the same medium, causing distortion?

Also, why does a light look star-like at night? Just out of curiosity."
"one question I had was about how if the wave is shifted to the right your phase constant is negative"
The displacement of a wave traveling in the positive x-direction is
Find frequency, wavelength and speed
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