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Published byPearl Stewart Modified over 9 years ago
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Example 6 A siren from a police car has a frequency of Hz and is moving with a velocity of 45.0 m/s. You are driving on the other side of the road towards the police car with a velocity of 36.0 m/s. What is the observed frequency from the police car? Assume the velocity of sound as 343 m/s.
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Going Faster Moving at the Speed of Sound Moving Faster than Sound
Moving Source Mach I Supersonic Pressure wave, piled up at nose Shock wave of constructive interference
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Sonic Boom When the wave of sound passes you – Sonic Boom!
Do you see the plane or hear it first?
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Sonic Boom High pressure Low pressure
Moisture condenses in low pressure region High pressure Low pressure
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Wave Interference SUPERPOSITION ! Wave interaction, described by
Superposition –constructive or destructive combination of waves if present at the same place, same time
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When Two Waves Meet… If two waves combine...
Constructive Interference – Result is a larger amplitude Destructive Interference – Result is a smaller amplitude Waves pass through each other afterwards.
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Interference by Position
If you have the same sound but from two different speakers… Constructive Interference– Waves must be a whole-number wavelength apart Destructive Interference– Waves must be a half wavelength apart
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Constructive/Destructive Interference – Applications
As the story goes, this man… …was on an airplane – trying to concentrate during take-off, but couldn’t due to the noise of the plane. So during the plane ride, this man came up with a design for something new…and… …he owned a company that could do it… Ten years later, the Bose Corporation introduced its first noise-cancelling headphones to the market
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Constructive/Destructive Interference – Noise Cancelling Headphones
Two Kinds of Noise-Cancelling Headphones Passive - Uses sound-absorbing materials to block sound Active – Uses destructive interference to cancel out outside noise High-density, sound absorbing foam
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