SOUND The Doppler Effect Created by Joshua Toebbe NOHS 2015.
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Transcript of SOUND The Doppler Effect Created by Joshua Toebbe NOHS 2015.
SOUND
The Doppler Effect
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• We know that sound travels at a
specific speed in particular mediums.• For air that speed is approximately
340 m/s, depending on temperature and humidity.
• This information can be very useful.
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• What are some things that require
knowing the speed of sound?• Ultrasound Imaging• Sonic Motion Detectors• Sonar (passive and active)• And of course: using them in a space
probe on an unknown planet to determine the atmospheric density and composition. Which is probably the most realistic (insert sarcasm) use of all.
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• We also know that the speed of sound
changes depending on the medium.• But there are other ways that it can
change as well.• The simplest way is if the source of
the sound is moving.
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• For example: A siren on a police car
emits a tone which moves at 340 m/s, but if the police car is moving forward at 38 m/s, then their speeds have to be added together. • To a person in front of the police
car, the sound is approaching at 378 m/s.
• To a person behind the police car the sound is approaching at 302 m/s. Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• At least that’s how you would think it
works, but it doesn’t. • You see, the speed of sound doesn’t
actually change. It still travels through the air at the same speed.
• Instead the wavelength changes.
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• This works because as the car moves
forward, each sound wave that gets emitted is farther forward than the previous one.
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• This process smushes (that is a
scientific term) the waves closer together in the front of the sound source.
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• Which results in a higher apparent
frequency in the front.
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• And a lower apparent frequency
behind.
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• And thus a change in wavelength.
Created by Joshua Toebbe NOHS 2015
λλ
Dop
pler
Effe
ct• The end result is a higher pitched
sound in front of the car, and a lower pitched sound behind it.
Created by Joshua Toebbe NOHS 2015
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Dop
pler
Effe
ct
Created by Joshua Toebbe NOHS 2015
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• I’m sure we have all experienced the change in pitch as a police car, ambulance, or fire truck passed by. Maybe it was train, or even a loud car.
Dop
pler
Effe
ct• This change in pitch is known as the
Doppler Effect. (change in wavelength based on movement of source)
Created by Joshua Toebbe NOHS 2015
λλ
Dop
pler
Effe
ct• Since distance is equal to speed
multiplied by time: The period of a wave multiplied by the speed of sound will give us the distance covered in one cycle. Otherwise known as the wavelength.
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• So the wave length in front, relative to
the source, is moving at the speed of sound, minus the speed of the source.
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• And the wavelength behind the car is
travelling at the speed of sound plus the speed of the source.
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• Since frequency is given as the speed
of sound divided by its wavelength (the wave speed equation): We have a new Doppler Equation.
• If the source is approaching (-)• If the source is departing (+)
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• Example #1:
• A source is moving past a sound sensor in air. The frequency detected in front of the source was 2800 cycles per second, and the frequency detected behind the source was 1800 cycles per second. How fast is the object moving?
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• Example #2:
• A source is moving past a sound sensor in an unknown medium. The frequency detected in front of the source was 2700 cycles per second, and the frequency detected behind the source was 1300 cycles per second. The object was clocked by a radar gun to be moving at 21 m/s. What is the speed of sound in the medium?
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• Example #3:
• If an object is moving in air at a speed of 18 m/s and emitting a tone at a frequency of 1870 cycles per second. What frequency should be detected in front of the car? And behind the car?
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• Example #4:
• A stationary sonar system in salt water (v=1530 m/s) emits sound waves at a frequency of 200 cycles per second toward a target. If the sound returns at a frequency of 450 cycles per second, how fast and in what direction is the target moving?
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• Example #5:
• A stationary sonar system in salt water (v=1530 m/s) emits sound waves at a frequency of 200 cycles per second toward a target. If the sound returns at a frequency of 167 cycles per second, how fast and in what direction is the target moving?
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• Practice #1:
• A stationary sonar system in salt water (v=1530 m/s) emits sound waves at a frequency of 253 cycles per second toward a target. If the sound returns at a frequency of 141 cycles per second, how fast and in what direction is the target moving?
Created by Joshua Toebbe NOHS 2015
Dop
pler
Effe
ct• Practice #2:
• If an object is moving in air at a speed of 13 m/s and emitting a tone at a frequency of 1757 cycles per second. What frequency should be detected in front of the car? And behind the car?
Created by Joshua Toebbe NOHS 2015