sound the doppler effect created by joshua toebbe nohs 2015

SOUND

The Doppler Effect

Created by Joshua Toebbe NOHS 2015

Dop

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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.


Dop

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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.


Dop

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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.


Dop

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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

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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.


Dop

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ct• This works because as the car moves

forward, each sound wave that gets emitted is farther forward than the previous one.


Dop

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ct• This process smushes (that is a

scientific term) the waves closer together in the front of the sound source.


Dop

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ct• Which results in a higher apparent

frequency in the front.


Dop

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ct• And a lower apparent frequency

behind.


Dop

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ct• And thus a change in wavelength.


λλ

Dop

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ct• The end result is a higher pitched

sound in front of the car, and a lower pitched sound behind it.


λλ

Dop

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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

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ct• This change in pitch is known as the

Doppler Effect. (change in wavelength based on movement of source)


λλ

Dop

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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.


Dop

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ct• So the wave length in front, relative to

the source, is moving at the speed of sound, minus the speed of the source.


Dop

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ct• And the wavelength behind the car is

travelling at the speed of sound plus the speed of the source.


Dop

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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 (+)


Dop

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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?


Dop

pler

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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?


Dop

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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?


Dop

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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?


Dop

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ct• Example #5:



Dop

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ct• Practice #1:



Dop

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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?


sound the doppler effect created by joshua toebbe nohs 2015

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