W8 Activity 3 - Doppler Effect

W8 Activity 3

Doppler Effect

By Nicholas Carrillo

The Doppler Effect happens when a wave source moves.

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The sound of a train approaching has a higher pitch, and a lower pitch when moving away.

Multiple Choice

You are standing on the side of a track when an approaching train blows its horn. Is the pitch (frequency) of the train horn that you hear higher or lower than the actual pitch of the horn?

• Higher

• Lower

• The same

Multiple Choice

The train from the previous question has now passed you by. Is the pitch (frequency) of the train horn that you hear higher or lower than the actual pitch of the horn?

• Higher

• Lower

• The Same

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We have two equations for calculating the frequency you hear.

​Moving Source (Stationary Observer)

​​Moving Observer (Stationary Source)

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​Moving Source (Stationary Observer)

​​Moving Observer (Stationary Source)

• ​Used when the source is moving and the listener is standing still

• Minus sign used when moving towards

• Plus sign used when moving away.

• ​Used when the source is still and the listener is moving.

• Minus sign used when moving away.

• Plus sign used when moving towards.

​A train blowing its horn at 150 Hz is approaching an observer at 25m/s.

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What pitch does the observer hear?​

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A train blowing its horn at 150 Hz is approaching an observer at 25m/s. (Speed of sound is 343 m/s)

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What pitch does the observer hear?​

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• ​Source is moving and listener is staying still.

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• Moving towards each other​ (use minus sign)

​A person is driving towards a train at 25m/s. The train is blowing its horn at 150 Hz.

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What pitch does the person hear?​

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A train blowing its horn at 150 Hz is approaching an observer at 25m/s. (Speed of sound is 343 m/s)

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What pitch does the observer hear?​

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• ​Source is stationary and listener is moving.

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• Moving towards each other​ (use plus sign)

​What happens when something moves faster than the speed of sound?

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A Sonic Boom​!

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​If something is moving faster than waves can travel. Then the waves in front will bunch up.

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A bow-wake will form​

The bow-wake that forms in air is the sonic-boom we hear and feel.

Multiple Choice

Bow-wakes form when a wave source is traveling...

• ...faster than the waves can travel

• ....slower than the waves can travel

Multiple Choice

A sonic boom happens when a sound source is moving...

• ...faster than the speed of sound

• ...slower than the speed of sound.

• ....at the speed of sound.

Multiple Choice

Which best describes what a sonic boom is.

• A bow-wake that forms in air.

• Waves bunching up in front of a sound source.

• The sound waves explode when a source sources travels faster than the speed of sound.

Work together to complete the remaining questions.

Fill in the Blank

A train blowing its horn at 150 Hz is approaching an observer at 30m/s. (Speed of sound is 343 m/s)

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What pitch does the observer hear?​

(Use the following equation)

$$f_{obs}\ =\ f_{source}\left(\frac{1}{1\ \pm\ \frac{v_{source}}{v_{sound}}}\right)$$  

Fill in the Blank

A train blowing its horn at 150 Hz is moving away from an observer at 30m/s. (Speed of sound is 343 m/s)

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What pitch does the observer hear?​

(Use the following equation)

$$f_{obs}\ =\ f_{source}\left(\frac{1}{1\ \pm\ \frac{v_{source}}{v_{sound}}}\right)$$  

Fill in the Blank

A car is moving towards a fire truck at 28m/s that is blowing its siren at 165Hz. (Speed of sound is 343m/s)

What pitch does the car hear?

(Use the following equation)

$$f_{obs}\ =\ f_{source}\left(1\ \pm\ \frac{v_{obs}}{v_{sound}}\right)$$  

Fill in the Blank

A car is moving away a fire truck at 28m/s that is blowing its siren at 165Hz. (Speed of sound is 343m/s)

What pitch does the car hear?

(Use the following equation)

$$f_{obs}\ =\ f_{source}\left(1\ \pm\ \frac{v_{obs}}{v_{sound}}\right)$$