# doppler effect

## what is the doppler effect

The Doppler effect is a change in the frequency of sound waves that occurs when the source of the sound waves is moving relative to a stationary listener. (It can also occur when the sound source is stationary and the listener is moving.) The Figure 1.1 shows how the Doppler effect occurs. The sound waves from the police car siren travel outward in all directions. Because the car is racing forward (to the left), the sound waves get bunched up in front of the car and spread out behind it. Sound waves that are closer together have a higher frequency, and sound waves that are farther apart have a lower frequency. The frequency of sound waves, in turn, determines the pitch of the sound. Sound waves with a higher frequency produce sound with a higher pitch, and sound waves with a lower frequency produce sound with a lower pitch.

## experiencing the doppler effect

As the car approaches listener A, the sound waves get closer together, increasing their frequency. This listener hears the pitch of the siren get higher. As the car speeds away from listener B, the sound waves get farther apart, decreasing their frequency. This listener hears the pitch of the siren get lower. Q: What will the siren sound like to listener A after the police car passes him? A: The siren will suddenly get lower in pitch because the sound waves will be much more spread out and have a lower frequency.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

the doppler effect can occur when the

``````a) sound source is moving and the listener is stationary.

b) listener is moving and the sound source is stationary.

c) sound source and listener are moving together.

-->  d) two of the above
``````

sound waves from a police car siren move away from the car in all directions.

``````-->  a. true

b. false
``````

sound waves that are closer together have a lower frequency.

``````a. true

-->  b. false
``````

as the frequency of sound waves gets lower, a listener perceives the sound to have a higher pitch.

``````a. true

-->  b. false
``````

a police car is speeding north with its siren blaring. the sound waves from the siren increase in frequency

``````-->  a) north of the car.

b) south of the car.

c) in all directions around the car.

d) none of the above
``````

if you are standing a few blocks north of the police car in question 6, how does its siren sound to you as the car gets closer to your location?

``````-->  a) the sirens pitch gets higher.

b) the sirens pitch gets lower.

c) the sirens pitch gets lower and then higher.

d) the sirens pitch does not change.
``````

after the police car in question 6 passes your location, how does the siren sound to you?

``````a) the sirens pitch gets higher.

-->  b) the sirens pitch gets lower.

c) the sirens pitch gets lower and then higher.

d) the sirens pitch does not change.
``````

## diagram questions

No diagram questions associated with this lesson