mini lab

The Doppler Effect on Car Horns

Name_________________________________________ Section______

Purpose: Use the Doppler Effect to discover the speed at which a car is traveling by the pitch change of his horn.

Theory: Simply put the Doppler Effect is the relative motion between the source of a sound and the listener/observer. The Doppler effect states that when the source of the sound is moving toward the listener the pitch will sound higher than it actually is. When the source is moving away from the listener the pitch will sound lower than it actually is. When the listener is moving toward the source of the Doppler effect the pitch of the noise sounds higher than it actually is. When the listener is moving away from the source of the sound the pitch will sound lower than it actually is. The pitch is higher because as the object move closer together the sound waves hit each other and make the pitch higher. By that same theory, the pitch sounds lower because as the objects move further away from each other the distance between the waves that bounce back and the waves coming toward each other lengthens and make the pitch sound lower.

Procedure:

(1) Measure the temperature outside where you will be driving the car.

(2) Prepare a video/audio recorder in a position where it will be able to record the car as it drives past.

(3) Have a car drive at a constant speed (setting cruise control may be a good idea).

(4) The driver will blow the horn about 20 meters before the position that the recorder is in, and continue to blow the horn as they pass the recorder.

(5) Return to the lab and figure the pitches using someone’s ears and G-tunes.

(6) Calculate (using the pitch found) the speed that the car was traveling using the formula given in class:

f = fo( u/( u -vs))

frequency =

original frequency * (speed of sound divided by(speed of sound- speed of source)

Solve for the speed of the car vs using your incredible algebra skills below: (show all work)

Data: Record the temperature outdoors where the car was driven.

Record the pitch observed by ear and the pitch by G-tunes or phone app.

Calculations:

1. Air Temperature (in (C) 10

2. The speed of sound (in m/sec) for that temperature.

(show work here)

3. Frequency when stationary Ear ____________ G tunes or app______360 Hz_______

4. Frequency when approaching Ear ____________ G tunes or app____375 Hz_________

By Ear

Speed of Source (in m/sec) (show work below)

Convert the speed of Source from Meters per Second to Miles per Hour.

(show work below)

By G-tunes or app

Speed of Source (in m/sec) 30 mph (show work below)

Convert the speed of Source from Meters per Second to Miles per Hour.

(show work below)

Percent errors from the speed given by the driver vs accepted ___________(show work below)

(By ear) (G-tunes or app)