The "Big Fat Hairy Deal"The Doppler Effect is a change in pitch that occurs when there is motion between the source of the sound and its receiver.
Wait... What?!?The Doppler Effect may seem scary, but you experience it every time you hear the screams on a roller coaster.
Have you ever been standing in line for a roller coaster ride when all of a sudden, the roller coaster cars zoom by, loaded with screaming people? If you listen closely, you will hear the pitch decrease as the car moves by you. This is due to... the Doppler Effect! |
Why Does it Work?Let's start with a diagram. After all, a picture is worth a thousand words!
Let's start off by replacing the fire truck with a roller coaster car. When the car is stationary, if someone screams, the sound waves all travel out at the same frequency in both directions. When both the source of the sound and the receiver are stationary, the Doppler Effect does not apply.
When the car is moving, however, the frequency - and the pitch - changes. Let's start with the sound waves going out behind the roller coaster. As the sound waves go out, the roller coaster car moves in the opposite direction. Therefore, the sound waves are very spread out. This is because the car moves away from the original sound wave before issuing another sound wave. Since the waves are far apart, there is a low frequency. When there is a low frequency, there is a low pitch. So, when you are standing behind the roller coaster car, the pitch is low. Now let's move on to the sound waves going out in front of the roller coaster. In this case, the roller coaster car is moving in the same direction as the sound waves. Therefore, the sound waves are very close together. The car moves closer to the original sound wave before issuing another sound wave. This results in the waves being close together, which means there is a high frequency, and therefore a high pitch. When a roller coaster flies by you, you are standing in front of the roller coaster, so the pitch is higher than usual. After, the roller coaster passes you, the pitch becomes lower. So, the pitch gets lower as the roller coaster car goes by you, and voila! we have the Doppler Effect. |
Words to KnowPitch is how high or low a sound is. Pitch is caused by frequency - the higher the frequency, the higher the itch, and the lower the frequency, the lower the pitch.
Frequency is the number of waves that pass a certain point in a given amount of time. Key PointsSound is a mechanical wave. A mechanical wave is a wave that requires matter to be transmitted.
The two types of mechanical waves are compressional and transverse waves. Sound is a compressional wave, also called a longitudinal wave. A compressional wave occurs when matter (usually air molecules) vibrates back and forth in the same direction that the wave is traveling. Bonus InfoThe opposite of a mechanical wave is an electromagnetic wave, which can travel through a vaccum. (A vacuum is a space that has no matter in it.) An example of an electromagnetic wave is a light wave.
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1, 2, 3... SCREAM!!
For many people, the screams on a roller coaster that can be heard all around the amusement park are the most memorable - and most terrifying - part of a roller coaster. But little do most of them know that the Doppler Effect is behind the very way they hear the screams.