# Doppler Effect: Airplane, Sound, & Frequency

• barryj
In summary, when an airplane is traveling faster than the speed of sound, there will still be a sound emitted but it will be distorted due to the Doppler effect. The frequency of the sound will also be affected, with the waves bunching up in front of the airplane. The Doppler equations can still predict this phenomenon, but they may result in negative frequencies when the airplane is traveling towards the observer. This concept has been used in science fiction, such as in the concept of Warp Drive in Star Trek.
barryj
This is not homework but rather a concept question.

If there was a siren on the front of an airplane that is traveling greater than the speed of sound, would there still be a sound emitted?

If the airplane was traveling toward me what would be the frequency? Would I heart it?

Where would the Doppler equations break down?

A sound is still emitted yes. For a stationary source the wavefronts are circles (in 2D). As the source approaches the speed of sound the circles stretch out behind the source and bunch up in front. At the speed of sound the waves bunch up at the tip of the airplane while at the back they stretch out further. At greater speeds, all the waves propagate backwards in the airplanes rest frame (but still radially in the observers rest frame).

You could hear the sound of the airplanes approach but not until after it has passed (i.e the plane would beat the sound). If the plane traveled towards you traveling at twice the speed of sound you would hear the song playing exactly backwards.

The Doppler equations don't break down, they predict negative frequencies when the plane travels towards you, so signals emitted later in time, arrive earlier in time.

"you would hear the song playing exactly backwards." This was exactly what I was wondering. If a person was speaking out the front of the plane, the words would come in reversed but then one could not understand a word played out backwards. Interesting concept.

Yeah it's very interesting, I suppose from this example you can understand a little better why traveling faster than the speed of light can be thought of as traveling backwards in time as well.

These effects could be the basis for a good si-fi movie?

barryj said:
These effects could be the basis for a good si-fi movie?

I'm not a Star Trek fan but I think this effect is what makes Warp Drive work haha

## What is the Doppler Effect?

The Doppler Effect is a phenomenon in which the perceived frequency of a wave (such as sound or light) changes when the source of the wave is in motion relative to the observer.

## How does the Doppler Effect relate to airplanes?

As an airplane moves toward an observer, the sound waves it produces are compressed, resulting in a higher frequency (pitch) being perceived. As the airplane moves away from the observer, the sound waves are stretched, resulting in a lower frequency being perceived. This change in frequency is known as the Doppler Effect.

## Does the Doppler Effect only apply to sound waves?

No, the Doppler Effect can also be observed with other types of waves, such as light. In this case, the change in frequency is perceived as a change in color.

## How is the frequency of a sound wave affected by the Doppler Effect?

The frequency of a sound wave is directly related to the speed of the source and the speed of the observer. If the source is moving toward the observer, the frequency increases. If the source is moving away from the observer, the frequency decreases.

## Can the Doppler Effect be used to measure the speed of an airplane?

Yes, by measuring the change in frequency of sound waves produced by an airplane, the speed of the airplane can be calculated. This is often used in radar systems to track the speed and location of airplanes.

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