What is the Doppler Effect?

There is an interesting phenomenon in life: when an ambulance is coming on, the sound is higher than the original; when the car leaves, the pitch of the sound is lower than the original. You may not realize that this phenomenon belongs to the same principle as the color Doppler ultrasound used in hospitals, and that is the "Doppler effect". ^[1]

The Doppler effect was also an accidental discovery. In 1842, a mathematician and physicist named Doppler in Austria. One day, he was passing by the railway crossing, just as a train was passing by him. He noticed that the whistle sounded louder and the pitch became sharper, and the whistle sounded weaker and closer when the train was near and far. Go low. He was very interested in this physical phenomenon and conducted research. It is found that this is due to the relative motion between the vibration source and the observer, which makes the frequency of the sound heard by the observer different from the frequency of the vibration source. This is the frequency shift phenomenon. Because the sound source is relatively

The Doppler effect states that the wave moves towards the observer at the source

The frequency relationship between the observer (Observer) and the source (Source) is (this formula does not apply to light waves, see the Doppler effect of light waves below):

Is the observed frequency;

Is the emission from the original emission frequency in the medium;

Is the travelling speed of the wave in the medium;

It is the moving speed of the observer. If it is close to the emission source, the operation symbol in the front is +, otherwise it is-.

It is the moving speed of the emission source. If it is close to the observer, the operation symbol in front will be-, otherwise it will be +.

Through this formula, we know the reason for the tone change when the train approaches you: the numerator in the formula is the sum of the speed of sound propagation and the speed of the observer (v + v ₀ ), and the denominator is the difference between the speed of sound propagation and the speed of the train (vv _s ), Then the original frequency of the sound source (

) Perform multiplication. Observation frequency

Higher than the original frequency of the train whistle, so the tone of the train whistle becomes higher. Conversely, when the observer and the train are far away, the numerator subtraction operation becomes smaller, the denominator addition operation becomes larger, and the calculated frequency becomes lower than the original sound frequency of the train whistle, so the hearing tone becomes lower.

The Doppler effect applies not only to sound waves, but also to all types of waves, including

Doppler effect of sound waves

In everyday life, we all have this experience: when a whistle train passes by

At 10 o'clock on March 24, 2014, Malaysian Prime Minister Najib held an emergency press conference. He said that according to new data analysis, Flight MH370 crashed in the South Indian Ocean.

The International Maritime Satellite Organization explained on the 24th that they used the Doppler effect theory to analyze the signal sent by Malaysia Airlines flight MH370 and believed that the aircraft fell into the South Indian Ocean.