Can a bullock cart also produce the Doppler effect? This may be a misunderstanding by a Yale professor

This article is from WeChat public number: Mr. Sai (ID: mrscience100), author: Wu Jinyuan, responsible for: Ye Shui sent

A while ago, I inadvertently saw Yale professor Craig Wright’s public lecture “Listening to Music” on the Internet, and after watching a number of episodes, I felt so benefited that I was ready to continue to finish it.

However, when the lecture talked about a musical work, there was a mistake in the explanation of a physics problem, and it is necessary to discuss it here.

Can a bullock cart also produce the Doppler effect? This may be a misunderstanding by a Yale professor

Listening to Music” by Professor Craig Wright of Yale University

The musical piece talked about in the lecture was “Picture Fair: Polish Ox Cart” by the Russian composer Mussorgsky. This short piece of music depicts an old Polish oxcart in a painting created by a friend of the composer. The music shows the oxcart slowly approaching from a distance, passing by, and moving off into the distance again.

In the orchestral version of Ravel’s arrangement, the bass tuba (Tuba) plays a low melody that progresses from weak to strong. Then the strings and percussion in the upper register join in, as if a bullock cart is passing by and we hear it creak and squeak. Finally, the bass tuba plays a low melody from strong to weak, as if the oxcart is gradually walking away (we recommend the reader to listen to this music on the B website).

It is a misunderstanding that the acoustic effect of the approaching cattle car is an acoustic phenomenon called the Doppler effect. The sound effect of the approaching cattle car is indeed an acoustic phenomenon, but not the Doppler effect, we will talk about this later.

It is necessary to say a few more words here, we are here to point out the misunderstanding in the physics of this public class, in no way “despise”, “crush” such meaning. On the contrary, the author has learned a lot from this lesson about music and still highly recommends it to the readers.

However, one should be responsible to all readers and viewers for speaking up about scientific issues. The author has searched the Internet and found a lot of articles about this music, including popular music articles, exam review syllabi, newspaper music reviews, and even children’s music textbooks, all saying that this phenomenon is the Doppler effect.

Can a bullock cart also produce the Doppler effect? This may be a misunderstanding by a Yale professor

I’m afraid we’ve all experienced ambulances from far and near as well as from near and far

On Zhihu, someone described this scene as “a perfect embodiment of the ‘Doppler effect'”. This public lesson has been available online for many years, and many young students have taken notes and posted them online for more than a decade. Unfortunately, all the ones I have seen have largely accepted the Doppler effect without question.

The Doppler effect is a very simple, yet relevant physics phenomenon to our lives. Therefore, I think it is very necessary to explain the relevant physics clearly.

  1. What is the Doppler effect?
Can a bullock cart also produce the Doppler effect? This may be a misunderstanding by a Yale professor

Christian Doppler was born in Salzburg in 1803 and died in Venice in 1853 at the age of 49.

The Doppler effect is a fluctuational phenomenon, which is a change in the observed frequency when there is relative motion between the source of the fluctuation and the observer. For example, when a car honks its horn and drives past, we will hear the horn sound is sharper when the car is approaching and then lower when the car is driving away.

Doppler effect produced by speeding cars honking their horns

If you don’t mind driving to do this experiment, you can also use your cell phone to do a simpler experiment. Let the phone play one or more fixed frequency sounds, and then let the phone quickly fling over your ear. (Or fling it over another phone with the recording function turned on). At this point, in addition to hearing the change in sound intensity, we can hear the change in frequency. In the video below, we use another phone instead of the ear, with an app called spectrum view installed on it, and we can even visually see the change in frequency when the phone is flung over.

Can a bullock cart also produce the Doppler effect? This may be a misunderstanding by a Yale professor

The author published an article on the Doppler effect and ultrasound for cardiac examination in The Intellectual a few years ago, the details of which will not be repeated.

At the time of writing this previous article, the author had visited Doppler’s birthplace, Salzburg, Austria. Later the author went to Venice and made a special visit to Doppler’s grave. In this article, we put the photos of his birthplace and tombstone at the same time, so that we can see the two ends of the life of this famous physicist.

Can a bullock cart also produce the Doppler effect? This may be a misunderstanding by a Yale professor

Doppler’s birthplace

Can a bullock cart also produce the Doppler effect? This may be a misunderstanding by a Yale professor

Doppler’s graveyard

  1. Acoustic phenomena during the approach and departure of the oxcart

Since “Picture Fair: Polish Bullock Carriage” has nothing to do with the Doppler effect, what kind of acoustic phenomenon does it show? It shows the attenuation of sound waves propagating in the air, and the frequency characteristics of this attenuation.

A sound source, at a relatively long distance, emits a sound that sounds weaker. And when it is closer, the sound emitted sounds stronger. In addition, the sound from a more distant source has more high-frequency components than low-frequency components lost, sounding very low. The sound from a closer source, on the other hand, has a relatively balanced high-frequency and low-frequency component, and thus a high-frequency component like the squeaking of a cattle car can be heard.

This phenomenon involves two mechanisms, the first mechanism is the acoustic inverse square rate decay. The sound emitted from a point source propagates as a spherical wave, and the farther the distance, the larger the area of the sphere. Thus the energy of the sound wave is dispersed over an increasingly large area, so the power of the sound wave per unit area becomes smaller and smaller, and thus the farther away the sound sounds weaker. Quantitatively speaking, the sound intensity is inversely proportional to the square of the distance. This mechanism has no significant frequency characteristics and works essentially the same for sound waves of different frequencies.

The second mechanism is the absorption of sound waves in the air attenuation. Sound waves propagating in the air, part of the mechanical energy will be due to the presence of viscosity of the gas, into heat energy loss. In addition, part of the mechanical energy will also become air molecules vibration and rotation kinetic energy, and eventually also become thermal energy loss. The role of this type of absorption and attenuation mechanism is related to frequency, different frequencies of sound waves, the attenuation rate in the air is different, the higher the frequency, the more attenuated.

Can a bullock cart also produce the Doppler effect? This may be a misunderstanding by a Yale professor

Picture Show: Polish Cattle Drive

In “Picture Fair: Polish Cattle Drive”, the piece goes from weak to strong and then from strong to weak, expressing the combined effect of these two mechanisms. However, the orchestration of the piece, starting with the bass tuba, followed by the high strings and percussion, and then returning to the bass tuba, is mainly a manifestation of the second mechanism.

The reader is reminded that whichever mechanism is mentioned above, it is only related to the distance between the source and the observer, not to the relative speed between the two, and thus the phenomenon we see cannot be the Doppler effect. The Doppler effect is only related to the relative velocity between the source and the observer, but not to the distance between the two.

Some readers may have questions, the bullock cart is clearly in motion, since the movement, should not have the Doppler effect? Yes, the movement of the bullock car will indeed cause the Doppler effect, but the movement of the bullock car is much slower than the speed of sound, so the Doppler effect is not significant.

Imagine an oxcart moving at 1 m/s, while the speed of sound in the air is about 340 m/s, corresponding to a Doppler frequency shift of about 0.3%. Assuming that the cart driver is holding up a cell phone that plays a 1000 Hz sine wave, when the bullock cart rushes toward us, we hear a sound of 1003 Hz, and when it flees us, the sound is 997 Hz, such a frequency difference most people can’t even tell. Moreover, this effect is not the phenomenon that the cattle car decays strongly in the high-frequency components at a distance, and thus the low-frequency components are relatively strong, as shown in the work.

  1. Acoustic phenomena in marching marching band performance

In this class, the professor explained this work with another example of the “Doppler effect”: the marching marching band performance at halftime of the stadium. When the band is playing facing the audience, the audience can hear the normal sound, but when the band turns its back to the audience, the highs are highly attenuated and the audience can only hear the drums or bass instruments.

This is indeed an interesting acoustic phenomenon, but still not a Doppler effect.

Can a bullock cart also produce the Doppler effect? This may be a misunderstanding by a Yale professor

Image from Qilu Evening News

When marching marching band players turn their backs to the audience, we can clearly hear the attenuation of the soprano section, a phenomenon that involves at least two mechanisms: the directivity of the instrument and the diffraction of sound waves.

Many wind instruments are directional, and the difference in sound intensity between a soprano instrument like the trumpet, played to the audience, and one played in the other direction, is very large. Wind instruments in the bass section, such as the bass tuba, euphonium, and alto horn, are played with the mouthpiece upward. Therefore, when the player turns around, the intensity of the sound heard by the audience does not vary as much, while the bass percussion instruments, such as the tuba, do not have significant directionality.

There are also some high pitched instruments, such as piccolo and flute, which are not very directional. However, when the player’s back is turned toward the audience, the sound from the instrument is obscured by the player’s head, just as an opaque object blocks light.

When any wave encounters an obstacle, it is blocked, but at the same time it may propagate to a part behind the obstacle, which is called the phenomenon of wave diffraction. The amount of energy diffracted by the wave to the back of the obstacle is related to its wavelength and the scale of the obstacle. The shorter the wavelength relative to the obstacle, the more significant the masking, for example, the wavelength of light waves is very short relative to the human body, so we stand in the sun will form a clear shadow, almost invisible diffraction phenomenon. On the contrary, the wavelength of sound waves is similar to the scale of the human body, so it is easier to diffract sound waves to the back of the human body.

The speed of sound in the air is about 340 m/s, so the wavelength of sound with a frequency of 340 Hz is 1 meter, while the wavelength of sound with a frequency of 3400 Hz is 10 cm, such a change in wavelength across the scale of the human body.

Thus, in the audible sound frequency range, there will be relatively significant differences in the diffraction or masking effects of the human body on low-frequency and high-frequency sounds. More high-frequency components are shaded, and more low-frequency components are bypassed. Thus, when the band plays with its back to the audience, we can hear much less of the high-frequency components, while the low-frequency components, although also reduced, are much stronger relative to the high-frequency components.

Interestingly, the band members seemed to know all about this effect of instrument directionality and sonic diffraction. When they need the audience to get a balanced acoustic effect, they will use lateral marching or even backward marching, trying to keep facing the audience. The method of turning around or playing with the back turned actually becomes an artistic device in their case. For example, in the third piece, in order to set off the poignant tone of the solo trumpet, the orchestra plays with its back to the audience, effectively attenuating the high frequency components and making the orchestra’s accompaniment seem more gentle and dreamy.

  1. Conclusion

Recently, I have seen from the Internet that people call for improving the quality of sensibility of all people, advocating the pursuit of beauty, popularizing music, etc. I fully agree with this.

At the same time, I think that, as scientific researchers, we can not only make efforts to improve our own quality of aesthetic education, but also explore the scientific issues in the process of popularizing aesthetic education and introducing music and paintings, so that science and art can complement each other.

After all, science, like art, is also the fruit of human civilization and should be available to all.

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