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Information about sound

The midnight bell rings to the passenger ship - talking about the propagation of sound and waves

The moon sets, the crows cry and the frost fills the sky, the river maple and the fishing fire are opposite to Chou Mian .

At Hanshan Temple outside Gusu City, the bell rang at midnight to reach the passenger ship.

This is the poem "Mooring at Maple Bridge at Night" written by Zhang Ji from the Tang Dynasty. Zhang Ji was a Jinshi in the twelfth year of Tianbao (AD 753). Not many of his poems have been handed down to the world, and he is not considered a first-rate poet in the poetry world. However, this poem of his was selected into the Tang poetry selection of all dynasties. Become a popular song.

Many people in history have commented on this poem and thought it was beautiful. Ouyang Xiu of the Song Dynasty raised a question in his "Shihua". He said: "Some people in the Tang Dynasty said: When the bell rings at midnight on the passenger ship, the speaker would be better if he said the same sentence. It is like the third watch, which is not the time to ring the bell." Ouyang Xiu affirmed. The poem is excellent, but whether the clock should be struck at the third watch has aroused some discussion among future generations. Wang Guanguo in the early Southern Song Dynasty wrote in "Xuelin": "When the world is doubtful in the middle of the night and the bells are not ringing, I watch the affairs of the country." Literary Biography of "Southern History" Qiu Zhongfu was a native of Wucheng, Wuxing. He was rarely eager to learn, and his reading was often limited to the sound of the bell in the sky. "But the midnight clock is inherent." Later in the Southern Song Dynasty, Ye Mengde said in his "Shilin Poetry": "When Ou Gong was sick, he didn't go to Wuzhong Temple in the middle of the night. Now the clock is ringing at midnight." "They explained that as early as the Southern Dynasty before the Tang Dynasty, and as late as the Southern Song Dynasty after the Tang Dynasty, there was a custom of ringing the bell at midnight in Suzhou. Ouyang Xiu's accusations are nothing more than rare and strange.

At the same time, people also found poems in Tang poetry that talked about the sound of the midnight bell, and there were many others besides Zhang. For example, Zhang Shuo, who was earlier than Zhang Ji, said in the poem "Hearing the Bell at Night on the Mountain": "Lie down at night and hear the night bell, and the quiet mountain will ring louder at night." Yu Hu's poem "Send the Palace Guests to the Road and Return to the Mountain" contains: "You must know that when you say farewell to your companions in the harem, you should listen to the midnight bell of Weishan Mountain." Bai Juyi said: "Under the shadow of the pine trees in the new autumn, after the midnight bell." Wen Tingyun said: "While traveling leisurely, I often look back and think about the midnight bell of Weishan Mountain." Chen Yu said: There is: "The melodious midnight bell across the water."

Reading these many poems, we can imagine that the melodious midnight bell can be heard from the mountains to passenger ships, and can be heard across the river to the other side. Going a step further, in Huangfu Ran's poem: "Autumn water is near the moon, and the mountain bell is across the middle of the night." This allows us to imagine that the melodious bells can even be heard across a mountain.

In Tang poetry, there are not only so many poems about the midnight clock, midnight clock, and midnight clock, but also about the sound of the flute and the piano at night. For example, Yu Hu said: "Where is the person playing the flute deeper? It is suspected that he is singing alone in the cold water." Bai Juyi said: "Who is playing the flute on the river at night? The sound seems to recall the spring in my hometown." Bai Juyi also has a famous long poem "Pipa Xing" "The first few lines of the poem use "Autumn Suosuo" and "River soaks in the moon" to explain the background of autumn and moonlit nights, and then say: "Suddenly I heard the sound of pipa on the water", and then "Looking for the sound and secretly asking who is playing it", indicating that Bai Juyi was the same. The person playing the pipa was still some distance away, so he had to "look for the voice and ask secretly", and finally he was able to "come out after calling for thousands of times, still holding the pipa half-hiding his face". It’s a story about how we must have known each other before we meet again. In Tang poetry, few people write about the sound of bells, flutes, and pianos during the day and at noon. This is by no means "following from one another" simply for the pursuit of beautiful words and phrases. People in the Song Dynasty said, "There must be someone to tell." This means that so many people wrote about the midnight bell, but they probably have their own reasons. It has been more than 1,200 years since Zhang Ji's "Night Mooring at Maple Bridge". During this long period of time, the development of science has proved that the writing methods of Zhang Ji and others are very consistent with scientific principles. In these many verses, a scientific fact is summarized: sounds at night travel far.

Why do sounds travel far at night? One way to say it is: It’s quiet at night and the background noise is smaller, making it easier for people to distinguish distant sounds. This is certainly a factor, but it is not the main reason. It starts with how sound is transmitted.

First of all, sound is the vibration of the sound source that disturbs the air, and the disturbance is transmitted out in the form of waves. Imagine that the sound source is a point above the ground. The waves in the air are transmitted outwards according to their different densities. If the sound speed of each point in the air is the same, the wavefront of the sound wave emitted from this point is a spherical surface, and the sound propagates The direction is considered to be the direction perpendicular to the wave front, that is, the direction of the radius. Now suppose that sound propagates at different speeds at different heights in the atmosphere. At this time, the wavefront no longer maintains a spherical surface and becomes distorted. Correspondingly, the sound propagation direction is no longer the direction of the radius of the ball, but turns a corner. This sound The phenomenon of bends in the propagation road is also called the phenomenon of acoustic refraction. The sound propagation distance is different during the day and at night, which is caused by this refraction phenomenon.

Secondly, in the air near the ground, the relationship between the speed of sound c (m/s) and the temperature t (℃) can be approximated as

c=(331.45＋0.61t/ ℃) m?s-1

That is to say, for every degree of temperature increase on the ground, the speed of sound increases by about 0.61 meters/second.

Our human activities are in the atmosphere close to the ground. At an altitude of less than 20 kilometers, the temperature changes in the atmosphere are very complex. During the day, due to the increase in temperature of the ground receiving solar radiation, the temperature of the atmosphere near the ground is higher than that of the air layer slightly higher, which means that the speed of sound near the ground is greater than that high in the sky. At this time, the sound propagation path turns to high altitude, and an acoustic quiet zone can be formed in an appropriate place, that is, no sound from a distance can be heard (Figure 1(a)).

At this time, since the sound propagation path is refracted to high altitude, if you sit on the balloon, you will hear the sound of the ground below the balloon very clearly. Zhang Ji, who is sitting in the balloon, may say, "The noon bell is ringing to the balloon." At night, the air near the ground gradually cools down, and the temperature above is relatively high. As a result, the speed of sound at high altitude is greater than that at the ground, so the sound will be refracted toward the ground (Figure 1(b)). This is why sounds at night are relatively far away. In cold weather, especially on frozen lakes or unfrozen water, even during the day, the effect of sound refraction toward the ground is very obvious due to the low ground temperature. "The moon is setting, crows are crying, and the sky is filled with frost." In the poem, Zhang Ji writes about the late autumn weather. It is not only the midnight bells, but also the midnight bells in the late autumn weather. Isn't it particularly clear? It can be said that "the sound of autumn is true in the middle of the night" (quoted from Qian Zhongshu's "Talk about Art"). It can be seen how carefully the poets of the Tang Dynasty observed. Due to the dual factors of "autumn" and "midnight", Huangfu Ran's poem: "Autumn water is near the moon, and the mountain bell is separated by midnight" seems very realistic. Only under such conditions, sound can be refracted from the mountain. Passed over there. Nowadays, people who live in downtown areas probably have this experience. During the day, except for the buildings facing the street, most of the annoying noise caused by the traffic on the road cannot be felt. However, late at night, even if there is only one car passing by, you can't feel it. It will disturb your sleep, and you can even hear it from a few buildings away. It can be said that "the noise disturbs the bed in the middle of the night". It is the same as "the bell rings at the passenger ship at midnight".

The use of modern scientific methods to study sound was about 1,000 years after Zhang Ji's poem "Mooring at Maple Bridge at Night". At that time, there was a saying in Europe: "The hearing situation in England is better than that in Italy." In 1704, two serious people: one was the British pastor W. Dele and one Italian, Avelloni, worked together to conduct actual measurements of the sound propagation conditions in the two places, and the results confirmed that there was not much difference in the sound propagation conditions between the two countries. The earliest person to measure the speed of sound was the Frenchman M. Mason, and then in 1738, the French Academy of Sciences measured the speed of sound more accurately.

When talking about the propagation of sound in the atmosphere, we should mention Aisin Gioro Xuanye (1654-1722), the Kangxi Emperor of the Qing Dynasty. He was an intelligent and knowledgeable politician. In his essay "Ji Xia Ge Wu Bian", he records an experiment he conducted on the sound of gunfire, titled "Thunder can't sound more than a hundred miles away". He said: "I compared it with an algorithm, and the sound of thunder cannot reach a hundred miles. The algorithm: According to the accurate size of the Huang Zhong, determine the weight line of one second, whether it is long or short, heavy or light, all have certain additions and subtractions. Try it first As for cannons and cannons, the sound will sound as soon as the smoke rises. The farther the sound is, the slower it will sound. Then calculate the distance of the thunder cannon, and then I will know. The sound of thunder is far and near. When I was stationed in Tianjin, the Eight Banners fired artillery at Lugouqiao. It was a northwest wind, and the sound of the cannon seemed not far away. It was about two hundred miles away, and the sound of the cannon was farther than the thunder. , no doubt." From Xuan Ye's words, it can be seen that he is very meticulous in his experiments. The so-called "Huang Zhong" is a standard scale in ancient times. Its tempered pipe is nine inches long and nine minutes in diameter, which can be regarded as the standard length. As for determining the weight line of 1 second, it is likely that the long period of the simple pendulum used is 1 second. Once the standard for measuring time is set, subsequent measurements will not be difficult. His experiment was similar to the roughly contemporary method used by the French Academy of Sciences to measure the speed of sound in 1738. It's just that Xuan Ye didn't put forward the concept of sound speed, but got the concept of proportion. The "accurate proportion" Xuan Ye said is the distance traveled by sound waves in unit time today, which is the speed of sound. Unfortunately he failed to note what percentage he got.

As for the phenomenon of sound refraction, it was not until the 19th century that European scholars quantitatively studied the relationship between temperature gradient and sound refraction effect. Later, people gradually realized that to understand the complex phenomenon of sound refraction in the atmosphere, it was necessary to have a map of the variation of sound speed along height. That is, the speed of sound as a function of height above the ground. According to current actual measurements and theoretical calculations, this functional relationship can be briefly expressed as Figure 2. From Figure 2 we can explain many interesting phenomena of sound propagation in the atmosphere. We see that within a few kilometers from point B to the ground, the velocity gradient is opposite from day to night. It can explain the reason why ground sounds travel farther at night than during the day, as mentioned before.

We also see that this curve has made several turns. Pay attention to point C, where the sound speed is the local minimum. If sound is made at this elevation, sound from any direction will be refracted and bent toward the horizontal. Because looking up from point C, its gradient is exactly the same as the gradient of sound speed on the ground at night. Looking down from point C, the speed of sound also becomes larger at a height away from C, so no matter what, the sound will bend towards the horizontal line passing C. That is to say, at this altitude, sound travels particularly far, which is called the vocal tract. On the contrary, when the sound propagates close to point D, which has the maximum value of the sound speed, part of it will be refracted back to the side where the sound wave came from, just like the reflection of the wave.

Thunder in summer is always followed by lightning. Lightning lasts only a moment, perhaps less than a thousandth of a second. But after a lightning strike, thunder often rumbles for a long time. The reason for this is due to sound reflection along the height, sometimes going back and forth several times, just like the constant echo heard when shouting in the valley. In fact, before thunderstorms in summer, the sound speed distribution is much more complicated than Figure 2. At this time, due to wind and clouds near the ground, the sound speed distribution not only changes along the height, but also changes along the level, which will cause extremely complex sound refraction phenomena.

During World War I, a strange phenomenon was discovered.

A cannon is constantly firing. When someone drives towards it from hundreds of kilometers away, they hear the rumble of the cannon at first, but as they drive closer, no sound of the cannon is heard for a section of the road. The reason is that the sound of the cannon heard at first is the wave reflected by the atmosphere, and closer to the quiet zone, the sound wave directly transmitted from the cannon is heard again.

Wind has an impact on the propagation of sound. The speed of sound is different when it is downwind and against the wind. When going downwind, the speed of sound c in still air is added to the wind speed, while when going upwind, the wind speed is subtracted. But the distribution of wind speed along different heights increases, and increases approximately exponentially. The wind speed is greater at high altitude and less close to the ground. Therefore, when the wind is against the wind, the sound speed at high altitude is less than the sound speed at the ground; when the wind is downwind, the sound speed at high altitude is greater than the sound speed at the ground. In this way, when the wind blows, the sound refraction when the wind blows is like nighttime, while the sound refraction when the headwind is like daytime. This is why when listening to people talking when the wind blows, standing downwind can hear it particularly clearly. This is what Xunzi said in "Encouraging Learning": "When you shout with the wind, the sound is not accelerated, but the listener is louder." reason.

Xuan Ye’s speed of sound experiment mentioned earlier was indeed very careful. He did not even ignore that the northwest wind was blowing when he heard the sound of the Lugou Bridge cannon in Tianjin. This shows that he was aware of the wind. Will have an impact on sound propagation. He was at a disadvantage, so he heard farther away. However, when there is thunder in the summer, the temperature of the sky happens to be low, and the sound is generally refracted towards the sky. Therefore, Xuan Ye cannot hear the thunder more than a hundred miles away. It is possible that he is in the quiet zone. Whether the sound is heard or not is not only related to the energy of the sound produced by the thunder and cannon, but also related to the place where the listener is located and the weather conditions. Assume that Xuan Ye is in the upper hand when he hears the sound of cannons, and the sound of cannons he hears may not be farther away than the thunder. Therefore, it cannot be said in general: "The sound of cannon is farther than thunder, no doubt."

The propagation of sound in water is also similar to the propagation in air. During World War II, it was discovered that sound channels existed deep in the sea, where sound waves could travel thousands of kilometers. This phenomenon has received great attention because it can be used to monitor the dynamics of enemy submarines. It is still an important topic in the application of hydroacoustic technology.

The propagation of sound in solids is more complicated, but it is nothing more than a phenomenon of refraction and reflection. Modern precision instruments can detect earthquakes and nuclear explosions occurring on the other side of the world. Its size and exact location can be estimated by measuring multiple points.

Using artificial explosions, the refraction and reflection of sound propagating in solids, and collecting these signals for analysis, can also be used for geological prospecting. Since the propagation of waves is related to speed, and speed is related to the density of the medium, the density of the medium can be analyzed by collecting sound waves coming from all directions. The application of this technology is called acoustic holography.

To get into the details of these techniques, there is a research direction in the discipline of mechanics called the inverse problem of waves and wave problems in layered media or inhomogeneous media. Sound is a kind of wave, and light is also a kind of wave. In uneven media, light waves will also be refracted. They are all the same. The "mirage" phenomenon is caused by light refraction.

"The midnight bell rings to the passenger ship" is a poem written more than 1,200 years ago. The scientific facts summarized in the poem have been continuously confirmed by subsequent scientific developments. Humanity's understanding of nature is gradually improving, and we are bathed in the glory days of scientific development. When we recite this beautiful verse repeatedly, how can we not marvel at the complete combination of the beauty of language and the beauty of science in this verse. Isn’t it true that science and technology, which have been developing day by day for thousands of years, have provided more sophisticated and detailed annotations to this verse?

Question: Compared with elephants and humans, who has a stronger sense of direction? Why?

Answer: There are three main reasons why two ears can distinguish the direction of a sound source. One is the difference in intensity felt by the two ears for the same sound. If the sound source is on the right, the sound heard by the right ear is stronger than the left ear. This difference in loudness forms a sense of direction in practice. At this time, we will turn our heads until both ears hear the same loudness. At this time, the sound source is directly in front or behind us. The second is that for the same sound, the two ears feel it at different times. If the sound source is on the right and the right ear is closer to the sound source than the left ear, the sound will be heard first by the right ear and then by the left ear. This difference in the time that the sound reaches the right ear and the left ear also forms the feeling of the direction of the sound source in practice. The greater the time difference, the easier it is to distinguish and the more accurate the feeling. In this regard, the elephant has an advantage over the human because the distance between its two ears is much greater than that of the human. Third, for the same sound, the vibration pace felt by the two ears is different. Since there is a time difference between the sound reaching the two ears, the pace of the vibrations felt by the two ears is also different, which will cause a sense of direction. However, this situation is more effective for low-frequency sounds, because high-frequency sounds vibrate quickly. Within the time difference between listening to the two ears, the sound is likely to vibrate for one or several full cycles. At this time, the vibration pace felt by the two ears is the same. , and the low-frequency sound has a long period and is difficult to synchronize.

Dogs can distinguish extremely small and high-frequency sounds, and have a strong ability to judge the source of sound. Its hearing is 16 times that of humans. When a dog hears a sound, due to the gap between the ears and eyes, It has a sympathetic effect, so it can see six directions with its eyes and hear all directions with its ears. At night, it maintains a high degree of alertness even when sleeping, and can clearly distinguish sounds within a radius of 1 kilometer. What is even more incredible is that it can It is distinguished that the number of vibrations of the metronome per minute is 96 or 100, 133 and 144 times. Dogs can establish conditioned reflexes based on changes in pitch and syllables for human commands and simple language. It is particularly important to note that there is no need to yell loudly at the dog. Excessively loud sounds or audio are a kind of adversity stimulus to him, causing him pain and fear. Of course, when he makes a mistake, he can raise his voice to discipline him.