Source: CCTV.com

05-30-2006 14:50

Cixian County, located in China's Central Plains, is about 30 kilometers south of Handan City. Several emperors and princes were buried there. Several tall mounds, concealing ancient tombs, are visible. The biggest mound is called "Tianzizhong", meaning an emperor's tomb. It was the tomb of an emperor named Yuan Shanjian, who was the only emperor of the Eastern Wei Dynasty (534 – 550 AD). He was killed by his prime minister, Gao Yang, who later established the Northern Qi Dynasty (550 – 577 AD). In ancient China, regicide was a great crime. In order to conceal his evil deed, Gao Yang built a grand tomb for Yuan Shanjian. The tomb is over 30 meters high and covers almost 1.4 hectares of land. In the vast plain, it looks like a hill. The locals call it Pingding Hill. Gradually, temples and buildings were built on the hill. On the north side of the hill, a flight of stairs was built leading to the hilltop.

The stairs were built over 20 years ago. The tomb was built in the Northern Dynasties (386 – 581 AD). It's one of China's nationally protected cultural relics. It was once a deserted mound. To allow tourists to get up there more easily, we built the stairs on its northern slope. But when people walk on them, the stairs make a strange sound, like water flowing.

This phenomenon has caused a sensation in the world. Many tourists have come to visit the tomb. People find that when they go up or down the steps, they don't hear their own footsteps but the footsteps of their companions several meters away.

This sound wasn't created on purpose. What causes it?

Since this phenomenon was first discovered, it's been reported frequently. Lots of tourists have come. Many experts and scholars have studied it. There are several explanations for it. Some people think that the tomb is vacant inside. When people walk on it, their footsteps echo.

Other people say there is a pool of water in the bottom of the tomb. When people walk on the steps, the water vibrates and emits a sound.

In ancient China, tombs were built in basically the same pattern. Their chambers were built of bricks and above them were mounds. Yuan Shanjian's ancestors were Xianbei people. They migrated from Northwest China to the Central Plains and established the Northern Wei Dynasty there. By the time Yuan Shanjian took the throne, the dynasty was split into the Eastern and Western dynasties. Does his tomb have a huge cave inside that can echo?

Xianbei people had migrated early to the Central Plains. The ruling class's lifestyle and habits were basically the same as the Han. Their tomb structures were the same as the Han, too.

What is the structure of Yuan Shanjian's tomb? No one knows because his tomb hasn't been excavated. He was poisoned and buried by his prime minister Gao Yang. However, Gao's tomb is only two kilometers away and was excavated in the 1980s. Through studying the structure of Gao's tomb, we can learn the structure of Yuan's tomb.

Local people used to take soil from the mound. When the mound was moved, the vault of his coffin chamber was exposed, so we did a salvage excavation of it. This is the vault of his coffin chamber. It was built of bricks. The vault is now preserved, so we can still see it and the bricks.

Was the mound built above the vault? Yuan Shanjian's tomb should be as big as Gao Yang's. Let's measure Gao's coffin chamber now. It's about seven meters long, four meters wide and over 10 meters deep. Its area is no more than 30 square meters.

Reporter: How high was the mound built above it?

Man: Over 30 meters high.

Reporter: As high as that tree?

Man: At least 3 times higher than the tree. When people walk on such a tall mound, even if there was a big cave below, their footsteps couldn't possibly echo.

Is there a pool of water in Yuan Shanjian's tomb? If so, are the echoes caused by the vibration in the water? The underground water lies in the surface layer, so it may vibrate because of the footsteps.

We actually did find a pool of water in Gao Yang's tomb. The pit is very deep and there is no light inside, so we can barely see the underground water. Let's throw a stone into it to see how deep the water is. When we were excavating the tomb, we used a pump to collect water. Once the pump stopped working, the pit was quickly filled with water. The underground water flow is large.

Reporter: This proves the pit is full of water.

Man: Yes. The entire pit is full of water.

Reporter: If we walked on the 30-meter-high mound

above it, would we make the water vibrate?

Man: No, we couldn't. We are now standing on the vault of Gao Yang's coffin pit. If we stamp our feet on it, we won't hear the echoes of the water from below. So if we walk on the 30-meter-high mound of Yuan Shanjian's tomb, our footsteps can't shake the water below it.

From the aspect of its outer appearance, the stairs on the northern slope of the imperial tomb don't seem different from common steps. They were built with gray bricks and consist of 109 steps. They're five meters wide and over 40 meters high. On both of their sides are two one-meter-high guard walls. The slope of the steps isn't large. But whether you walk on them or stomp on them, you'll produce echoes.

Two years ago, some physics professors from Hengshui College began to study the echo. They have surveyed the steps several times.

Reporter: When we hit the inside of the wall, it echoes. How about if you hit the top?

Teacher: When we hit the top of the wall, it doesn't produce any echoes. When we hit the root of the wall, it produces remarkable echoes. Prof. Li, I'll go uphill to hit the steps, you listen for echoes.

Teacher: How about when I clap my hands?

Professor: No echo, either.

Teacher: How about when I clap my hands

high in the air?

Professor: This doesn't echo, either.

I can't hear anymore.

Teacher: When I clap my hands above the walls, it doesn't echo. When I clap the top of the wall, it doesn't echo, either. So we can initially conclude that the echo is from our footsteps or by hitting the walls and steps.

The professors used equipment to measure the frequencies of our footsteps and their echoes and then analyzed them.

Professor: From the results, we found the frequency of the footsteps is about 240 Hz and their echoes is about 370 Hz. So we see the steps change a low-frequency sound into a high-frequency one.

Teacher: Acoustically, a sound of 240 Hz should produce an echo of 240 Hz after it's reflected by something. As people walk on the steps or clap their hands, they produce different frequencies of sounds. The sounds are mixed together. Some of them are absorbed by the walls and steps. Some form echoes. So the echoes should be different from the sound sources.

The data show that the frequencies of the sound sources are different from that of the echoes. It's the difference that creates this strange phenomenon. But, what factors have changed the frequency of the echoes?

Why can't steps in other areas produce such echoes? What differences do the steps here have from steps elsewhere? First, the steps were built with gray bricks that have grooves on their surfaces. These grooves reflect and refract sound waves and alter their frequencies. Second, the two walls on both sides of the steps are different from common ones. They are solid and don't have holes. Sound waves can't pass through them. In addition, their surfaces aren't flat. There are engravings and grooves on their surfaces. So they can reflect and refract sound waves and change their frequencies.

On the south slope of the imperial tomb are two flights of stairs, which look the same as the one on the north slope. They were built with gray bricks and have brick walls on both sides. Do they also produce echoes that sound like water?

These stairs, built on the southern slope, were built of the same material as the ones on the northern slope. Why do the stairs on the northern slope produce the water sounds?

It requires special conditions to turn a low-frequency sound wave into a high-frequency one through reflection and refraction. It needs a relatively long distance to spread the sound wave. If the steps are too narrow, the range of the sound wave will be limited, and its frequency can't be changed.

The steps on the southern slope of the tomb are a little more than one meter wide, and their guard walls are less than one meter high. But the steps on the northern slope are five meters wide, and their guard walls are one meter high.

When the steps were being built, they didn't have a specific design. After they were built, people found they produced the sound. So this strange acoustic phenomenon was formed by chance. If the guard walls were a little lower or the steps were a little narrower, it wouldn't have occurred.

 

Editor:Ge Ting