Temple of Heaven - Sound Transmission Magic or Ancient Technology?

The Temple of Heaven is the place where ancient emperors offered sacrifices to heaven and prayed for a good harvest.

It is particularly famous for the concept of "the sky is round and the earth is square".

The main buildings of the Temple of Heaven, such as the Hall of Prayer for Good Harvests, the Circular Mound Altar and the Imperial Vault of Heaven, are all circular structures, symbolizing the "roundness of the sky."

But who would have thought that behind this symbol lies the secret of ancient science and technology, which gives the Temple of Heaven many acoustic wonders.

1. Echo Gallery: Ancient Telephone

The wall surrounding the Imperial Vault of Heaven is called the Echo Wall. Its design and construction cleverly use the principle of acoustics to produce a magical effect: as long as two people whisper against the wall, they can hear each other clearly even through the hall in the middle, just like making a phone call. This wonderful phenomenon stems from the precise design of the Echo Wall. It turns out that the wall is not simply formed into an approximate circle, but a super-precise circle. The wall is made of Chengjiang bricks produced in Linqing, Shandong during the Ming Dynasty. The texture of this brick is very fine. It is said that "there is a sound when knocked, and there is no hole when broken". It is really a masterpiece of ancient materials. Whether it is hardness, density, or its smooth surface, it provides an ideal foundation for the acoustic effect of the Echo Wall. The choice of materials is perfect.

Because the curvature of the wall is extremely standard and the wall surface is smooth and flat, a circular sound field is formed, so that the energy loss of sound waves is very small when they are reflected along the wall, so even a whisper can be transmitted that far.

In modern ice sculpture art, people use ice bricks with similar properties and the same structure to reproduce the magical acoustic phenomenon of the Temple of Heaven. Of course, ice bricks that melt when the weather warms up can only reproduce the ingenuity of the ancients for a short time, and naturally cannot preserve the acoustic wonders for hundreds of years like Chengjiang bricks.

2. The Echo Magic of the Three-tone Stone

In front of the Imperial Vault of Heaven, there are three special stone slabs, called Heaven Stone, Earth Stone and Human Stone. Don't underestimate them, stand on them and talk - you will immediately feel the "magic" of acoustic design.

The "human stone" is also called the three-sound stone. If you speak quietly on it, you can hear a loud echo. The ancients described this phenomenon as "the whispers of the people on earth are heard by the heavens like thunder", which means that people's voices can be transmitted to the heavens. Although the ancients gave this echo a mysterious symbolic meaning, in fact, this effect is entirely due to the multiple reflections of sound waves.

The Three-Sound Stone is located exactly in the center of the Echo Gallery. When you stand on the Three-Sound Stone and speak, the sound waves will bounce back and forth within the wall, continuously reflecting along the curved surface of the Echo Gallery, and finally transmitted back to the position of the stone slab, and gradually return to your ears.

If you clap your hands once while standing on a rock, you can even hear three echoes! These three echoes actually come from one sound, but they travel different distances after being “split into several groups”, resulting in different times when they reach your ears.

Simply put, sound waves return to the center of the circle through different paths, resulting in a tiny time difference. Your ears will be "tricked" and unable to distinguish echoes that are too close together, and will merge them into several groups of loud echoes.

After precise measurements and calculations, modern people have discovered that the first echo is formed by the superposition of reflected waves from the walls and foundations of the two side halls of Huangqiongyu, while the second and third echoes are formed by the convergence of the first and second reflected sound waves from the echo wall.

These acoustic wonders reflect the ancient craftsmen's profound understanding of acoustics and materials. Following these wonderful phenomena, we can discover the ingenuity of the ancients. Through clever design and exquisite craftsmanship, the ancients not only created buildings with aesthetic value, but also applied scientific principles to integrate the laws of sound propagation in nature into architecture to achieve a perfect fusion of beauty and technology.

Therefore, even ancient buildings are full of technological elements.

Author: Hikka Slate Restoration Group

Reviewer: Liu Jie, Associate Professor, School of Architecture and Art, Beijing Jiaotong University

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.