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# Why can the soundwall transmit sound?

In Beijing, Thien Dan is famous in the world because of its majestic architectural art, but what attracts tourists is that there are an echo wall and a very strange three-tone stone. Those who have come to Thien Dan cannot stop weeping the amazing phenomenon of its sound transmission.

We know that we don’t hear clearly when we talk every day, five or six meters away. Yet standing on one side of the wall of the Thien Dan echoing wall spoke softly; the person next to the opposite wall could always hear clearly. Between them apart, more than 50 m away there! There is an even more astonishing thing: if you stand on the three-tone rock in the center of the wall, you can applaud once. You can hear two and three applause at once. Why is there such a phenomenal sound propagation phenomenon? It is thanks to the help of the reply! The true triple sound stone is located in the center of the wall’s circle surrounding the reverb wall. The sound emitted on the triplet will uniformly propagate to the retaining wall’s parts and be reflected by the retaining wall. Also, go through the circle’s center, so standing on the three-tone stone can hear clear echoes. The echo after the reflection passes through the circle’s center, resuming its propagation along the circle radius. When they hit the opposite wall, they were reflected again. So we hear the second and third echoes right away.

The reverb wall tile in Thien Dan is firm and smooth, a very good acoustic reflector. As shown in the figure: when a person stands talking at point A on one side of the wall, the sound will follow the wall to point 1, then from point 1 will bounce off, follow the wall to point 2, again to points 3, 4, etc., finally to point B on the other side of the feedback wall. Because the brick wall absorbs very little sound, the sound is constantly reflected on the surrounding wall, unlike when transmitted in the air, it is easy to spread out and attenuate.

The sound emitted from point A has traveled a very long distance, but when it comes to point B, it is still very clear; it seems that the sound is transmitted from point C that is so close by.

James Smith
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