Whispering Corridor

Dad, have you ever been to St. Paul's Cathedral in London?

Ah, that was quite a while ago, wasn't it? Are you asking about the Whispering Corridor?

I see. I was wondering why I could hear such a quiet whisper from so far away on the other side.

It's easier to explain with a diagram. The cloister of St. Paul's Cathedral in London is circular when viewed in plan at the base of the dome. That circular shape is the cause of this phenomenon.

Is the spherical part on top of the dome irrelevant?

An important condition is that it forms a perfect circle when viewed from above. The cross-sectional shape is bell-shaped, as shown in this diagram. Of course, because it is a concave surface when viewed from the inside, unique phenomena such as sound focusing occur, but these are limited to specific conditions based on the relative positions of the sound source and the receiving point.

Why does a phenomenon like a "whispering corridor" occur when the circle is perfect?

The phenomenon of the whispering corridor is that a whispered sound can be heard even at a distance along the corridor. Look at this diagram. If there were only one reflection path, attenuation would occur with distance, so it wouldn't be heard at a distance. The characteristic of this circular reflection is that there are many reflection paths with only a small difference in reflection path, and because they reach the receiving point at almost the same time, the sound waves overlap and are heard.

So, because the sound is reflected repeatedly and many of those waves overlap, it can be heard even if it's attenuated due to distance?

That's true. However, while spaces with concave surfaces on the inside can produce interesting phenomena like this, caution is needed when designing concave surfaces in auditoriums where clear sound transmission is essential, or in concert halls where smooth sound decay is desirable.

Note?

In a hall, repeated reflections, like a "singing dragon," can ruin the natural acoustics, right? That's why it's necessary to design the shape and select interior materials in a way that prevents such interference.

Come to think of it, I remember hearing that the ceiling of the Nikko Crying Dragon is also slightly concave.

Ah, you mean "concave." So, when an architect wants to create a concave curved surface for design purposes, they respect the shape and typically use sound-absorbing materials on that surface, or they create a large concave shape but then incorporate small convex reflective walls to diffuse the sound.

Hmm. I'd like to try the Whispering Corridor. I wonder if there's a circular corridor-like place nearby.