Whispering Corridor

Oto-kun

I see. I was wondering why a small whisper can be heard from a distance on the opposite side.

father

It would be quicker to explain it with a diagram. The cloister of St. Paul's Cathedral in London is circular when viewed flatly under the dome. This circular shape is a factor in this phenomenon.

Oto-kun

Is the spherical part on top of the dome not relevant?

father

It is an important condition that it forms a beautiful circle when viewed flat. The cross-sectional shape is bell-like, as shown in this diagram. Of course, since it has a concave surface when viewed from the inside, phenomena such as sound focus occur, but these are limited to the relationship between the sound source and the listening point.

Oto-kun

Why does a phenomenon like a whispering gallery occur with a beautiful circle?

father

The phenomenon of the whispering gallery was that sounds whispered softly could be heard even at distant places in the corridor. Look at this diagram. If there is only one reflection path, sound will attenuate as the distance increases, making it inaudible at far locations. The characteristic of this circular reflection is that there are many reflection paths with slight differences in the path lengths, which reach the listening point almost simultaneously, allowing the sound waves to overlap and be heard.

Oto-kun

Does it mean that because the reflections occur repeatedly and the waves overlap a lot, they can still be heard even if they are far away and have attenuated?

father

That's true. However, while spaces that include concave surfaces can create interesting phenomena, caution is necessary when designing concave surfaces in auditoriums, where clear sound transmission is required, and in concert halls, where smooth sound decay is desirable.

Oto-kun

Attention?

father

In a hall, if reflections are repeated like a crying dragon, the natural sound will be compromised, right? Therefore, it is necessary to design shapes and select interior materials that prevent such obstacles.

Oto-kun

By the way, you mentioned that the ceiling of the crying dragon in Nikko is also slightly concave.

father

Ah, it's 'mukuri.' Yes, so when architects want to design with concave surfaces, it is common to respect the shape and incorporate sound-absorbing materials, or to have a large concave form but use finely detailed convex reflective walls to diffuse sound.

Oto-kun

Hmm. I want to try the whispering corridor. Is there a circular corridor nearby?