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

Dad: It was a long ago. You want to hear about the whispering gallery?

Ken: Yes. I wonder why you can hear a whisper from the opposite side at a distant.

Dad: It is faster to explain with a diagram. The gallery of St. Paul's Cathedral in London is circular in a plan view. This circular shape is the reason of this phenomenon.

Ken: Does the spherical shape of the roof of the dome have nothing to do with it?

Dad: Showing a perfect circle in the plan view is important. It is a bell shape in a cross-sectional view as shown in this diagram (Diagram 1) (Roll cursor over here for picture). This inside concave shape, of course, causes peculiar phenomenon like focusing a sound, but it only occurs at a specified point with a special relation between a sound source and a listening point.

Ken: How come if the circle is perfect, a phenomenon like the whispering gallery occurs?

Dad: The phenomenon of whispering gallery is that you can hear whispers at the gallery even at distant places in the gallery. Look at this diagram (Diagram 2) (Roll cursor over here for picture). If there is only one reflection path and when the distant is further, attenuation occurs so you will not be able to hear it from afar. The characteristic of this circular shape reflection is that there are many reflection paths with just a slight difference, and the reflections arrive at the receiving point almost at the same time so you hear the sound waves overlapping.

Ken: Because the small reflections are repeated and the reflected waves are overlapping, you can still hear the attenuated voice from afar, correct?

Dad: Yes. The spaces with concave interior may create interesting phenomenon like this. However, in an auditorium where voices have to be clear or concert halls where an attenuation of smooth sound is desirable, designing concave surface must be done with caution.

Ken: Caution?

Dad: If repetitive reflections like the Nakiryu occur in a hall, natural vibration would be lost. Therefore, careful design of the interior shape and deliberate selection of interior materials are necessary to avoid such problems.

Ken: I remember you said that the ceiling of Nakiryu in Nikko was slightly concaved.

Dad: Ah, it's called "mukuri". Anyway, if an architect wants to design a concave surface, common measures to be taken are to use the sound absorbing material on that surface or to make many small convexes on the concaved surface to diffuse the sound.

Ken: I want to experience the whispering gallery. I wonder if there is a circular gallery nearby.