Ken: The other day, I went to the Nikkou-Toushou-Gu for my school excursion. Under the Nakiryu in the Yakushidou, a monk used wooden blocks to make noise so I could hear the dragon’s voice.

Dad: That’s good. Did they explain why the dragon cries?

Ken: Yes. They said it’s from continuous reverberation between the ceiling and the floor, but now that I think about it, it’s strange that you can only hear it where the dragon is.

Dad: I have detailed information regarding the Nakiryu so let me explain. The dragon was originally drawn by Kanou Eishin Yasunobu, but the Yakushidou that housed the Nakiryu was burned down in 1961. In 1968, the artist Katayama Nanpu restored the drawing.

Ken: Was the dragon known to cry, even before the fire?

Dad: Yes. Model experiments were done in order to reproduce the sound, so not only the drawing but the sound was also restored.

Ken: I see. I wonder how the experiment was done.

Dad: Listen In a document (1) published in 1965, the following was written describing the result measured based on the actual structure from 1953, which took place before the model experiment. "Since the number of continuous reverberation wave is 29 times per second, taking 2.5 seconds to attenuate 60dB, and 6 cm of indentation (ceiling is 2D concaved surface), the sound made by the clapping of hands in the center causes a long echo from repetitive reverberation between the ceiling and floor."

Ken: But even if there isn’t an indentation, if there is a parallel surface, wouldn’t a repetitive reverberation occur?

Dad: Good question. When the ceiling is concaved, it is reflected in the direction it enters, so the reverberation wave is concentrated at the sound source. But if the ceiling is flat, sounds entering straight up continuously reflect at that spot and the floor, and if it enters at an angle, it reflects outward at that same angle which extends away from the middle. That is why if the ceiling is concaved, the number of reflected waves are greater and making the time longer.

Ken: But I was surprised that the sound that was reflected was different from the sound that comes from the wooden block.

Dad: That is the other mystery of the Nakiryu. The reason why the sound becomes repetitive and sounds like an echo was explained earlier, but there is a different reason why the original sound is different from the sound that is heard. First of all, when a human hears a sound with short duration (click sound : called pulse)continuously, if the time between each sound is less than 50 ms (0.05 second), the sound is not separated but heard as a single noise. Vice versa, if the time between the sounds is longer than 50 ms, it sounds separated. When the original wave is a pulse, it is called a flutter echo and makes a fluttering sound.

Ken: Sound travels about 340 meters in one second. So in 0.05 seconds, two sounds seventeen meters away will arrive separately, but if it’s lower than that would it arrive as one sound?

Dad: To be precise, the location where Ken is listening is the sound receiving point. The two sounds with a time difference of 50ms or distance difference of 17 meters in arrives at the receiving point; it may sound like two or a group of sounds. So with the Nakiryu in Nikkou, from the distance of the reflection, each reflecting sound should not be heard separately. *(2) There is a sample of a repetitive sound so listen to it. This is a single frequency(called simple tone) and it is a sample of a 1000Hz sound in a cycle with different change times given, and is started by slowly changing but gradually increasing the change time. Even with a continuous sound, if the cycle exceeds 50ms (20Hz), you are able to grasp the cycle.

Sample of repetitive sound <Flash>

Ken: So this frequency indicates how many times it changes within one second.

Dad: That is correct. Did you notice something when you heard a sound once every second in order from 1Hz?

Ken: So as you said father, from around 20 Hz the sound does not separate and any frequency higher than 50 Hz made a muffled sound.

Dad: This is the same reason why you hear a different sound than the original sound source under the Nakiryu. If there is a difference in time during the same cycle, people find the frequency (modulation frequency) within the cycle of around 70Hz to be the most rough. Ken, you said that you felt the sound was muffled, but if you rephrase, you meant that the sound felt rough.

Ken: The Nakiryu at Nikkou was mysterious, but this was the reason for it.

 

*(1) Takeo Sato, Kiyoteru Ishii, Tomohiko Hirano "Research of the reproduction of the "Nakiryu" in the Nikou Toushou gu. Japan Architect Society Essay Report Collection Sept. 1965

*(2) :In Essay (1) it states, "29 reverberation waves in 1 second" and if it is an even interval the time would be about 35 ms. Aurally, the reason why sounds are heard separately like the flutter echo is because in a certain duration, concentrated groups of reverberation waves are occurring, and if the interval between the waves are over 50 ms, it is presumed the sound is separately heard.

◆Graph, picture, sound file of nakiryu was provided by Professor Tachibana of Tokyo Univerity.