singing dragon
Oto-kun
I went to Nikko Toshogu Shrine on a recent school trip. Under the crying dragon at Yakushido, a temple staff member used clappers to make a sound, and I was able to hear the dragon's voice.
father
That's good, isn't it? Could you explain why the dragon roars?
Oto-kun
Yes. They explained that the sound is repeated due to the reflection between the ceiling and the floor, but upon further thought, I find it strange why it only sounds in the presence of the dragon.
father
There is detailed information about the Crying Dragon, so let me explain it to you. The dragon painting was originally said to have been created by Kanou Eishin Yasunobu, but the Yakushido, which houses the Crying Dragon, was burned down in a fire in 1961. In 1968, the dragon painting was restored by the artist Katayama Nanpu.
Oto-kun
Were dragons known to roar before a fire occurred?
father
That's right. Therefore, in order to reproduce that sound, they conducted model experiments to confirm its effects, and it seems they restored not only the visuals but also the sound.
Oto-kun
Wow. I wonder how that model experiment was conducted?
father
In a paper published in 1965[1] ※, it is written about the results measured in real life in 1953 before the model experiments. "The number of repeated reflected waves is 29 times/second, it takes 2.5 seconds to attenuate by 60 dB, and due to the 6 cm dome (the ceiling being a two-dimensional concave surface), the sound of clapping hands in the center resonates for a long time due to the back-and-forth reflection between the ceiling and the floor," it says. As shown in this figure (Figure 1), the results observed with the oscillograph at that time show that the pulse of the reflected wave appears continuously, representing the phenomenon described as the dragon's roar. In the model experiments (1/4 scale), it is stated that the differences in waveforms due to the size of the dome and the position of the sound source were observed, clarifying what is necessary to reproduce it.
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Naki Falls Before Restoration
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Crying Dragon After Restoration
Oto-kun
But if there are parallel surfaces without 'mukuri,' wouldn't that lead to reciprocal reflection?
father
That's a good question. When the ceiling is a concave surface, sound waves that enter at an angle reflect back in the direction they came from, concentrating the reflected waves at the position of the sound source (listening point). However, when the ceiling is flat, sound that enters directly from above undergoes multiple reflections parallel to the floor, but when it enters at an angle, the path of reflection moves away from the center by that angle. Therefore, with a concave ceiling, the number of repeated reflected waves is greater, and the duration is longer.
Oto-kun
It's strange, but the sound of the clapper seemed to ring out like a 'beep.' It wasn't the repeated sound of a 'clang,' so I was a bit surprised.
father
That is another mystery of the crying dragon. The principle of sound being heard repeatedly and becoming echo-like is as I explained earlier, but there is another reason why the original sound and the altered sound are perceived differently. First, when humans hear a series of short-duration sounds (like a snapping sound, referred to as a pulse), if the interval between the sounds is shorter than 50 ms (0.05 seconds), the sounds do not seem separate and are heard as one sound. Conversely, if the interval between the sounds is longer than 50 ms, they are perceived as separate. When the original sound wave is a pulse, it is called a flutter echo, resulting in a flapping sound.
Oto-kun
Sound travels about 340 meters in one second. So, if the time is 0.05 seconds, does that mean that two sounds arriving from 17 meters apart can be heard separately, but if they are closer, they will be perceived as one sound?
father
To be precise, the point where the sound is heard is called the sound reception point. When two sounds reach that reception point with a time difference of 50 ms and a distance difference of 17 m, they can be heard as two separate sounds or as one cohesive sound. Therefore, considering the length of the reflection path, each reflected sound from the sunlight's singing dragon should not be heard separately. (Note 1)
There is a sample of continuous sound, so give it a listen.
This is a sample of a sound with a single frequency (called a pure tone) of 1000 Hz, which has periodic time variations applied to it, ranging from slow to gradually faster variations. Even with continuous sound, once the period exceeds 50 ms (20 Hz), it becomes possible to capture that period.
There is a sample of continuous sound, so give it a listen.
This is a sample of a sound with a single frequency (called a pure tone) of 1000 Hz, which has periodic time variations applied to it, ranging from slow to gradually faster variations. Even with continuous sound, once the period exceeds 50 ms (20 Hz), it becomes possible to capture that period.
Oto-kun
The frequency here indicates how many times it changes in one second.
father
That's right. When you listen to it starting from 1 Hz, which is once per second, did you notice anything?
Oto-kun
As my father said, sounds become indistinguishable at around 20 Hz, and at frequencies higher than 50 Hz, they sound somewhat muddy.
father
The sound of the clapper under the picture of the crying dragon did not sound like the original 'kahn', but rather like a 'bean'. When temporal fluctuations occur at the same cycle, humans perceive the frequency of the time cycle (modulation frequency) around 70 Hz as the roughest sound. I mentioned that the sound felt murky, but in other words, it means that the sound was heard as rough and gritty.
Oto-kun
There was something mysterious about the crying dragon of sunlight, but there was a reason for that.