Photo/Illutration Yuto Shibasaki, left, and Michiru Tsurumaru at Tokuyama Senior High School in Shunan, Yamaguchi Prefecture, on Jan. 17 (Natsuno Otahara)

SHUNAN, Yamaguchi Prefecture—High school students here have figured out the mechanisms of a medieval eavesdropping device that was among the most important tools for ninja on espionage missions.

The students at Yamaguchi Prefectural Tokuyama Senior High School will present their research results at the pre-college competition of the International Science and Engineering Fair (ISEF) in the United States in May. They will be members of Japan’s national team.

Ninja are believed to have been active from the Period of the Northern and Southern Dynasties (1336-1392) through the Edo Period (1603-1867).

The primary mission of ninja, also called “shinobi” (the stealthy), was to gather intelligence for their lords, often using a metal plate called “saoto-kikigane,” which literally means “metalwork for listening to faint sound.”

Made of brass or other materials, the plate typically measured about 4.5 centimeters long, 3 cm wide and 3 millimeters thick.

The gadget is mentioned in “Ninpiden,” a book on “ninjutsu” (art of ninja) from the early Edo Period.

A complete, contemporary Japanese edition of “Ninpiden,” translated and annotated by Atsumi Nakashima, is available from publishing house Kokushokankokai Inc.

One passage describes saoto-kikigane as a “ninja tool that was likely more useful than anything else for the stealthy tribe.”

Another passage says, “When you have sneaked into a house and found people’s voices are inaudible, you are advised to dangle this tool and bring it close to the side of your ear so you can listen more clearly.”

But how the device was used and what effect it had remained a mystery.

A test on how a “saoto-kikigane” dangled in the air changes sound waves emitted from a speaker (Natsuno Otahara)

Three Tokuyama Senior High School science club members with a soft spot for ninja began studying the matter in spring 2021 after checking out Nakashima’s book at their school.

They were particularly interested in a passage that said, “I have never made an experimental model for the device, so I cannot explain about its effects.”

The students enlisted the help of Nakamura Metal Works Co., based in Shunan, Yamaguchi Prefecture, to produce an experimental model made of brass.

The model saoto-kikigane measures 7 cm by 3 cm and weighs 48 grams.

Lab experiments showed the saoto-kikigane does not effectively amplify sounds but rather lowers the volume of high-pitched tones, with frequencies of 5,000 hertz or more, around it.

Their study was taken over this academic year, which began in April, by year-younger students Michiru Tsurumaru and Yuto Shibasaki, who accumulated more precision data.

The second-year students applied sound on the saoto-kikigane and made measurements on its back side. They found that the higher the pitch, the more the plate diminished the sound volume, especially near both ends of the device.

Tsurumaru and Shibasaki said physical phenomena produced the result.

Sound waves in higher-pitch ranges are less likely to be diffracted, or turn around corners, onto the back side of the device, they said.

In addition, they said, sound waves interfere with each other and cancel each other out.

The pair concluded the saoto-kikigane made human voices more audible by blocking the high-pitched sounds through a “noise-canceling” effect.

Their lab tests showed the device made human voices twice more audible.

An artist’s rendition shows how a ninja presumably dangled a “saoto-kikigane” from a hairpin by a thread and applied an ear to one of its ends. (From a research paper by Michiru Tsurumaru and Yuto Shibasaki)

“A ninja presumably used a saoto-kikigane by dangling it by a thread so that one end roughly fell near an earhole,” they wrote in their research paper.

Tsurumaru and Shibasaki were curious about what motivated ninja to block such high-pitched tones. They realized the sound of falling rain and the chirping of insects were likely major sources of background noise when there were no cars or cellphones around.

Measurements showed that bell crickets chirped at high-pitched frequencies of around 5,000 hertz in a rice paddy not far from their school.

The students assumed such ambient noises were a nuisance for ninja on eavesdropping missions.

Eager to bring ninja technology back to life, the students used a 3-D printer to make a device featuring a small piece of metal that resembles a saoto-kikigane.

They showed that bringing the equipment closer to a microphone allowed a human voice to be captured more clearly through the noise-canceling effects.

Tsurumaru and Shibasaki presented their study at the 20th Japan Science & Engineering Challenge (JSEC 2022), a competition of independent research results by high school and technical college students. They were awarded a JFE Steel prize, one of the top eight awards, during the final screening held last December.

They are currently translating their research paper into English ahead of their presentation at the ISEF in Dallas in May.

“It’s great that ninja used a tool that had such a theoretical basis,” Tsurumaru said. “They are like scientists.”

“I want the world to know how fascinating ninja are,” Shibasaki said.

Nakashima, the 78-year-old translator of the ninjutsu book that inspired the research, commended the students.

“As far as I know, this is the first time that anyone has demonstrated that a saoto-kikigane effectively blocks high-pitched noise,” he said. “The enthusiasm of the high schoolers who pursued their curiosity through and through is wonderful.”

Nakashima, a long-time student of ninja and ninjutsu, serves as president of an International Ninja Research Association.

“It really makes me wonder how ninja took notice of this scientific effect and began using the device,” he said.