High speed train silently slices through air
In Japan, the 500-series bullet trains could travel 300 km/hour (200 mph) but the sound levels exceeded environmental standards. One source of noise was an atmospheric pressure wave forced in front of the train as it traveled through a narrow tunnel, creating a loud "tunnel boom" at the exit. The bullet-shaped nose was part of the problem. Another source of noise was the pantograph, a protrusion that extends above the train to receive electricity from wires overhead. The structural adaptations of the Shinkansen train help to minimize these concerns.
Eiji Nakatsu, an engineer with JR West and a birdwatcher, used his knowledge of the splashless water entry of kingfishers and silent flight of owls to decrease the sound generated by the trains.
Kingfishers move quickly from air, a low-resistance (low drag) medium, to water, a high-resistance (high drag) medium. The kingfisher's beak provides an almost ideal shape for such an impact. The beak is streamlined, steadily increasing in diameter from its tip to its head. This reduces the impact as the kingfisher essentially wedges its way into the water, allowing the water to flow past the beak rather than being pushed in front of it. Because the train faced the same challenge, moving from low drag open air to high drag air in the tunnel, Nakatsu designed the forefront of the Shinkansen train based on the beak of the kingfisher.
Engineers were able to reduce the pantograph's noise by adding structures to the main part of the pantograph to create many small vortices. This is similar to the way an owl's primary feathers have serrations that create small vortices instead of one large one.
Read more about the bioinspiration behind the Shinkansen Train in Zygote Quarterly: