Echo Filter

The is a technology used to see beyond obstacles using ultrasonic waves.

Principles
It is similar to a the ultrasound that is used on pregnant women to look on the fetus inside a womb, however, the output was stronger.

Originally, the idea of using ultrasound to scan beyond obstacles was derived from research in applications dealing with counter-terrorist operations. However, since the images often end up vague as a result (making it difficult to tell who the person that the ultrasound revealed), it had gone for many years without becoming widely used.

The device can apparently be used in a smart phone.

Shinyaku Toaru Majutsu no Index

 * Main article:Baggage City Arc

The Echo Filter is first used by Kihara Enshuu in order to figure out what's behind a certain room's contents. Here, she discovers that Ayles Bigant is behind the door, and later uses a remotely controlled crane to smash down on his hiding place, crushing him, though not killing him.