Echo Key Beams.
The Echo Key Beam system is a terminal in Metroid Prime 2: Echoes that interacts with a sonic pulse from a combination of the Annihilator Beam and Echo Visor to open Echo Gates. Three terminals are found with each Echo Gate, each one emitting a different pitch of sound, and they must be shot in a certain order to open the gate.
These systems are found in the Main Gyro Chamber and Sentinel's Path in the Sanctuary Fortress, and the Profane Path in the Sky Temple Grounds. There are a total of nine Echo Key Beam systems; one of which is not used, as the Echo Gate linked to it only uses two different tones in its unlocking sequence.
Scan[]
"Object scan complete.
This is an Echo Key Beam system.
Sonic detection gear needed to interface with this system. Shoot the Echo Key Beam emitter with a sonic pulse to activate it. It will then fire a blast of focused sound at an Echo Gate lock."