Sculk Sensor

The sculk sensor is a block that can detect vibrations and emit a redstone signal.

Obtaining
Sculk sensors can currently be obtained only from the creative inventory or commands.

Breaking
Sculk sensors can be mined with any tool, but hoes are the quickest.

Light
Sculk sensors have a light level of 1, which is very inconspicuous. However, when active it visually shows a brighter yellowish light illuminating the surroundings.

Vibration detection
Sculk sensors can detect vibrations in an 8 block radius around it. A vibration is anything that causes physical motion, although there are some motions that are undetectable to sensors by sneaking; these sneak-friendly vibrations include walking, falling to the ground or throwing/shooting projectiles.

Wool has a special interaction with sculk sensors. If a wool block is in the way of a vibration source, the sensor will not be able to detect it.

When a vibration is detected, a signal is sent from the source location to the sensor at a speed of one block per game tick (20 blocks per second). Other vibrations cannot be detected by a sensor when a signal is already travelling to it. When the signal has arrived, the sensor is activated for 40 game tick. While activated, the sensor cannot detect other vibrations.

Sculk sensors have a cooldown period of 1 tick after being placed or after deactivating. During this short cooldown period, it cannot detect vibrations. This is to prevent it from activating itself when a contraption it is powering is being unpowered.

Sculk sensors don't listen to vibrations that are directly created by other sculk sources.

Redstone emission
Sculk sensors emit a redstone signal when they are activated. The strength of the redstone signal is inversely proportional to the distance the vibration signal traveled – the closer the vibration is to the sculk sensor, the stronger the redstone signal is, so it reaches the maximum redstone signal strength when the vibration is directly on top of the sensor.

Vibration frequencies
Sculk sensors have a unique interaction with comparators. Each vibration in the game falls under a certain frequency value, and this value can be measured with a comparator as redstone output. With the right contraption, this allows the player to detect when a certain action has occurred nearby.

Trivia

 * "Sculk" is derived from "skulk," meaning "keep out of sight, typically with a sinister or cowardly motive".