Sudden Motion Sensor
The Sudden Motion Sensor is a type of active hard-drive protection developed by Apple, used to protect hard disk drives in their notebook computers. The system first appeared in 2005 on the refreshed PowerBook line and is present on all of Apple's non-SSD notebooks since then, including the MacBooks. The last MacBook with HDD was released in 2012.
With a triaxial accelerometer, the shock detector detects sudden acceleration, such as when the computer is dropped, and prepares the relatively fragile hard disk drive mechanism for impact. The system disengages the disk drive heads from the hard disk platters, preventing data loss and drive damage from a disk head crash. When the computer is stable, the drive operates normally again. A clicking noise can be heard when the sudden motion sensor activates.
Broadly speaking, there have been two types of Sudden Motion Sensor. The sensor used in the G4-based laptops resolved shifts of 1/52 g, while the sensor used in the later Intel-based laptops have an 8-bit resolution. In at least one model of Intel-based laptop, the MacBook Pro 15", Apple uses the Kionix KXM52-1050 three-axis accelerometer chip, with dynamic range of +/- 2g and bandwidth up to 1.5 kHz.