Tremor Pak
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The Tremor Pak is a third-party version of the Nintendo 64's official Rumble Pak, a device designed to vibrate the N64 controller during selected games to enhance realism. For example the Tremor Pak might cause the controller to vibrate when a gun is fired or when the player takes damage.
The Tremor Pak was considered to be superior to the Rumble Pak by some due to its reliance on AA batteries as opposed to the AAA batteries used by the Rumble Pak (AA is a more popular, and cheaper size) and the addition of two levels of vibration, low and high.
Variant versions were even released with a built in controller pak (a memory storage system) and some models even used the N64 console itself as their primary power source, eliminating the need for batteries (though the vibrations were weakened as a side-effect).