|This article does not cite any references or sources. (December 2009)|
Vibrating alerts are primarily used when a user cannot hear the ring tone (a noisy environment or through hearing loss) or wants a more discreet notification. However when the device is placed on a hard surface it can often be as loud or louder than a ringtone.
Most 21st-century mobile phones are fitted with a vibrating alert, one of the exceptions being the Nokia 9500: due to the large size of this phone a larger motor would be required to make the phone vibrate. The phone itself would absorb too much of the vibration and therefore the motor required to make it vibrate was too big and could damage the phone.
The vibrating alert is produced by a small electric motor connected to an eccentric (unbalanced) weight.