In both tapping and rubbing, a voice coil, such as one found in a standard 3.5-inch hard disk drive, was used. To "rub," a long arm was mounted on the actuator, so that the arm would "swing" through a 30 degree angle or so. A nub was then mounted orthogonally to the arm, so the rubbing could be felt on the bottom of a user's palm as he rested it on the frame. Tapping was much simpler: all the users needed to do was to mount a "hammer" onto the head, so that its movement created a tapping sensation. ... In a series of small tests, users were asked to experience a series of taps and rubs, and to determine which of a pair was strongest, and then which of a series was the hardest or fastest. The group was also asked to determine which taps or rubs felt the most natural. The researchers determined that the softer taps felt more natural, while faster taps blurred into vibration. Interestingly, the "rubbing" technique that the researchers used couldn't actually generate enough pressure to feel natural.