Abstract: A sieve (10) has abase (3), a sieve screen frame (4) and a sieve screen (12) mounted in the frame so as to be substantially horizontal. A first vibrator (1) vibrates the frame relative to the base in a generally horizontal direction at a first frequency less than 400 Hz. A second vibrator (2) has a vibration transmission member (6) attached directly to the sieve screen and an excitation source which activates the vibration transmission member to vibrate the sieve screen (12) relative to the frame (14) in substantially vertical direction at a second frequency less than 400 Hz. The resulting activity of the screen reduces blinding in use.