Abstract: A separator for effecting separation of solid materials from liquids has a filter screen arranged so that in operation liquid passes through that screen and solid materials are retained on and travel along that screen in a first direction. Generator means is provided for subjecting the screen to acoustic pressure waves, the generator means being arranged close to but not touching the screen so that in operation liquid passing through the screen couples acoustic pressure waves from the generator means to the screen. The generator means is an acoustic pressure wave transducer assembly that stretches at least nearly the whole way across the filter screen in a second direction transverse to said first direction. Preferably the generator means produces ultrasonic pressure waves and comprises a casing and a plurality of individual ultrasonic transducers within the casing so that a relatively uniform field of ultrasonic waves is produced over the entire width of the screen.