The invention covers the field of positioning of small particles (58, 59) in a fluid (55). Disclosed is a method by which small particles (58, 59) are positioned by a sound field. For generating the sound field an apparatus contains a plate (51) excited to vibrations by a transducer (52) and a reflecting body with a rigid surface (54). A mounting (53) holds the vibrating plate (51) and the transducer (52). The particles will be concentrated at predetermined positions either at the rigid surface of the reflecting body (58) or levitating in the fluid (59). Due to the fact that the sound waves are emitted by the vibrating plate particles can be positioned in entire area between the vibrating plate and the reflecting body.