Abstract: A device for performing the manipulation of particles suspended in a fluid is disclosed. The device comprises a duct for the flow of a fluid in which particles are suspended, an acoustic transducer, and a reflector for establishing an acoustic standing wave field across the width of the duct. It has been unexpectedly found that optimum performance of the device occurs when the spacing between the transducer and a reflector is 300 microns or less. The device seeks to overcome prior engineering difficulties that prevented one from observing the individual particle bands or separating the individual particle bands from the duct.