Apparatus (1, 2) for examining particles (WBC, RBC, PLT) in a liquid to be examined, having a process stage for separating and positioning the particles (WBC, RBD, PLT) to be examined, comprising: - a flow passage through which the liquid to be examined is moved at a first flow rate (v s), comprising at least one inlet (E3, E4), through which at least one sheath liquid (PF1, PF2) flows into the first flow passage at at least one second flow rate (v 3, v 4) such that the at least one sheath liquid (PF1, PF2) forms at least one sheath flow (MS1, MS2) in the flow passage and the liquid to be examined flows through the flow passage longitudinally in relation to the at least one sheath flow (MS1, MS2), - a wave-producing device for piezo-acoustic production of sound waves that propagate through the flow passage transversely to the flow direction of the liquid to be examined and that form wave nodes in an observation plane such that, on account of the effect of pressure of the sound waves in the transversal direction, particles (WBC, PLT, RBC) to be examined of the liquid to be examined are displaced transversely into the observation plane and are accumulated there, wherein the value of the at least one second flow rate (V 3, v 4) is chosen in such a way that the at least one sheath flow (MS1, MS2) has a predetermined cross section such that the observation plane extends through the at least one sheath flow (MS1, MS2) and the particles (WBC, PLT, RBC) to be examined are accumulated in the at least one sheath liquid (PF1, PF2), wherein the apparatus is configured to form standing waves by arranging reflection elements opposite to the wave-producing device, the apparatus further comprising drive elements for producing the individual flows of the at least one sheath liquid (PF1, PF2) and of the liquid to be examined, the apparatus further comprising a control unit for choosing or controlling the flow rates, wherein the control unit is configured to calculate the values for the individual flow rates in advance and match these to one another, and to control the drive elements for producing the individual flows of the sheath liquids and of the liquid to be examined in accordance with the ascertained flow rates. - a flow passage through which the liquid to be examined is moved at a first flow rate (v s), comprising at least one inlet (E3, E4), through which at least one sheath liquid (PF1, PF2) flows into the first flow passage at at least one second flow rate (v 3, v 4) such that the at least one sheath liquid (PF1, PF2) forms at least one sheath flow (MS1, MS2) in the flow passage and the liquid to be examined flows through the flow passage longitudinally in relation to the at least one sheath flow (MS1, MS2), - a wave-producing device for piezo-acoustic production of sound waves that propagate through the flow passage transversely to the flow direction of the liquid to be examined and that form wave nodes in an observation plane such that, on account of the effect of pressure of the sound waves in the transversal direction, particles (WBC, PLT, RBC) to be examined of the liquid to be examined are displaced transversely into the observation plane and are accumulated there, wherein the value of the at least one second flow rate (V 3, v 4) is chosen in such a way that the at least one sheath flow (MS1, MS2) has a predetermined cross section such that the observation plane extends through the at least one sheath flow (MS1, MS2) and the particles (WBC, PLT, RBC) to be examined are accumulated in the at least one sheath liquid (PF1, PF2), wherein the apparatus is configured to form standing waves by arranging reflection elements opposite to the wave-producing device, the apparatus further comprising drive elements for producing the individual flows of the at least one sheath liquid (PF1, PF2) and of the liquid to be examined, the apparatus further comprising a control unit for choosing or controlling the flow rates, wherein the control unit is configured to calculate the values for the individual flow rates in advance and match these to one another, and to control the drive elements for producing the individual flows of the sheath liquids and of the liquid to be examined in accordance with the ascertained flow rates.