A method of washing particles in an acoustic chamber (104, 1104, 2104, 3104), the method comprising: a) flowing a suspension of particles in a suspension medium through a standing wave generated in a standing wave volume (133, 1133, 2133, 3133) of the acoustic chamber (104, 1104, 2104, 3104) to accumulate within the acoustic chamber (104, 1104, 2104, 3104) at least some of the particles as raw particle concentrate; and b) flowing a wash medium through the acoustic chamber (104, 1104, 2104, 3104) to wash within the acoustic chamber (104, 1104, 2104, 3104) at least some of the particles of the raw particle concentrate, and retaining within the acoustic chamber (104, 1104, 2104, 3104) at least some washed particles; wherein some particles of the raw particle concentrate settle within a settling volume (112, 1112, 2112, 3112) of the acoustic chamber (104, 1104, 2104, 3104), and wherein some of the washed particles settle within the settling volume (112, 1112, 2112, 3112), the settling volume (112, 1112, 2112, 3112) located below the standing wave volume (133, 1133, 2133, 3133); characterized in that the acoustic chamber (104, 1104, 2104, 3104) includes a first bottom port (120, 1120, 2120a, 3120) below the settling volume (112, 1112, 2112, 3112), a top port (128, 1128, 2128, 3128) above the standing wave volume (133, 1133, 2133, 3133), and a side port (1129, 2129a, 3129) above the settling volume (112, 1112, 2112, 3112), and wherein during step (b), the wash medium flows through the acoustic chamber (104, 1104, 2104, 3104) from the first bottom port (120, 1120, 2120a, 3120) to the top port (128, 1128, 2128, 3128) through the raw particle concentrate, and wherein the method further comprises at least one of: i) during step a) flowing the suspension medium through the acoustic chamber (104, 1104, 2104, 3104) from the side port (1129, 2129a, 3129) to the top port (128, 1128, 2128, 3128) through the standing wave; ii) after step (a) and prior to step (b), evacuating at least some supernatant suspension medium from the acoustic chamber (104, 1104, 2104, 3104), and replacing the evacuated supernatant suspension medium with the wash medium, wherein the supernatant suspension medium is evacuated from the acoustic chamber (104, 1104, 2104, 3104) through the side port (1129, 2129a, 3129) and/or wherein the wash medium flows into the acoustic chamber (104, 1104, 2104, 3104) through the side port (1129, 2129a, 3129) to replace the evacuated suspension medium. a) flowing a suspension of particles in a suspension medium through a standing wave generated in a standing wave volume (133, 1133, 2133, 3133) of the acoustic chamber (104, 1104, 2104, 3104) to accumulate within the acoustic chamber (104, 1104, 2104, 3104) at least some of the particles as raw particle concentrate; and b) flowing a wash medium through the acoustic chamber (104, 1104, 2104, 3104) to wash within the acoustic chamber (104, 1104, 2104, 3104) at least some of the particles of the raw particle concentrate, and retaining within the acoustic chamber (104, 1104, 2104, 3104) at least some washed particles; i) during step a) flowing the suspension medium through the acoustic chamber (104, 1104, 2104, 3104) from the side port (1129, 2129a, 3129) to the top port (128, 1128, 2128, 3128) through the standing wave; ii) after step (a) and prior to step (b), evacuating at least some supernatant suspension medium from the acoustic chamber (104, 1104, 2104, 3104), and replacing the evacuated supernatant suspension medium with the wash medium, wherein the supernatant suspension medium is evacuated from the acoustic chamber (104, 1104, 2104, 3104) through the side port (1129, 2129a, 3129) and/or wherein the wash medium flows into the acoustic chamber (104, 1104, 2104, 3104) through the side port (1129, 2129a, 3129) to replace the evacuated suspension medium.