Disclosed is a device, suitable for use as an acoustic resonator, including a base adapted to be coupled to at least one acoustic wave generator, a spacer including an aperture and a reflector, wherein the base includes a protruding part having a thickness t; the aperture of the spacer is complementary to the protruding part of the base; the device further includes a housing having an aperture complementary to the protruding part of the base and wherein the inner edge of the aperture has the same thickness t than the protruding part; and the housing is positioned between the spacer and the reflector, such that the thickness of the inner edge of the spacer defined the thickness of a cavity between the protruding part and the reflector. Also disclosed is a method of trapping particles in a fluid.