| 摘要 |
Fluid, in which particles are suspended, is passed along a duct (12, 14, 16, 18) having means (20, 22) for establishing an acoustic standing wave field so that the particles are displaced to form parallel bands. The duct includes an expansion in width (16) so that the spacing between the bands of particles is increased, so that these bands are easier to observe and/or to separate from the duct.
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