Abstract: A flow cytometer has an aperture in which an inlet chamber and/or an outlet chamber have predetermined geometric relationships with respect to the aperture for decreasing electronic edge effects and increasing the sensitivity of electronic particle volume measurements. In one embodiment, the aperture is triangular and is formed by assembling a plurality of truncated pyramids with their truncated surfaces defining aperture walls. At least one of the elements forming the aperture walls is either part of a lens system or a transparent cover plate so that simultaneous optical and electronic cell volume measurements can be performed. On one embodiment, a piezoelectric transducer is provided to sonically clear any aperture clogging.