The primary difficulty inherent in this approach is that high-frequency localisation and clarity relies on the diaphragms approaching true coincidence. By stacking the capsules vertically, perfect coincidence for horizontal sources is obtained. However, sound from above or below will theoretically suffer from subtle comb filtering effects in the highest frequencies. In most instances this is not a limitation as sound sources far from the horizontal plane are typically from room reverberation. In addition, stacked figure-8 microphone elements have a deep null in the direction of their stacking axis such that the primary transducer in those directions is the central omnidirectional microphone. In practice this can produce less localisation error than either of the alternatives (tetrahedral arrays with processing, or a fourth microphone for the Z axis.)