An omnidirectional (or nondirectional) microphone's response is generally considered to be a perfect sphere in three dimensions. The smallest diameter microphone gives the best omnidirectional characteristics at high frequencies. Many engineers are afraid of using omnidirectional microphones in a multi microphone setup with several musicians or sound sources. "Leakage" seems to be the buzzword that is often heard in such situations and, without even trying anything else, directional mics are by habit chosen. A cardioid may be the right choice, but often an omni would give a better performance, because of its sonic qualities, low handling-, wind- and pop-noise and lack of proximity effect. The omni, however, can be moved closer to the source, without the penalty of the proximity effect that occurs with a directional microphone (pressure gradient transducers). The leakage that still occurs - in an acoustic setting it cannot entirely be avoided - is at least of neutral tonal quality and may add some beautiful "air" around the instrument.