For most applications, an inexpensive low-impedance, omnidirectional microphone, such as a lavaliere microphone works adequately. Some installers have used boundary microphones with good success (they’re unobtrusive). Other installers have mounted microphone cartridges (available from Mouser Electronics in the U.S.) inside electrical boxes equipped with a single-holed cover. In smaller rooms or acoustically live spaces, a directional microphone is a good choice, because you can use its position to favor the ambient sound and minimize pickup from the sound system loudspeakers. If the sense mic is positioned a relatively long distance from the ambient noise source(s), a directional microphone may improve system performance.