A boundary microphone (or pressure zone microphone) is one or more small omnidirectional or cardioid condenser mic capsule(s) positioned near or flush with a boundary (surface) such as a floor, table, or wall. The capsule(s) are typically mounted in a flat plate or housing. The arrangement provides a directional half-space pickup pattern while delivering a relatively phase-coherent output signal. The boundary microphone can be used as a piano mic by placing it inside the piano lid, an approach which can obtain better pickup of the piano's mix of sharp percussive transients and gentle undertones than other microphone options. Boundary mics are used on hockey boards for body check sound effects. They are also commonly used to record full room sound, such as in a conference room, by being mounted on a wall or table. When used to record a soloist or small musical ensemble along with the room acoustics (e.g. reverberation), a boundary microphone prevents phase interference between direct and reflected sound, resulting in a natural sound with a flatter frequency response than can be obtained with a stand-mounted microphone at the same distance (explanation below). Boundary mics are usually less expensive than other mics, although there is nonetheless a range of price points that varies, depending on the number of capsules and the absence or presence of additional features, such as on-off switches, wireless capabilities, and levers to reposition the capsule(s). As they are condenser mics, they need power. Boundary mics have a relatively flat profile, they have the advantage of being less visually obtrusive, such as when they are placed in a conference table or the floor or a musical theater stage. At the same time, the placement of the mic on a table or floor may increase the likelihood of unwanted sounds from contact of items or body parts on those hard surfaces. Development and function In the early 20th century, physicist Ludwig Prandtl studied air movement and aerodynamic forces between air (or fluid) and an object. On August 8, 1904, he delivered a groundbreaking paper, Über Flüssigkeitsbewegung bei sehr kleiner Reibung (On the Motion of Fluids in Very Little Friction), at the Third International Mathematics Congress in Heidelberg. In this paper, he described the boundary layer and its importance for drag and streamlining. The paper also described flow separation as a result of the boundary layer. He discovered that some molecules of air "stick" to the