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Boundary microphone
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) is/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.
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 object. The place right next to the object was called the "boundary layer", because it was between the object (no movement of air) and the free-flowing stream of air beyond it.
A conventional microphone placed on, or a few inches above, a hard boundary surface will pick up the desired direct sound as well as delayed sound reflecting off the boundary surface. The direct and delayed reflected sounds will combine at the microphone to create comb filtering, with constructive and destructive interference causing peaks and valleys in the frequency response. The delay time of the reflection for most microphones would be in the range of 0.1 to 1 milliseconds, corresponding to cancellation frequencies of a few kilohertz and octave multiples. Since these frequencies are audible, the cancellation effects are also audible and are said to "color" the resulting audio signals. An approach was described by Roger Anderson and Bob Schulein in 1971 using a microphone stand to place the microphone near the floor Another implementation of this approach was patented in 1973 by Lou Burroughs of ElectroVoice
In 1978, audio engineers Ed Long and Ron Wickersham studied the effects of the boundary layer in sound recording. In a study of flush-mounted microphones, they realized if the mic is only a few millimeters away from a large surface, there is a boost in coherence of sound signals, as they are still in phase after being stopped at the boundary. The point where the waves were stopped at the boundary was called a "pressure field" or "pressure zone". They also noted that using a mic in this set-up leads to a six dB boost in sound pressure, and improved uniformity of response Only two years after Long and Wickersham's patent application, in 1980 Crown Audio developed and marketed the first boundary mic.
By placing the diaphragm of the microphone capsule parallel to and facing the plate boundary provided by the microphone package, the reflected sound delay is reduced, and the resulting comb filter interference frequencies are high enough that they are outside the audible range.
The terms "boundary microphone" and "pressure zone microphone" (or PZM) refer to the same general design. PZM is a trademarked by Crown. Crown's trademark on this approach is "Phase Coherent Cardioid" or "PCC," but there are other makers who employ this technique as well. Radio Shack and Tandy also use the PZM term for their particular design of a boundary mic, in which the condenser capsule was positioned slightly higher than the base plate. One 1986 source uses the alternate term "barrier microphone" to refer to the boundary microphone approach.
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Boundary microphone AI simulator
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Boundary microphone
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) is/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.
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 object. The place right next to the object was called the "boundary layer", because it was between the object (no movement of air) and the free-flowing stream of air beyond it.
A conventional microphone placed on, or a few inches above, a hard boundary surface will pick up the desired direct sound as well as delayed sound reflecting off the boundary surface. The direct and delayed reflected sounds will combine at the microphone to create comb filtering, with constructive and destructive interference causing peaks and valleys in the frequency response. The delay time of the reflection for most microphones would be in the range of 0.1 to 1 milliseconds, corresponding to cancellation frequencies of a few kilohertz and octave multiples. Since these frequencies are audible, the cancellation effects are also audible and are said to "color" the resulting audio signals. An approach was described by Roger Anderson and Bob Schulein in 1971 using a microphone stand to place the microphone near the floor Another implementation of this approach was patented in 1973 by Lou Burroughs of ElectroVoice
In 1978, audio engineers Ed Long and Ron Wickersham studied the effects of the boundary layer in sound recording. In a study of flush-mounted microphones, they realized if the mic is only a few millimeters away from a large surface, there is a boost in coherence of sound signals, as they are still in phase after being stopped at the boundary. The point where the waves were stopped at the boundary was called a "pressure field" or "pressure zone". They also noted that using a mic in this set-up leads to a six dB boost in sound pressure, and improved uniformity of response Only two years after Long and Wickersham's patent application, in 1980 Crown Audio developed and marketed the first boundary mic.
By placing the diaphragm of the microphone capsule parallel to and facing the plate boundary provided by the microphone package, the reflected sound delay is reduced, and the resulting comb filter interference frequencies are high enough that they are outside the audible range.
The terms "boundary microphone" and "pressure zone microphone" (or PZM) refer to the same general design. PZM is a trademarked by Crown. Crown's trademark on this approach is "Phase Coherent Cardioid" or "PCC," but there are other makers who employ this technique as well. Radio Shack and Tandy also use the PZM term for their particular design of a boundary mic, in which the condenser capsule was positioned slightly higher than the base plate. One 1986 source uses the alternate term "barrier microphone" to refer to the boundary microphone approach.