Boundary microphone
A boundary microphone, engl. " Boundary layer microphone" or " PZM / pressure zone microphone" ( name protected by Crown International ) is a special design of microphones, in which the housing is part of the transducer.
Principle
Of the microphone body is a plate on which a pressure microphone capsule is usually inserted flush. The polar pattern is thus hemispherical. The transducers are usually capacitors or Elektretkondensatoren. This design was developed to exploit the advantageous acoustic properties occurring at reflecting surfaces, without affecting the sound field itself. The microphone is on a large sound-reflecting surface, such as on the floor laid. It obtains the maximum sound pressure with reduced space sound components, which leads to a balanced frequency response and acoustically good impression of the room:
- At reverberant surfaces occur no disturbing reflections on, as they are generated here only
- In rooms whose resonances are less absorbed by this microphone; By placing a microphone to a boundary surface are no klangfärbenden comb filter effects as they occur within the space. When moving sound sources, no sound color differences arise.
- Space signals R are compared with the direct signals D attenuated by 3 dB, which means a preference of the direct sound.
- Similarly, flat frequency response of the direct sound and room sound. This is not the case with conventional pressure microphones. The timbre varies with neither the distance nor the direction of sound incidence.
- Hemispherical neutral frequency directivity.
- A 6- dB sound pressure level profit at the interface causes an improvement of the signal to noise ratio than comparable spherical microphone capsules that are free-standing.
- Due to the positioning of the microphone body, the sound field is not affected.
Often the special space signal attenuation is accepted and boundary microphones just used for receiving signals from space. This has the reason that so better rooms " sound " since come to fruition in a boundary layer microphone resonances and comb filter effects of room acoustics less. In an AB stereo arrangement of these microphones is of course still comes to the usual comb filter effects when the signals are mixed or is bugged in mono.
Pressure accumulation effect and surface dependence
The principle of the congestion pressure is increased when boundary microphone with pressure microphone capsule through the acoustically active area up to low frequencies. There can therefore be no different diffuse-field and free- field equalized boundary microphones, since the cut-off frequency of this height increase varies with the bearing surface. The diffuse-field frequency response is hereby extended linearly to low frequencies. The cut-off frequency at which this pressure accumulation effect comes in place, is the extent of the reverberant surface on which the boundary microphone is placed .. Prior statements of Dickreiter call as necessary diameter of a circular bearing surface ( d = λ / 2 [m]) for dependent the cut-off frequency. Add new editions, however, the frequency f [Hz ] is given by d = λ [m ], from which the maximum pressure accumulation effect with 6 dB is measured ..
Or
It is the speed of sound c = 343 m / s at 20 ° C.
Variants
In the first interface microphones the membrane to the interface showed; by the microphone body hindered the sound access. This system has been patented and is " sold " the restricted access of sound as an advantage because it will jam the pressure inhibited at high frequencies. In reality appear as artifacts significantly harmful interference, and a wavy frequency response. Rarely boundary microphones also be implemented with a Druckgradientenmikrofonkapsel. The polar pattern is then a "lying" kidney.
Microphones, which can be built according to the principle of the interface to have an upwardly capsule which is flush with the surface. The polar pattern is thus "half sphere". Another variant is PZMs the capsule are indeed embedded in the boundary surface, the directional characteristic, however, "Half kidney" or " Half- supercardioid " is. The 0 degree Einsprechrichtung the capsule is not directed upward in this variant but is either nearly parallel to the interface and 15 to 20 degrees above. Externally, this can be seen by a flattened front (as in the photo above right), this page is aimed at the sound source. Advantage of this variant is the suppression of background noise, such as the sound system, so if a directional microphone is used on a stage.