Boundary microphone

PZM is a redirect to this article. This abbreviation also stands for Münsingen Psychiatry Center .

A boundary layer microphone or PZM / pressure zone microphone (protected name of Crown International ) is a special type of microphone in which the shape of the housing determines the acoustic properties.

Boundary microphone

principle

The microphone body is a plate on which a pressure microphone capsule is usually embedded flush. The directional characteristic is thus hemispherical. The transducers are mostly condenser or electret condenser microphones. This design was developed in order to take advantage of the advantageous acoustic properties that occur on sound-reflecting surfaces without impairing the sound field itself. The microphone is placed on a large sound-reflecting surface, e.g. B. on the floor. It thus receives the maximum sound pressure with reduced room noise , which leads to a balanced frequency response and an acoustically good spatial impression:

• No annoying reflections occur on reverberant surfaces , as this is where they arise
• In rooms, their natural resonances are picked up less by this microphone; By placing a microphone on a boundary surface, there are no sound-coloring comb filter effects that occur within the room. With moving sound sources there are no differences in timbre.
• Room signals R are attenuated by 3 dB compared to direct signals D, which means that direct sound is preferred.
• Same, flat frequency response of direct sound and room sound. This is not the case with conventional pressure microphones. The timbre does not change with distance or with the direction of sound incidence .
• Hemispherical frequency-neutral directivity.
• A 6 dB gain in sound pressure level at the interface improves the signal- to -noise ratio compared to comparable spherical microphone capsules that are set up freely.
• Due to the positioning, the microphone body does not affect the sound field.

The special room signal attenuation is often accepted and boundary microphones are used to pick up room signals. The reason for this is that rooms "sound" better with it, since natural resonances and comb filter effects of the room acoustics are less important with boundary microphones . With an AB stereo arrangement of these microphones, the usual comb filter effects continue to occur when the signals are mixed or monitored in mono.

Pressure accumulation effect and area dependency

The principle of pressure build- up in the boundary microphone with pressure microphone capsule is enlarged by the acoustically effective surface down to low frequencies . There can therefore be no different diffuse field and free field equalized boundary microphones, since the cutoff frequency of this treble increase varies with the support surface. The diffuse field frequency response is linearly extended to low frequencies. The cut-off frequency from which this pressure accumulation effect comes into play depends on the extent of the reverberant surface on which the boundary microphone is placed. Earlier statements by Dickreiter cite the necessary diameter of a circular contact surface ( d = λ / 2 [m] ) for the cutoff frequency. In new editions, however, the frequency f _c [Hz] is specified with d = λ [m] , from which the maximum pressure back-up effect can be measured with 6 dB.

${\displaystyle d={\frac {c}{f_{c}}}}$

or.

${\displaystyle f_{c}={\frac {c}{d}}}$

The speed of sound is c = 343 m / s at 20 ° C.

f _c = 30 Hz then must be d = 11.43 m
f _c = 100 Hz then must be d = 3.43 m
f _c = 1000 Hz then must be d = 0.34 m

variants

In the first boundary microphones, the membrane pointed towards the boundary; as a result, the microphone body obstructed sound entry. This system was patented and the narrowed sound access was “sold” as an advantage, because it prevented the pressure build-up at high frequencies. In reality, clearly disruptive interference and a wavy frequency response appear as artifacts . Boundary microphones are rarely also implemented with a pressure gradient microphone capsule. The directional characteristic is then that of a "lying" cardioid.

Microphones that are built according to the interface principle have an upward-facing capsule that is flush with the surface. The directional characteristic is thus "half sphere". Another variant are PZMs whose capsules are embedded in the boundary surface, but whose directional characteristic is "half cardioid" or "half supercardioid". In this variant, the 0 degree direction of contact of the capsule is not directed upwards, but is either almost parallel to the interface or 15 to 20 degrees upwards. Externally this can be recognized by a flattened front (as in the photo above right), this side is directed towards the sound source. The advantage of this variant is the suppression of background noise, e.g. B. the public address system , if such a directional microphone is used on a stage.

swell

1. ↑ Thomas Görne, Microphones in Theory and Practice, 2nd edition 1996, page 95 ff.
2. ↑ sengpielaudio.com - Lower limit frequency for the boundary microphone (PDF file; 309 kB).
3. ^ Michael Dickreiter, Handbook of Tonstudiotechnik, 6th edition 1997, Volume 1, page 187 ff, KGSaur-Verlag.
4. ^ Michael Dickreiter, microphone recording technology, 3rd edition 2003, volume 1, page 107, Hirzel-Verlag.
5. ^ Long, Edward M., Wickersham, Ronald J .: Pressure recording process and device. In: U.S. Patent 4,361,736. November 30, 1982. Retrieved July 10, 2010 .  
6. ↑ sengpielaudio.com - Two different boundary microphones (PDF file; 132 kB)

Web links

• History of the PZM on the manufacturer's website (English; PDF file; 7.7 MB)

See also

Stereophony | Miking | Support microphone | Runtime stereophony | Distance law