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The GFM 132 is a
boundary layer microphone. Through computer simulation Neumann
optimized the design to be free of any comb filter effects due to
reflections, typical of other such microphones. The smooth
frequency response for all angles of incidence exhibits a rise in
the upper frequency range. This assures that all sound sources,
even distant ones, will be recorded with clarity and presence.
Therefore, typical applications are for live recordings, such as in
the orchestra pit of opera houses, theaters, and on stage.
The back of the microphone has non-slip pads for its use in a
horizontal or inclined position, and holes for wall suspension. It
is supplied with a wooden case and a wind screen.
Background
Boundary-layer microphones are generally characterized by the
following features:
- They have an identical flat frequency response in the
diffuse-field and free-field.
- They have a hemispherical polar pattern, independent of the
frequency.
- They have a 6 dB higher output level through pressure doubling at
the boundary surface.
Until recently, neither the potential of identical diffuse- and
free-field response, nor the ideal hemispherical polar patterns
throughout the entire frequency range have been achieved by any
known boundary-layer microphone. Circular, square, or rectangular
plates were used to mount the acoustic transducer and to provide
the "live" sound reflecting surface for pressure doubling at high
frequencies.
However, such shapes have disadvantages: The sound pressure level
at the position of the transducer depends on the frequency and the
incidence angle. The incoming primary sound field is superimposed
upon the secondary sound field resulting from diffraction at the
edges of the plate. As a result, boundary-layer microphones using
circular, square or rectangular shaped plates generate linear
distortion, such as comb filter effects, of frequency and polar
response.
The Neumann solution
The GFM 132 boundary-layer microphone has a unique, computer
generated shape that totally avoids these disadvantages. The path
lengths from each edge point to the center of the transducer are
distributed evenly for all wavelengths within the frequency
range.
This design eliminates any possible linear distortion of
frequencies caused at the location of the electro-acoustic
transducer by the interaction of the incoming primary sound field
with the secondary sound field from diffraction at the edges of the
plate.
The operating range of the plate reaches from the lowest frequency
causing a pressure doubling in front of the plate to the upper
limit of the audible range.
The microphone features a smooth frequency response for all angles
of incidence, with a slight rise in amplitude in the upper
frequency range. This assures that all sound sources, even dis-tant
ones, will be recorded with clarity and presence.
Acoustic features
- The microphone provides high output voltage through pressure
doubling at the boundary surface.
- Identical diffuse- and free-field frequency response. Its
advantage is that the apparent tonal balance of a moving sound
source is independent of the distance and direction.
- The special geometric shape prevents angle-dependent coloration
in the vertical and horizontal planes.
- There are no comb filter effects in typical applications, for
example on a speaker's desk, as they would occur through
reflections, using conventional microphones.
- The hemispherical polar pattern is independent of the frequency,
producing a spatial sound with presence and excellent
transparency.
- As is common for a pressure transducer, the microphone is
insensitive to structure borne noise and air movements.
- The microphone reproduces with great accuracy very low
frequencies if the boundary layer is adequately large.
- In surroundings with good acoustics the GFM 132 creates
incredibly realistic AB-stereo recordings, taking advantage of
delay and intensity differences in the audio signal.
Electrical features
The GFM 132 uses transformerless circuitry and operates on 48 V
phantom power. The usual output transformer is replaced by an
electronic circuit.
As with traditional transformers, this design ensures good common
mode rejection and prevents RF interference that may influence the
balanced audio signal. The microphone features high output
capability and extremely low self noise. It provides exceptionally
clean sound reproduction free of coloration.
Attenuation
The microphone has a 10 dB attenuation switch to prevent the input
of the following unit from being overloaded. The switch is located
next to the cable connector at the side of the GFM 132 boundary
layer microphone.
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