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
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
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
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.
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.
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
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
- Boundary layer microphone
- Pressure transducer
- Frequency independent hemispherical directional characteristic
- Identical diffuse- and free-field response
- No angle dependent coloration through patented triangular form
- No comb filter effects Insensitive to structure-borne noise
- Acoustical operating principle: Pressure gradient transducer
- Directional pattern: Hemispherical
- Frequency range: 20 Hz...20 kHz
- Sensitivity at 1 kHz into 1 kohm: 18 mV/Pa
- Rated impedance: 50 ohms
- Rated load impedance: 1000 ohms
- Equivalent SPL CCIR 468-3: 24 dB
- Equivalent SPL DIN/IEC 651: 14 dB-A
- S/N ratio CCIR 468-3: 70 dB
- S/N ratio DIN/IEC 651: 80 dB
- Maximum SPL for THD 0.5%: 137 dB
- Maximum SPL for THD 0.5% with preattanuation: 147 dB
- Maximum output voltage: 10 dBu
- Dynamic range of the microphone amplifier DIN/IEC 651: 123 dB
- Supply voltage: 48 V +/- 4 V
- Current consumption: 2 mA
- Matching connector: XLR3F
- Weight: 460g
- Diameter: 213 mm
- Length: 168 mm