The company was founded in 1928 by Georg Neumann to supply microphones to the German national broadcasting organization. In 1943, it moved from Berlin to a village south of Leipzig where work could continue, uninterrupted by Allied bombing. The company was known throughout the world for precision condenser microphones for studio and measurement applications, phonograph record cutting lathes and other studio equipment. After the war, most functions of the Neumann company moved back to West Berlin, but a few key people stayed in Gefell, and that section of the company remained there through forty years of East German government. The measurement microphones made in Gefell were adopted throughout eastern Europe and the USSR. Since the Berlin wall came down, the products of the Gefell factory are once again available in the world market and since 1991 Josephson Engineering has brought Gefell's unique measurement microphones to North America.
A unique feature of the 1/2" and 1" MTG microphones is their built-in calibration electrode, which is integrated into the protection grid on the front of the microphone. Now a complete electrostatic frequency response calibration can be done using only a small DC power supply and signal generator, no special chamber or spacers are required. A range of capsules from 1/4 inch to 1 inch, and with different sensitivity and damping options are available. The larger capsules are more sensitive, allowing measurements to be made at lower sound levels. The smaller capsules provide less output, but have uniform response to higher frequencies, up to 40 kHz for half-inch and 100 kHz for quarter-inch capsules.
The diaphragm of Gefell instrumentation mics is made of pure nickel grown in their own lab, and is galvanically attached to its support ring of the same material. The process is effectively a chemical weld entirely made of nickel, where the molecular structure continues from the diaphragm through the bond area and into the support ring. The diaphragm assembly is stretched over a circular ridge in the outer housing and conditioned to assume the correct resonance frequency, which is stable over a wide range of temperature. The backplate is secured on a quartz disk which provides insulation and precise positioning. A tiny capillary passage is provided to allow pressure inside and outside the capsule to equalize, so that performance is the same at any altitude or air pressure. The capsule is operated with a 200 volt DC polarization bias applied to the backplate.
One Inch Measurement Microphones
MK102/103 For measurements in quiet environments, the Microtech Gefell one inch capsules provide optimum performance. With an output of 50 mV/Pa, the useful sound pressure range is at least 16 dB to 146 dB (A) SPL. The noise floor of these mics with the MV203 preamp is an extremely low 11 dB SPL, A-weighted. The two types differ only in the details of the pressure equalization vent, which controls the low frequency rolloff. The MK102 uses a side vent and has flat response down to 10 Hz, while the MK103 uses a rear vent adjusted for flat response to below 2 Hz. Both types have flat response up to at least 18 kHz in free field conditions.
The basic half-inch capsule type is the MK202, with flat freefield response between 10 Hz and 40 kHz, and an output of 14 mV/Pa. With the MV203 preamp, the MK202 covers an operating range of 24 to 158 dB SPL (noise floor plus 5 dB, to overload point) while the MK221 has flat response from below 3.5 Hz to above 20 kHz, and a sensitivity of 50 mV/Pa. This additional sensitivity allows an IEC operating range of 20 to 146 dB SPL, as the complete microphone then has an equivalent noise floor around 15 dBA. A special version of this type, the MK222, extends the low frequency limit to below 0.5 Hz for measurements in the infrasonic range. Finally, the MK231 provides flat response from 3.5 Hz to 8 kHz in pressure and diffuse field conditions. Pressure/diffuse microphones are used where the response to sound must have an average frequency response that is flat for sounds arriving from all directions. The response toward the front of the microphone shows some high frequency boost in order to achieve this characteristic.
Quarter Inch Measurement Microphones
For the ultimate in resolution of high frequency sounds, impulse measurements and acoustic modeling, the MK301 1/4" capsule provides flat response from 5 Hz to above 100 kHz, and allows linear operation up to 168 dB SPL. It may be used with an adapter on the standard 1/2" preamplifier, or with a special 1/4" preamp, the MV302.
When budgets require precise sound measurements at lower cost, the Microtech Gefell MK360 constant-current-powered measurement microphones are the ideal choice. They are self-polarized and offer flat frequency response from 20 Hz to 20 kHz within 2 dB. They are powered by IEEE 1451.4-compatible instrumentation systems from a variety of makers and meet the requirements for IEC 61672 "Class 1" sound level meters. Each microphone comes with a traceable calibration curve.
Measurement Microphone Preamplifiers
Condenser measurement microphones are used together with preamplifiers that convert the very high microphone output impedance to a level that will drive the measuring equipment being used. Circuitry is assembled with individually laser-trimmed microcomponents bonded to a stable ceramic hybrid substrate. The MV203 is equipped with threads for use with any MTG or other standard 1/2" capsule. Adapters are available for use with 1" and 1/4" capsules. The A-weighted noise level with a typical 1/2" capsule is less than 2.5 µV, equivalent to 9 dB(A) with the high output 1/2" capsules, or 19 dB(A) with the low output 1/2" capsules. The increased capacitance of the 1" capsule reduces the noise level to 1.3 µV, or about 3 dB(A). The output stage is a push-pull type for best linearity, with an output impedance of 80 ohms. This allows long cables to be driven at full output. The MV203 also includes a heater which allows the continuous use of rear-vented capsules in very high humidity conditions. For applications where minimum acoustic shadowing is important, the MV301 1/4" preamp is available. Specifications are identical except that the noise level is typically 6 dB higher (about 5 µV A-weighted) due to the lower capacitance of the 1/4" capsule. Both preamps operate on 120V DC and provide unity voltage gain from 1 Hz to 1 MHz ±0.5 dB, at levels up to 33V rms without distortion. These specifications essentially insure that the circuitry has no effect on the measurement over a dynamic range of more than 140 dB. Permanently attached superflexible cables terminating in standard Lemo connectors are provided.
A wide range of accessories is available for MTG microphones.
The Microtech Gefell measurement microphone preamplifers require 120 V DC power and 200 V DC polarization voltage for the capsules. The MN900 power supply provides low noise, tightly regulated power for two microphones at these voltages and also powers the MV203 heater. Other specialized supplies are available for arrays and outdoor microphones.
Frequency response measurements of microphones are ideally made in a large anechoic chamber with a servo-controlled sound generator. Condenser mics may also be calibrated by placing them in a controlled electrostatic sound field which drives motion of the diaphragm. Generally this involves removing the protection grid, so that it is impossible to measure the grid's effect on the frequency response, which is substantial at higher frequencies. Microtech Gefell 1" and 1/2" microphones are unique in having a built-in calibration electrode integrated into the front surface of the protection grid. With a 400 V DC power supply and a 30 volt AC signal coupled to this grid, complete pressure response measurements can be made. A simple wire lead may be used, or the preamp and microphone can be installed in the EMK1 measurement chamber, which is made of heavy metal and filled with felt to attenuate external noise by about 40 dB. The EMK1 can be used with the MV203 or other standard half-inch preamps, and includes a spring-loaded gold pin which makes contact with the calibration electrode.
Cables and Connectors
Mating Lemo connectors and extension cables, C88.1 10 meters long and C88.2 20 meters long, are available for use with the MG measurement microphone preamplifiers. Special flat cables and weatherproof enclosures are also available.