Josephson Engineering Instrumentation Microphones

Microphones from Josephson Engineering are available for every acoustical measurement purpose. The nickel diaphragm microphones made for us by Microtech Gefell in Germany are the finest anywhere, and they meet the stringent environmental requirements for IEC Type I and Type 0 sound measurement. We're proud to have maintained stock and support for Gefell instrumentation mics in America for more than fifteen years. A complete range of capsules and accessories is available for work from infrasound through ultrasound, and we offer applications support, systems engineering and calibration by our own lab in California. The Josephson C550 series is a lower cost alternative providing similar accuracy, over a narrower frequency and sound level range. Complete sets of equipment including preamplifiers, power supplies, cables and other accessories are available so these microphones can be used with any kind of sound measuring equipment. We also provide applications assistance to help users develop the best approach for any acoustical measuring project. For information, prices and availability call us at +1 831-420-0888 or email

In 1943, the Georg Neumann Laboratory for Electroacoustics was moved from Berlin to a village south of Leipzig where they could continue working 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. Josephson Engineering is pleased to bring their measurement microphones to the U. S. A unique feature of the 1/2" and 1" MG 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 1/2 inch and 100 kHz for 1/4 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, which is also made of nickel. Many other manufacturers use solder for this critical attachment, but the Gefell 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.

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 from 16 dB to 146 dB (A) SPL. The noise floor of these mics with the MV203 preamp is an extremely low 11 dBA SPL. 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.

MK202/221/222/231 Most acoustic measurements can be accomplished with the standard range of half inch capsules, which offer a good compromise between output and frequency response. The MG 1/2 inch capsules meet all the requirements for IEC 651 Type 1 sound level meters. Typical applications include environmental sound measurements, loudspeaker and microphone testing, room acoustics analysis and evaluation of machinery noise.

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 5dB, 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, called 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 miccrophones 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.

MK301

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 MV301.

MV203/301

Condenser measurement microphones are used together with preamplifiers, whose main function is to convert the very high microphone output impedance to a level that will drive a cable and the measuring equipment being used. For best performance, the input impedance of the preamp should be as high as possible. The Microtech Gefell preamps are the highest impedance units available today, at 20 GW and a shunt capacitance of less than 0.2 pF. The circuitry is assembled with individually trimmed microcomponents bonded to a stable ceramic hybrid substrate. The MV203 is equipped with threads for use with any MG 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 µ V, equivalent to 9 dBA with the high output 1/2" capsules, or 19 dBA with the low output 1/2" capsules. The increased capacitance of the 1" capsule reduces the noise level to 1.3 µ V, or about 3 dBA. 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 swing. The MV203 also includes a heater which allows the 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 µ V 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.

MN900 Power Supply

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.

The A63 adapter allows the older Microtech Gefell 1" microphones to be used on standard 1/2" preamps, while the A63.1 fits the newer (ANSI standard) 1" mics. The A67 allows all standard 1/4" microphone capsules to be used on 1/2" preamps, while the A69 allows 1/2" capsules to be used on 1/4" preamps. The 1409 adapter allows the MV203 and MV301 preamplifiers to be used with current B&K measuring equipment, without a separate power supply.

All microphones exhibit some acoustic shadowing at higher frequencies. This can be offset somewhat through the use of nose cones, which replace the standard microphone protection grid. Nose cones are also streamlined to greatly reduce noise from wind. The NK63 nose cone fits 1" mics, while the NK65 is used with 1/2" mics.

Probe microphones can be used for a variety of applications such as measurements in the ear, measurements of sound absorbing material and inside machinery, even at high temperatures, up to 100C. The requirements for length and width of the probe will vary with the application, but it's best to use as short and wide a probe as possible so that the effect on frequency response is minimized. The probes are made with thin wall stainless steel tubing attached to adapters that screw on to the microphone capsules in place of the normal protection grid. Complete sets of probe attachments are available, the SV63 for 1" mics (five probes: 2x200, 3x200, 4x400, 5x400 and 6x500mm) and the SV65 (four probes, 4x250, 2x250, 1x250 and 0.5x100mm) for 1/2" mics. The probe sets come in fitted wooden cases.

Preamplifier performance is checked by removing the capsule and substituting a test adapter having the same capacitance as the capsule. The K65 shown here is used for testing the MV203 and other standard 1/2" preamplifiers, and the K67 is used with 1/4"units.

Sensitivity calibration of measurement microphones can be made with a pistonphone. The MG 5001 uses two pistons which are driven by a cam, powered by a quiet speed- controlled DC motor. The sound pressure level is dependent only on the displacement of the pistons, the air chamber volume and the air density, all of which can be controlled so that the device is essentially an absolute calibration. A 250 Hz tone at a sound pressure of 124 dB,

In humid weather conditions, moisture can become trapped between the diaphragm and backplate, causing noise. Often, the built-in heater of the MV203 preamplifier is enough to dispel this moisture. The TA202 dehumidifier can also be inserted between any rear- vented 1/2" capsule and its preamplifier. Silica gel inside the TA202 absorbs moisture from the air spaces inside the capsule. There is a small window through which the color indicator of the gel can be seen. When the gel turns from its normal blue color to white, the dehumidifier can be recharged by placing in a 100 C oven for an hour.

Round open cell foam windscreens are available for the three mic sizes, W1 for 1", W2 for 1/2", and W3 for 1/4" mics.

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 ½‘ preamps, and includes a spring- loaded gold pin which makes contact with the calibration electrode.

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.