Microtech Gefell KEM 970
Nov 1, 2003 12:00 PM, John McJunkin
Unorthodox. Odd. Maybe even a little weird. Microtech Gefell's KEM 970 microphone is anything but run-of-the-mill. On the other hand, the words excellent and superb also describe this microphone. I have seen microphones that seem to have arrived in a flying saucer with an alien named Trebor from the planet Livocs in the Anozira quadrant, and the KEM 970 definitely ranks up there on the “what the?” scale. Nonetheless, this mic is based on a brilliant idea.
The KEM 970 has a smooth burnished metal tube 25 mm in diameter and 355 mm long. Connected to the tube along its length by 8 posts is another tube 40 mm in diameter and also 355 mm long. This tube has smooth burnished metal end caps but is otherwise formed of a black metal mesh. Essentially, you have two tubes, side-by-side, one slightly larger than the other.
The aforementioned posts are approximately 9 mm in diameter and 12 mm long. Four of these posts are centered on the lengthwise midpoint of the tubes in a straight line on centers of approximately 25 mm. About 30 mm further along the length in either direction are two more posts, and then the final two of the eight are located proportionally farther from the midpoint, about 25 mm from the ends of the tubes. The point is that these posts are not evenly distributed along the length of the microphone, which is important because each corresponds to one of the microphone's eight condenser elements. This is, in fact, a line-array microphone!
The condensers correspond, in terms of position, exactly with the posts that connect the two tubes together. The condensers are approximately 10 mm in diameter and thus cannot be described as large diaphragm, but that is not the point of this design. A half-circle of black plastic slices through the black mesh tube in a perpendicular fashion nearly a third of the length from the top. The flat half of this semicircle faces the speaker and features two green LEDs to indicate to the speaker when the mic is on. Finally, at the bottom end is a finely machined flange that couples with the mic's proprietary 7-pin connector. The connector also serves as a standard mount for use with a tabletop stand. Actually, because the mic's pattern on the vertical axis is symmetrical, the mic could also be suspended upside down if necessary.
The idea of using multiple condensers in a microphone is nothing new. Arraying eight of them vertically, however, is simply not a common design. Nowadays the idea of arranging specialty loudspeakers in a vertical line to help gain control over the directionality of acoustical energy dispersion is just an accepted fact of life. Here that concept is reversed. The vertical arrangement of eight condensers with the correct spacing and phase relationships yields a nice tidy pattern for the capture of sound rather than its dispersion. Pretty clever idea! In this case, the pattern is 30 degrees wide in the vertical plane and 120 degrees wide in the horizontal plane. The KEM 970's two tubes are normally oriented vertically with the black mesh facing the speaker, and the pattern emanates outward in a 30-degree-wide supercardioid pattern along the vertical reference axis. The pattern from top to bottom is 120 degrees in what Microtech Gefell describes as a club shape.
The point of having a pattern that differs in its horizontal and vertical axes is that podium speakers tend to move about, some more than others. A particularly animated speaker may from time to time move outside the pickup pattern or at least into an area where the sensitivity is low enough to significantly attenuate the signal. The traditional solution is to use a wider cardioid pattern to encompass the entire area where the speaker may be. A drawback to that solution is that wider patterns increase the propensity for feedback, and the speaker may be awash in an ocean of the other sounds picked up by the wider pattern. The benefit of the KEM-970 is that the pattern can be directed specifically toward the person speaking while allowing him or her the freedom to move within that pattern. It simply enables more precise directionality than a traditional microphone with a spherically symmetrical pattern.
I test-drove this microphone with speech and conducted some pseudoscientific experimentation to make some determinations about the pattern. The results were interesting, to say the least. First of all, the quality of the signal with the speaker dead-on center in both axes is impeccable. The manufacturer's published frequency range is from 40 Hz to 18 kHz, and it's all there, clear and clean. The published self-noise rating is a dead-quiet 15 dBA. With the pad switched to line position, the mic can handle 116 dB SPL. It's not likely this mic will wind up in front of a Marshall stack, but it has the capability.
Here's what's particularly interesting about the KEM 970: the two condensers located at top and bottom of the array do not yield high frequencies. In fact, they sound quite dull. I moved the microphone up and down on the vertical axis, and it became apparent that the brightest part of the pattern was indeed dead-on center. I was not provided with a spectral plot for this mic, but I'm going to guess that there's a pretty significant bump in the higher intelligibility frequencies and extending even a bit higher. This bump is significant enough that were the pattern any larger or less restricted, a whole lot of monitor wringing would be necessary. But that's the point here; the mic yields plenty of the intelligibility frequencies of human speech but in a controlled pattern that helps tremendously to solve the feedback issue that is typically associated with that range.
I moved the mic vertically in front of a white-noise source (with highpass filtering to make it more directional). The point was to test for any audible phase shift. There was a barely audible amount toward the ends, as one might expect, but it was only scarcely audible and even then only in the highs and high mids. With human speech, the phasiness would be practically inaudible and even then only if the speaker were literally jumping up and down.
The KEM 970 is a specialized microphone, intended mainly for podium use, and it exhibits a high-quality signal. It simplifies the job of audiovisual professionals who seek a no-muss, no-fuss solution. In most applications, EQ will not be necessary at all. At $8,500 it's not cheap, either, and that's to be expected in consideration of the fact that this is a world-class microphone. You're much more likely to see this microphone on the floor of the United Nations than at the city council meeting in West Podunk, Wyoming. If you need a top-quality microphone with maximum ease of use and money is no object, the KEM 970 is the choice for you.
John McJunkinis the principal of Avalon Audio Services in Phoenix. For more than 20 years, he has engaged in studio recording, live sound, and broadcast. He consults in design and commission for numerous studios and artists.
Company: Microtech Gefell, distributed in the United States by C-TEC (JP CableTek Electronics); www.gefell-mics.com
Product: Microtech Gefell KEM 970
Pros: High-quality reproduction. Well-defined coverage pattern.
Cons: Too expensive.
Applications: Vocals, such as podium speakers, choirs, and so on.
Pattern Cardioid plane 120-by-30 degrees
Response 40 Hz-18 kHz
Sensitivity 775 mV/Pa
Level/Rolloff -10 dB/-20 dB pad, -10 dB @ 90 Hz
Self-Noise -15 dBA
Signal-to-Noise 79 dBA
Maximum SPL 136 dB with 10 dB pad