microphone Neumann KM 88

Both mics, along with the stereo mics SM2 and SM23, used the same basic capsule design ” which itself was very similar to the KM54 capsule. According to Klaus Heyne of German Masterworks, the FET KM88 was designed in order to use up the company`s surplus of these capsules. In the KM56 and KM88, a pair of small-diameter capsules were joined back-
microphone Neumann KM 88 - schematic

to-back with a gap between them. The gap was created to increase the time difference between the arrival of low-frequency signals at the two membranes. Typical dual-diaphragm capsules have a very small gap between front and back membranes. Long, low-frequency signals are nearly in phase at both diaphragms simultaneously. When the polarity of the rear diaphragm is reversed to create a figure-of-8 pattern, the resulting low-frequency response is attenuated because the two signals ” reversed in polarity but nearly identical in amplitude ” cancel out. The diaphragms in the KM56/KM88 capsule are made of 0. 7-micron nickel ” approximately 1/10 the thickness of typical (~6-micron) condenser capsule membranes. The initial design of this capsule used a relatively loose diaphragm tension, and was subject to damage from high SPLs. [When] the diaphragms were repeatedly overloaded with SPLs to a point that the metal diphragm bottomed out against the electrically charged backplate, a tiny (arc) hole [can burn] into the diaphragm at the point of contact. In 1982, Neumann changed the tensioning of the nickel diaphragm, in order to make the capsules more durable in close-miked vocal applications (such as German broadcast). This change altered the frequency response of the microphone ” specifically, in Cardioid and Figure-of-8 patterns, low-frequency response was lost ” 5dB @ 40Hz in Cardioid, and ~11dB @ 40Hz in Figure-8. See the...

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