Sennheiser MKH 40
A cardioid condenser built for the digital age - designed to stay quiet when the source gets loud.
Overview
Sennheiser ran a pitch in the British studio press through the mid-to-late 1980s, and it was direct about why the MKH 40 existed. Digital recording was spreading through professional rooms. The noise floor of the tape machine stopped being the limit, and the microphone's own self-noise and overload behavior moved to the front of the chain. Sennheiser's advertising in Studio-Sound (July 1986, p.31) framed the MKH 40 as the answer: a cardioid condenser engineered for extremely low inherent noise and for taking high sound pressure levels with low distortion. The same copy ran, essentially unchanged, for years - Studio-Sound (January 1987, p.71), Studio-Sound (March 1987, p.114), and Studio-Sound (June 1987, p.46) all carried the identical argument. That's marketing, and it should be read as marketing. The technical literature does tell a consistent story about how the microphone was built.
The MKH 40 P48 belongs to the family of condenser microphones using a symmetrically arranged capsule, a design approach described in the period technical documentation. In a symmetrical capsule, both sides of the diaphragm face a fixed but acoustically transparent counter-electrode held at the same voltage. The electrical forces of attraction on the diaphragm cancel each other out, so the diaphragm isn't unilaterally biased. That reduces the second-order harmonic distortion that would otherwise come chiefly from that initial tension. The symmetry also helps at high sound pressure levels. Distortion from non-linear air behavior inside the backplate's drilling and slots is reduced, because two identical counter-electrodes compensate for each other. And because the moving, electrically biased diaphragm generates alternating voltages in both counter-electrodes, the output voltage is doubled with appropriate circuitry, which improves the signal-to-noise ratio correspondingly. That's the engineering behind the "low noise, high SPL" claim. It's a specific capsule topology.
The MKH 40 is a cardioid. It rejects what's behind it while still capturing a natural off-axis picture, which is why it turns up in stereo arrays where a spaced omni pair alone would leave a hole in the middle. The most concrete documentation of the microphone in real use comes from the Hollywood Bowl, where sound designer Joseph Magee built a system around the MKH 40. Magee's approach was to use as few microphones as possible - typically fewer than a dozen Sennheiser, Coles, and Schoeps mics for a performance that might put nearly 100 musicians on stage. His main stereo system used two outer MKH 20 omnis spaced 20 feet apart and panned hard left and right, paired with two near-coincident MKH 40s set center stage, also panned hard. The MKH 40 capsules were spaced only about 12 inches apart. Magee ran the outer omni pair about 6 dB hotter in the mix than the inner cardioid pair. He described the near-coincident MKH 40 system as having an image very close to the widely spaced omni pair in terms of phase and image accuracy.
The MKH 40 also shows up in touring and broadcast equipment lists of the period. A documented system inventory from the era lists two Sennheiser MKH 40s alongside six Sennheiser MKE 2 capsules, two Beyer M400s, and six Micron diversity radio mics - a compact mic complement for a large-scale production. The same inventory lists the rest of the signal chain: Klark-Teknik graphics and parametrics, Yamaha delays and reverbs, Dolby SR and dbx noise reduction, and a Nexo and Martin speaker system. The MKH 40 was one component in professional systems where the microphone was expected to be the quietest, most predictable link in the chain.
Period review coverage is thinner than the advertising, but it exists. Audio (January 1988, p.118) ran listening tests on the MKH 40 P48 and confirmed that the small bass roll-off was insignificant. It also found what the reviewer described as a bit of extra brightness. So the microphone is not flat-flat, and the top end has some color. The same coverage grouped the MKH 40 with a discussion of a consumer-type electret microphone with an integral battery, and advised that a fresh battery be installed before any critical recording session. That applies to a battery-powered variant, not the P48 phantom-powered version.
According to the available documentation, the MKH 40 has no published original list price, no confirmed production start and end years, and no documented common failure modes. Collectors and working engineers tend to regard the MKH series as buy-it-for-life gear, and the MKH 40 is the cardioid workhorse of that family - the pattern you reach for when you need rejection without the tightness of a supercardioid. No reliable current market pricing or parts availability information is present in the source material, and none should be assumed. Used examples circulate.
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