Sennheiser MKH 815 T
A 55-centimetre interference-tube condenser that shrugged off a gale while the competition's gun mic came back unintelligible.
Overview
The MKH 815 T is a slim, sand-blasted nickel tube, 550 mm long and 19 mm in diameter, and it weighs approximately 370 grams. A boom operator can point it all afternoon without cursing. Studio-Sound (January 1975, p.68) tested the microphone at length and found it "extremely elegant, small enough not to be conspicuous, and beautifully finished," with the sand-blasted nickel finish chosen so that studio lights wouldn't bounce off it if the mic wandered into shot.
This is a gun microphone, and the design is the classic interference-tube approach. Sound arriving straight down the barrel passes largely unimpaired to the capsule. Sound arriving from the side enters through a row of closely-spaced slots running some 45 cm down the tube and reaches the capsule out of phase with the on-axis wave, so it cancels itself out. That leaves a single large lobe pointed where the operator aims it, narrowing at high frequencies. The slots are backed by a fine metallic gauze, and the transducer diaphragm sits roughly 6 cm back from the rear of the aperture row, so rear-entering sound sees a shorter interference path of its own.
The amplification scheme is what separates the 815 T from a conventional capacitor gun mic. Sennheiser used RF techniques rather than an FET front end with a DC polarising voltage. An 8 MHz crystal-controlled oscillator feeds a discriminator tuned by the capsule's capacitance, and the resulting audio is amplified, buffered and filtered. The voltage across the capsule stays down around 10 V AC, which helps in humidity. Output impedance is low - under 20 ohms between 500 Hz and 20 kHz, per the manufacturer's own figures - so long cable runs barely touch the frequency response, and the recommended amplifier input impedance is anything above 200 ohms. The mic is not modular. Transducer and amplifier live in one housing, which is part of why it's shorter and slimmer than a modular rival.
Output level is unusually hot for a professional condenser. Studio-Sound (January 1975, p.68) measured sensitivity at 5 mV for 74 dB SPL at 1 kHz - within the upper limit of Sennheiser's spec once measurement error is allowed for - and noted the mic can deliver up to half a volt at high sound pressure levels. That is both convenient and a trap. It will happily feed the auxiliary input of a Revox unbalanced, but the review warned that preamps need to handle the level without overload, and attenuators may be needed in line. The published spec sheet lists sensitivity at 4 mV/µbar ±1 dB, frequency response 50 Hz to 20 kHz, maximum SPL 150 µbar, operating voltage 12 ±2 V at approximately 6 mA draw, and a temperature range of −10°C to +70°C.
Studio-Sound (January 1975, p.71) measured equivalent internal noise at just 22 dB(A), and observed that combined with the high output, very good noise performance is achievable even through mediocre microphone amplifiers. The reviewer's only quibble was tonal. The residual hiss carried slightly more high-frequency content than a competing AKG gun mic's, making it marginally more noticeable, though both were judged very good.
On axis the response rises at the top and rolls off at the bottom, and the bass roll-off is what makes the mic so tolerant of handling noise. Off axis, high frequencies fall away fast, with a pronounced dip around 6 kHz at moderate angles. Studio-Sound (January 1975, p.70) found that within roughly ±15° of alignment this presents no problem in practice. Subjective listening matched the curves: a slight lack of bass and a noticeable high-frequency lift, clean and clear but not always as natural as the AKG CK9, which the reviewer preferred on balance in good conditions. The lift can be tamed externally with EQ, and doing so trims the hiss too.
The 815 T pulled well ahead of the field in rough weather. Studio-Sound (January 1975, p.71) took it out in a gale that was stripping branches off a beech tree. Recordings made without any windshield were perfectly intelligible and cleanable with a steep high-pass filter, while the AKG gun mic recorded at the same moment was so badly blasted as to be unintelligible. Swung violently through the air, it tolerated more abuse than any sane boom operator would supply. Handling noise was minimal even without the optional shock mount, and no hum or RF breakthrough appeared during testing - the absence of an output transformer likely helping there.
Directivity was judged broadly comparable to the AKG combination, with the Sennheiser giving a very slightly broader acceptance angle for speech and a somewhat faster high-frequency fall-off around 30° off axis. Between 90° and 180° rejection was very good. The reviewer declined to pick a winner between the two, calling the gun mic a specialist tool whose choice should follow the specialist's needs, but concluded the 815 T deserved serious consideration for film work, sound reinforcement or sound gathering, indoors or out.
The connector is a three-pin DIN socket to DIN 41 524. The MKH 815 T-U variant substitutes a three-pin Cannon XLR3. Sensitivity can be trimmed by ±1 and 2 dB by changing fixed attenuator values ahead of the head amplifier's buffer transistor, with values given in the instruction booklet, which includes a circuit diagram. The mic shipped in a padded wooden case with that booklet and its own measured axial response curve. Studio-Sound (January 1975, p.72) found the case hinges a little flimsy, one having come adrift in transit, and noted the supplied curve sat just outside Sennheiser's own tolerance in the 8-15 kHz region. Sennheiser offered a foam windshield (MZW 813) and a water-resistant type (MZW 804) claimed to cut wind noise by 26 dB against the foam's 16 dB, plus pistol grips and shock mounts. Neither windshield nor mount came with the review sample.
The available documentation doesn't establish production years, original price, or a catalogue of common failures, and no market pricing should be assumed. What is documented is a beautifully made, unusually wind-proof directional condenser with a hot output and a bright top end — the kind of specialist tool that earns its keep precisely where conditions are worst.
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