Western Electric 640AA (1938)

A twelve-inch chrome rocket that weighed five pounds and told the truth for a living - the calibration microphone that RCA, Altec, and the National Bureau of Standards all pointed to when they needed to know what a sound actually was.

Overview

The Western Electric 640AA is not a microphone you buy to flatter a vocal. It is a measurement instrument that happens to look like something Wernher von Braun sketched on a napkin. It is twelve inches long, about five pounds, finished in bright chrome and shaped like a supersonic rocket. It was built to answer one question with as little argument as possible: what is the actual sound pressure in this room, right now, regardless of the temperature, the humidity, or the barometric pressure that day. Owners describe it as a tool rather than a character. Its omni pattern and lack of expressive coloration make it useless for sweetening a mix and indispensable for anyone who needs a number they can trust.

Western Electric built it out of a specific frustration. The company needed a reliable calibration standard, and the earlier 394 condenser was dismissed as too bulky for routine lab work. Engineers went small instead, introducing a one-inch diaphragm design first known as the D96436, tested successfully in 1932 and later re-designated the 640A. By 1942, Bell Labs had confirmed what users already suspected - the 640A drifted with temperature, pressure, and humidity. The 640AA was the answer. The aluminum alloy diaphragm of the 640A was swapped for one made of highly corrosion-resisting steel, and internal components were revised to minimize environmental drift. That is the whole story of the "AA" suffix.

The reputation was earned in silence, in calibration rooms, government labs, and the corners of Bell Labs, not on record sleeves. The 640AA became the mainstay calibration tool for RCA, Altec, and the National Bureau of Standards, and it was still showing up in acoustical engineering textbooks as late as the 1970s. According to documentation from the period, the Bureau of Standards used it as a reference microphone to check other microphones. That is a very specific kind of fame.

It did see life outside the lab. Broadcast, recording, and measurement applications all made use of it. One was spotted in Symphony Hall, Boston, suspended from the ceiling - likely long after its prime, eventually replaced by Telefunken/Neumann U47s. There is a famous photograph of Miles Davis with a 640AA hovering overhead. And in the mid-1950s, a "Soundorama" concert demonstration at Constitution Hall used a 640AA suspended about twenty feet above the stage and slightly behind the podium, calibrated by the Bureau of Standards, to pick up the National Symphony Orchestra. The engineers rigged two additional mics in the same spot as emergency backups in case the principal circuit failed. When the playback of Rimsky-Korsakov's Spanish Caprice came through the house, the balance was good and the sound was realistic and natural. A comparison take made with improperly placed stage mics had the drums crashing through and overriding the lighter sections. The audience of four thousand reportedly could not reliably tell the live orchestra from the tape.

Key Features

A Steel Diaphragm Where Aluminum Used to Be

The biggest change from the 640A to the 640AA was the diaphragm material. Aluminum alloy went out. Highly corrosion-resisting steel went in. This was not a voicing decision. It was a stability decision. A calibration microphone that changes its response when the weather changes is not a calibration microphone. The steel diaphragm, combined with revised internal components, was Western Electric's attempt to make the capsule's behavior predictable across conditions. The one-inch diaphragm size carried over from the D96436 lineage, and the capsule is a medium-diaphragm condenser design paired with a vacuum tube amplifier in the full system.

The RA-1095 Amplifier and the 200-Volt Polarizing Supply

The 640AA was never sold as a bare capsule in its working configuration. It came as part of a system built around the RA-1095 amplifier (also referenced in some sources as the RA-1090), which supplied the 200v polarizing voltage the condenser element needs and handled the 600-ohm output impedance. That integration is what made the system consistent. It is also what makes reviving one today such a headache. The capsule-to-amplifier connection relies on a hard-to-find connector, and owners and restorers describe losing that piece as a roadblock to revival. A 640AA capsule without its amplifier and cable is a chrome paperweight until the missing connector turns up.

Twelve Inches of Chrome Rocket

At twelve inches long and about five pounds, the 640AA is a substantial object. The bright chrome finish and the supersonic rocket silhouette are not decoration. The housing has to contain the capsule, the shielding, and the connections while minimizing interference, and the shape follows from that. It is still one of the most photogenic measurement instruments ever built.

Omnidirectional, and Proud of It

The 640AA's pickup pattern is omni, and period engineering literature treats that as a feature rather than a limitation. A general-purpose microphone for orchestral pickup, one source notes, preferably has nondirectional characteristics as typified by the Western Electric 640AA or the 633 Type. The omni pattern means the mic does not impose a directionality on the sound field it is measuring. That is what you want when the job is to characterize a room, a source, or a calibrator rather than to capture a performance.

Flat Response and a $195 Price Tag

Documentation of the 640AA is thin. Audio (October 1967, p.8) cites the 640AA variant with flat response and a $195 price. That same issue notes a calibration tolerance of ±0.3 dB at 500 Hz and ±0.5 dB at other frequencies for the Western Electric 640AA or equivalent microphone, with carrying case and microphone adapters included.

Influence That Outlived the Product

The 640AA's design outlived Western Electric's own microphone production. The Altec-Lansing M21 used a condenser element that was either identical or very similar to the 640AA's, and given Altec's role as a successor to Western Electric in certain product lines, this reads as continuity. When Altec took over production of other WECO mics like the 639 and the "saltshaker" models, the 640AA's engineering lived on in labs and broadcast booths. When Western Electric eventually decided to buy calibration mics rather than build them, they turned to Brüel & Kjær, who developed the 4160 to meet Western Electric's exacting specs. Once the 4160 was in production, 640AA capsule production ceased.

Historical Context

The 640AA sits at the end of a specific line of problem-solving inside Western Electric. The 394 condenser was too big and bulky for routine lab use. The D96436, a miniature one-inch diaphragm condenser system, tested successfully in 1932 and became the 640A. The 640A then revealed its weakness, environmental drift confirmed by Bell Labs in 1942, and the 640AA was the fix. The 640AA set the standard for measurement microphones and influenced their development for many decades, making its mark on 1-1/2-, 1/4-, and 1/8-inch microphones as well as on calibration techniques and calibrators. The known documentation record for the mic spans 1949 to 1967, though production years themselves are recorded as unrecorded. The 1938 date in the title reflects the model's introduction, not a confirmed production window.

Its use was not confined to the metrology lab. In 1955, White demonstrated a reciprocity calibration technique using a Western Electric 640-AA condenser microphone, and a 640-AA 1-inch condenser microphone was first calibrated by White in an NSMRL report. In 1956, recordings made at the Forum Theatre in Los Angeles used custom microphones with Western Electric 640AA condenser heads. In acoustic research, 640AA pressure probe microphones were used for measurements at the lips and in open air. One capsule's box carries an inscription stating it was calibrated by NBS in 1982, decades after the mic left production.

Collectibility & Value

The 640AA is a niche collectible. It is not a studio-status symbol the way a U47 is. It is a measurement instrument that happens to be beautiful, which narrows the buyer pool. Current market prices run from roughly $270 to $849 depending on condition and completeness, and an eBay listing for a "Western Electric Type 640AA Microphone Capsule, in Case, Nic" has appeared at $300. Capsules show up more often than complete systems, and that is the central problem. Without the RA-1095 amplifier and the hard-to-find capsule-to-amp connector, a capsule alone cannot be put back into service easily. Collectors note that some Altec capsules may fit, and the old literature is the place to check compatibility. The dimensions of the microphone are such that it will adapt into any fixture designed to accept the current industry-standard Western Electric 640 AA condenser microphone.

Because the 640AA was a reference instrument, its value is bound up with its calibration history. A capsule with documented NBS calibration, or one that has been recently checked against a modern standard, is worth more than a bare chrome shell. Buyers should expect to see capsules in cases, sometimes with original documentation, and should treat the connector and amplifier as the make-or-break components. Common failure modes and repair procedures are not well documented in the surviving literature. This instrument was serviced by Western Electric and the Bureau of Standards, not by hobbyists.

eBay Listings

See Western Electric 640AA on eBay

As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.