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AES Section Meeting Reports

Los Angeles - July 30, 2013

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Summary

The July AES-LA meeting featured a technical presentation from Audio Engineering Associates Marketing Manager (and
former mic and preamp design engineer) Julian David on the topic of "Ribbon Microphones: Beyond the Figure-8
Pattern." The focus was geared towards recording with ribbon microphones, exemplified by live demonstrations with
guitar performed by local musician Douglas Showalter. However, the presentation began with a short history of ribbon
microphone designs and overview of the engineering work it takes to create a unidirectional version.
Ribbon microphones were first developed in the early 1900's when there was a strong need for low-noise, highsensitivity
transducers. An approximately 1.8 micron thick aluminum ribbon diaphragm clamped between two poles of a
magnet provided a solution, but not without introducing its own engineering challenges. Ribbon microphones, in contrast
to capacitive microphones, are tuned such that their resonant frequency is in the low end of the audible frequency
spectrum, e.g. in the 15-65 Hz range. This leads to some issues in designing microphones with flat frequency responses
throughout the human hearing range. In their simplest form, without an acoustic back cavity, they behave as pressure
gradient sensors, leading to a spatially bi-directional response, or figure-8 pattern. With the addition of a back cavity, or a
second omnidirectional diaphragm and electrical summing, various first order polar patterns are achievable within a limited
frequency range (spacing and diffraction effects limit control of the polar pattern at higher frequencies). The
bidirectional pattern has the most pronounced proximity effect, followed by the super-cardioid, cardioid, then
omnidirectional (omni showing no proximity effect).
The history of ribbon microphone designs began in Germany in 1920 with a design by Schottke and Gerlach that was
commercialized in the Siemens-Telefunken M201/1. The patent for the mic was purchased by RCA and leveraged in
many early microphones designed by Harry Olson in the 1930's. Included in these were the RCA 77A, a dual-diaphragm
"poly-directional" design, the RCA 77-DX, a ribbon mic with a rotating acoustic shutter to adjust the polar pattern, the
RCA Varacoustic, of similar design to the 77-DX, the RCA KU-2A (a.k.a. "the skunk"), a unidirectional boom mic, and the
RCA KU-3A, a design considered one of the best ribbon microphones. The RCA ribbon microphone designs presented
acoustic challenges since an enclosed air volume used as a back cavity operates as an acoustic spring where an
acoustically resistive load is desired. To get around this issue, RCA developed a labyrinthine tube behind the diaphragm
that was then filled with material that had absorption across a broad frequency range. The best material for the job at
the time was cow hair (which lead to large manufacturing variability)!

RCA was not the only manufacturer to design unidirectional ribbon microphones
at the time. The Western Electric / Altec 639A (a.k.a. "the birdcage")
was a 1938 design incorporating a moving coil diaphragm along
with a ribbon, letting the user select between 3 polar patterns. Current ribbon
microphone designs include the Beyerdynamic m160, a double ribbon
piston with hyper-cardioid pattern, the Samar Audio AF11, a soon to be
released fixed cardioid design, the Samar Audio TF08, a dual-diaphragm
ribbon and condenser design, and the AEA KU4, a super-cardioid interpretation
of the RCA KU-3A.
The presentation was concluded with a short Q&A session where it was
noted that dealing with power to dual transducer designs is a complicated
engineering challenge. A conversation regarding transient response of
different microphone diaphragms revealed that it is poorest with heavier
moving coil designs and improved with condenser and ribbon designs (but
experiments are needed to see how this changes with unidirectional responses).
The importance of transformer design for ribbon microphones
was discussed, noting that the transformer must be carefully engineered
since it interacts with both the ribbon & the microphone preamp. Guitarist/Producer Douglas Showalter
After the presentation, Douglas Showalter brought his guitar on stage to show off the differences between the
RCA 77-DX, AEA KU4, RCA Varacoustic, and RCA BK-5 in a live recording demonstration. Experiments with
the proximity effect in different polar patterns, pointing the null of the bidirectional pattern in different directions,
and pointing the back of the microphone toward the guitar were all done live. The audio recordings from the
demo are available from Julian David for those who couldn't attend.
The July meeting was an excellent review of the engineering challenges involved with microphone design, the
rich history of ribbon microphones, and the interesting recording possibilities that come about through the use of
directional microphones. The future in this space looks bright as companies like AEA and Samar Audio continue
the tradition of designing and manufacturing new ribbon microphones for recording and broadcast applications.

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