History & Development

 A microphone by definition has two meanings: it is an
instrument for intensifying weak sounds. And it is a device
for transforming sound waves into electrical impulses.
While the first definition might seem to have no bearing
here, it was Sir Charles Wheatstone, in 1827, who used it to
describe an all-mechanical vibration stethoscope - the
origin of the word
microphone to describe the transducer.

 We would attempt to give you a brief incite into the
evolution of early microphones and how they have
progressed over the years. This however could not have
been possible without prior consent and permission of
Testa Communication
and the Author Jesse Klapholz
jesse@klapholz.com) from the September 1986 issue of
Sound & Communications magazine. For information Go to

 Although Testa communications has given us permission
to use the entire text, we feel that excerpts and portions of
it along with microphone images would help to demonstrate
the history and development of early microphones.

 In the mid-1920 the development of the condenser microphone
and the electronic vacuum tube amplifier paved the way for
sound on film recording, the first high quality, wide range
condenser microphone was developed by E. E. Wente at Bell
Labs as a measurement standard in the late 1910s. In order to
satisfy the high-quality microphone requirements of the rapidly
growing radio broadcast and recording industries, Western
Electric introduced the 394 condenser microphone;
subsequently RCA came out with the
4AA condenser
mic. With the introduction of condenser microphones,
the problems of signal-to-noise ratio and frequency
response associated with the carbon microphones,
then in general use, were overcome.

 The omni directional dynamic microphone was developed by
Wente and Thuras in the late 1920s, and introduced as the
Western Electric 618-A.  Actually, the original dynamic or moving
coil techniques were patented by Ernst Siemens in 1874. He
even specified the diaphragm to be the frustrum of a cone, which
was used in many other inventions. Nonetheless, the 618-A was
the first practical dynamic microphone. Because of the simplicity
of the 618-A as compared to the condenser microphone and
amplifier, the omni-dynamic mic proved to be more practical for
many applications, although the 618-A was considered to an
omni mic, the microphone becomes very directional in
high-frequency range. Western Electric developed a dynamic
mic in the late 1930s that was omni directional to 15 kHz. Called
the 630A, it was better known as the
Eight-Ball mic,
because it resembled an eight-ball right off a pool

 Even though there were several advancements
in microphone technology the early models had not
proved themselves reliable enough for broadcast
work. But, in 1929 and 1930, at NBC's installation of new audio
facilities for its Chicago Civic Opera House broadcasts, the 18
carbon microphones were replaced by three parabolic sound
reflectors used in conjunction with a condenser microphone.
This was the first application of highly-directional microphone
techniques. NBC subsequently used these parabolic dishes in
both the Philadelphia and New York Metropolitan Opera Houses
and in its Time Square studios. In 1939, Mason and Marshall of
Bell Labs reported on a design of a tubular microphone which
used a single element and acoustical tubes of varying lengths to
achieve a highly directional pickup pattern. This tubular design
paved the way for the shotgun mics we are familiar with today.

The Olson/RCA Legacy

 Perhaps the two most famous microphones to be
commercialized were developed by Harry F. Olson at RCA. They
were the
44A, B, BX velocity ribbon microphone series
                 (1930-1940), and the
77A, B, C, D, and DX
                 unidirectional ribbon microphone series
                 (1931-1937). These vintage microphones
                 are still in great demand; the 44BX could
                 be found in many NBC studios, and the
                 77DX is currently used on NBC's
                 "Tonight show" and on "Late Night with
David Letterman."

                     77A                                                77D                                                77DX          

 When Olson developed the velocity mic it was a large step
forward in microphone technology; it was the first high quality
directional microphone. The effective solid angle of sound
reproduction for the figure-eight velocity mic is one-third that of
the omni directional mic. This means a reduction of 5 dB on the
effective sound pickup of reverberation and other unwanted
sounds. The directional properties of the velocity microphone
were found to be useful in reducing effects of reverberation and
increasing the intelligibility of reproduced speech.

 The next logical step was the development of the unidirectional
or cardioid pattern. Olson's 77A, which was introduced in 1933,
consists of mini- and bidirectional capsules whose outputs are
combined so that they yield the cardioid pattern. The cardioid
pattern also affords the same effective angle of sound reception
as the figure-eight, and hence the same advantages with the
addition of a front-to-back rejection characteristic.

Sound Reinforcement

 While RCA's microphone developments efforts were
concentrated more towards broadcasting/recording applications,
Western Electric, in 1939, introduced the
unidirectional microphone with sound reinforce-
ment applications in mind. The 639A consists of a
ribbon velocity element and a dynamic pressure
element, whose outputs are combined so that
they yield a cardioid pattern. Marshall and Harry
of Bell Labs reported in 1941 that due to the reproducing
characteristics of monaural sound systems in use, directivity
needed to be supplied by the microphone in order to produce a
more natural balance of direct-to-reverberant sound. They further
stated, "This [feedback in a reinforcement system] is merely a
special case of extraneous noise, and its effect can generally be
reduced by directivity in the microphone."

 Marshall and Harry implemented two field tests on the 639A
mic, one of a broadcast of a 30-piece orchestra and another of a
sound reinforcement system. In the studio, the 639A allowed for
an ideal acoustical location of the mic and it was commented that
the bass reproduction was much clearer than with other mics.
Marshall and Harry attributed the clarity of bass to the
suppression of reverberant bass energy pickup, where the
studio's acoustical treatment was deficient. The 639A was
installed at the House of Representatives, in Washington, D.C.,
where feedback conditions were so severe that other types of
microphones had proven inadequate in providing sufficient
reinforcement. The
639B has a six-position switch that yields
                 omni, cardioid, several types of hypercardioid, and
                 figure-eight patterns. In the House of Represent-
                 atives, the hypercardioid afforded an increase of 5
                 dB in the system gain -- wherein was the difference
                 between success and failure of the entire

 In the late 1930s, Benjamin Bauer of Shure Brothers developed
a new cardioid dynamic microphone that used a single element
and acoustic means to achieve its directional pattern. The
Unidyne made it debut in 1941
and was a turning point for microphone design,
manufacture, and reinforcement applications.

Crystal Microphones

                 In 1880, Jacques and Pierre Curie discovered the
                 piezoelectric effect. Piezoelectric crystals were
                 first used by Langevin in 1917, in connection with
                 his research efforts in underwater acoustics using
                 ultrasonic transducers. In 1919, using Rochelle Salt,
Alexander Nicolson first demonstrated a variety of piezoelectric
devices, including loudspeakers, phonograph pickups, and
microphones. Problems of manufacturing crystals with
uniformity and the necessary shapes prevented the commercial
production of any of these devices. Almost 10 years later, C.B.
Sawyer and C.H. Tower developed processes to manufacturer
uniform complex-shaped piezo crystals. This led the way for
many piezoelectric or crystal transducers, as they were first


 Work on the electret condenser microphones dates back to as
early as 1928. These microphones used permanently polarized
wax plates. Eventually, microphones with wax electrets were
offered commercially by Bogen (1938 to 1940) under the name
Velotron. The first large-
scale application of electric transducers was during
WWII, when wax-electret microphones were used in
Japanese field equipment. The wax-electrets, how-
ever, did not catch on due to their instability and very
small capacitance which complicates the mic-preamp
design. From 1948 through the early 1960s, work
continued in electret microphone technology, turning up
materials such as acrylics, ethyl cellulose, polystyrene, vinyl
polymers, and ceramic electrets.

 In 1962 and 1965 electret microphones in which the diaphragm
was composed of a metalized thin foil of Mylar or Teflon,
respectively, which has been converted into an electret were
proposed. Finally, in 1968, Sony brought out the finest electret
condenser microphone. Later, around 1971, Primo Company Ltd.
introduced an electret mic with a monolithic IC preamp.
Foil-electrets are manufactured in countless numbers; the
Japanese production of electrets alone is estimated to exceed 20
million units per year.

The Future

 Once the past has been clearly laid out before us, the future is
easy to imagine. Many inventions of the future will be stolen
from early predecessors. Those who worked in the labs in the
1800s and early 1900s left us with a long list of inventions to be
implemented with modern materials, and new electronic and
manufacturing technologies. These
new devices can be
categorized into mechanical and non-mechanical transducer
systems. Diaphragms made of materials yet to be patented will
use various modulation and sampling techniques to convert their
motion into data. Optical A/D microphones are currently being
developed. On the horizon we see the technologies of fiber
optics, lasers, and interferometers applied to the electrical and
digital transduction of acoustical phenomena.

 "There is room for improvement in even the best
microphones....Possibly an understanding of the limitations can
be had by considering that, for perfection, a microphone should
possess no inertia, and should produce an output directly
proportional to the air pressure applied." Although this quote is
from the book,
Public Address Systems, by James R. Cameron
published in 1935, it still holds true today.