Bell Laboratories and the Development of Electrical Recording
Fletcher-Munson Curve from Bell Labs audio research
The Development of Electrical Recording at Bell Laboratories
Radio broadcasting in the United States began in November, 1920 when
KDKA, Pittsburgh was granted its call letters and began broadcasting
This development represented a challenge to the phonograph, since free
music and entertainment was available "off the air".
Phonograph recording companies sought to respond by improving the
quality of their recordings by adopting new technology.
A key innovator of this technology was Bell Laboratories.
Bell Laboratories was the research division of the US telephone
monopoly, American Telephone & Telegraph. Research had
previously been done by Western Electric, the manufacturing arm of
AT&T, and also by Bell System division until
"...On January 1, 1925, AT&T established Bell Telephone
Laboratories, Inc., in New York City as a joint venture with Western
Electric. Gifford [President of AT&T] appointed Frank Jewett
president of the new company in charge of 4,000
workers..." 2. Thereafter, a steady flow of
ground-breaking technology was developed by Bell Labs during the next
Bell and Western Electric research had been working since at
least 1914 on technologies for long-distance telephone transmission
to span the US. This research also provided the components of
what later became the Westrex electrical recording system.
The first complonent was the condenser microphone, developed in 1916
by the brilliant electrical engineer Edward Christopher
"E.C." Wente (Ph.D. from Yale in 1918). The condenser
microphone became practical with the development of the vacuum tube
to amplify the low-level signal output of the condenser microphone
Prior to the condenser microphone, since the earliest days of Bell's
telephone, the carbon microphone had been used as a transmitter (and
continues to be used today). The carbon microphone is basically
a generator of voltage, and requires no amplification, at least for
telephone transmission over distances up to several hundred miles
(or kilometers). However, it does not produce an
electrical signal sufficient for transcontinental
telephone transmission. The condenser microphone connected to
an amplifier was found to produce strong, transcontinental signals.
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The condenser microphone design is innovative. It includes
two parallel plates, each having an electrical charge, The
front plate moves with the audio signal, while the rear plate is
fixed. Sound waves moving the front plate cause a slight
change in the capacitance between the plates, which allows a change
in current flow in the electrical circuit connected to the
microphone transmitter. The amplifier circuit then amplifies
this low-level signal to a useable level 12.
This condenser microphone, although first developed for long-line
telephone transmission, when the US was beginning transcontinental
telephone service, had remarkable acoustic performance. In
its initial versions with thicker steel plates stretched to give
stiffness (so as to raise its resonant frequency above the audio
range), it had nearly flat
frequency reproduction up to about 6,000 Hz. With improved,
thinner plates of aluminium, this reproduction was expanded up to
15,000 Hz, at a time when the acoustic recording process did not
record much above 2,400 Hz 9.
Edward C. Wente of Bell Laboratories holding his Condenser Microphone
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Earlier Development of Electrical Recording Technology
Prior to the Bell Laboratories research, a number of attempts had been
made to develop an electrical recording process that involved microphones
and electrical disk cutting. This was to replace the
acoustic or mechanical recording technique used since Edison,
which involved no electricity or amplification.
An early example of electrical recording was the work of
Lionel Guest and Horace O. Merriman in Britain. They had
developed and demonstrated an electrical recording system using carbon
microphones connected to a moving coil recording head
15. Their system produced the earliest issued
electrical recording which has come down to us. This was the
1920 recording of the burial of the Unknown Soldier in Westminster
Abbey, London on November 11, 1920 14. Guest and
Merriman placed 4 carbon microphones inside Westminster Abbey
15, with their recording apparatus outside.
The resulting recording was issued by Columbia Graphophone Co.
However, the sound quality of the Guest and Merriman system was
not satisfactory. British recording companies, including
Columbia Graphophone, after evaluation decided not to adopt the
Guest and Merriman system. They subsequently went to the
U.S., but with similar results, and their system was not further
The sound quality of the 1920 Westminster Abbey
recording is sufficiently poor that it is difficult to discern if only
instruments are playing, or if there is also singing. The result
is significantly poorer than an acoustic recording of the period, except
perhaps that it would be difficult for the acoustic horn to capture such
a large group. You may hear and judge the sound quality of this
1920 recording for yourself by clicking on the link below. This
reproduces the music Abide with Me from this 1920 Westminster
Click here to listen to 'Abide with Me' recorded electrically by Guest and Meriman in 1920
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Joseph P. Maxfield and Henry C. Harrison of Bell Labs Develop
In about 1920, two teams at Bell Laboratories were created to pursue
electrical recording. Joseph P. Maxfield and Henry
C. Harrison, two of the leading developers of modern electrical
components at Bell Labs, were charged with developing a phonograph
recording system. E. C. Wente, Donald MacKenzie and Irving
Crandall (who had originally hired Wente in 1916) 19 were
assigned to develop a sound system for films to be shown
in cinema theaters5. Both groups were overseen by
Dr. Harvey Fletcher
, who previously joined Bell research in 1916.
Joseph P. Maxfield and Henry C. Harrison of the Bell Laboratories
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Dr. Harvey Fletcher of Bell Laboratories
Dr. Harvey C. Fletcher was a brilliant physicist born into a Mormon family
in Utah on September 11, 1884. Fletcher studied and worked at the
University of Chicago with Nobel Prize winner Robert A. Millikan.
In 1911, Fletcher received his Ph.D. in Physics from the University of Chicago
summa cum laude. After creating and heading the Physics Department
at Brigham Young University in Utah, Fletcher in 1916 became Director of Research at Bell
Laboratories, where he oversaw three decades of research and improvement
in sound, hearing, transmission, and reproduction.
At Bell Laboratories, Fletcher and his predecessor Irving B. Crandall
oversaw research in this area by Joseph P. Maxfield (1887-1977),
Henry C. Harrison (1887-1971), Edward L. Norton (1898-1983),
K. P. Secord, Donald MacKenzie, Rogers H. Galt (1889- )
16, and Harold Black (1898-1983). Harold Black had
invented the negative feedback amplifier in 1927. Fletcher also supervised
Arthur C. Keller (1901-1983), who worked extensively on recording improvements,
including binaural and stereo recording (as described below).
Harvey Fletcher, K. P. Secord, and Rogers H. Galt at the
Bell Laboratories in New York City
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Western Electric Carbon Microphones
Microphone design was a key part of the Bell Laboratories development of
electrical recording technology. Carbon microphones, using
loose carbon granules were employed from the earliest days of telephone
instruments until today. The carbon microphone generates its
own current from acoustic vibration, so requires no electrical
amplification (although it will require a circuit with voltage).
In the 1920s, Bell Labs developed improved carbon
microphones, including the so-called "double button" carbon
microphone which kept the carbon powder from touching the microphone
The famous Western Electric double-button carbon microphone in the
Western Electric 1B microphone housing circa 1925.
The double-button carbon microphone reduced noise and
also greatly reduced harmonic
distortion. These improved carbon microphones and later the condenser
microphones described above were manufactured by Western Electric
(the manufacturing arm of the Bell system). The
Western Electric double-button carbon microphone in the 1B and
1C housing was widely used in radio broadcasting. In
broadcasting, the carbon microphone was connected directly to the
engineering control panel and then to the radio transmitter.
A radio studio from late 1924 with this microphone arrangement
can be seen in in the photograph below.
Art Gillham (with glasses) standing to left of Will Rogers in
November 4, 1924 during the Eveready Hour, broadcast by WEAF New
York. AT&T sold WEAF in 1926 as part of a settlement with
GE/RCA to exit broadcasting, and GE and RCA committed to transmit
all their network broadcasts via AT&T long distance lines
18. (note the Western Electric carbon microphone in
the 1B housing on a stand)
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The Matched-Impedance Electrical Recording System
Harrison and Maxfield made a number of developments which, combined
together, allowed the creation of an electrical disk sound recording
system. Already described is the condenser microphone.
Another of these key developments was the matched-impedance
amplification system, having such a condenser microphone, linked to a
tube (valve) amplifier, with the amplified output voltage driving a
moving magnet (called also a "moving armature'") cutting
head to scribe the sound into a wax master disk.
Bell Laboratories had early licensed the patents covering Lee de
Forest's Audion tube.  Using this device, Bell Laboratories
engineer Harold Arnold had by late 1914 improved the Audion by
replacing gas by a vacuum and redesigning the electrodes and
filaments, producing an amplifier having low distortion and
linear amplification 20. Such an amplifier
was the basis for the matched-impedance
electrical recording system which was developed by Bell Labs.
The early performance was encouraging. Initially, it had a
a recording bandwidth from 50 Hertz to about 6,000 Hertz, beyond
which its high frequency sensitivity declined11.
Frequency response of the electrical amplification and
condenser microphone of the Westrex system 11
This frequency reproduction range was noticeably superior to the
highly variable acoustic process, which (with the best equipment
and with the best recording engineers) could reproduce from
about 250 Hz to about 2,400 Hz, with rapid reproduction
fall-off thereafter. This wider bandwidth
added another octave of sound reproduction, along with reduced
harmonic distortion and a generally more realistic sound image,
including recording of higher harmonics of the musicians. The
dynamic range of the sound recording and the background noise were
both also somewhat improved.
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Light Valve Vacuum Tube for Movie Film Recording
At the same time, Edward C. Wente and his team had developed the
"light valve", a light-emitting vacuum tube that converted
audio voltage into variable levels of light to expose movie film.
This light output produced a variable density sound pattern
on the film. Wente was granted a patent on this technology
in 1923, and it was later the basis for the Western Electric
Western Electric 47A Condenser Microphone with 394 transmitter at top and a single
vacuum tube (valve) amplifier in base
The initial condenser microphones using the Model 361 transmitter were
replaced by improved condenser microphones such as the Western Electric
47A having the Model 394 transmitter beginning in late 1926 and into
1927-1928 in both movie sound recording and phonograph disk recording
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A Recording System based on the Theory of Electrical Filters
Electrical recoding systems previous to and contemporaneous to the
Bell Laboratories development may be fairly described as being mostly
empirical or trial-and-error development. In contrast, Bell
Laboratories based their design on scientific concepts from electrical
theory, reflecting the advanced electrical engineering eduction of these
scientists. In their development of an electrical phonograph disk
recording system, the Maxfield and Harrison team based their system
on the theory of a many-sectioned electrical filter
5. Bell Labs had applied filters to the
improvement of long-distance telephone transmission. The
"theory of electrical filters" was well-developed and
mathematically well-characterized. This allowed them to apply
these known mathematical solutions to the recording and reproduction
problems they were attacking.
Using this approach, a mechanical equivalent of each electrical
component of such a filter was identified. This approach was
used both for the electrical recording system and for sound
reproduction systems such as loud speakers.
For example, the air in an enclosed cabinet could be
considered to function like a capacitor. Since the
mathematical solutions in the theory of electric filters
had already been solved, this accelerated a scientifically-based
development of sound reproduction.
In the new Bell Labs recording system, the microphone was
connected to the matched-impedance amplifier (described
above), whose electrical output was connected to
and controlled the electrical disk cutting mechanism.
Henry C. Harrison and Norman H. Holland examine a phonograph tone arm
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The "Rubber Line" Recorder
For the disk cutting mechanism, the Bell engineers developed a
"moving magnet", also called a "moving armature"
electromagnetic device. In this system, the electrical output of the
matched-impedance amplifier was fed to an electro-magnet, causing the cutting
stylus to move according to the changes in the magnetic field of the
electro-magnet. In this way, the music, in the form of the varying
electrical output of the amplifier, going to the cutting stylus via the moving
magnet would inscribe the music wave form into the wax master. So, in
this electrical system, the microphone and amplifier, moving the stylus,
replaced the old acoustic horn mechanically vibrating a sound box which would
move the stylus. Since the microphone and amplifier had a broader
frequency range and less harmonic distortion and less acoustic resonances,
the recording cut into the wax was dramatically better, as could immediately
be heard in the first electrical disks released by the Victor Talking
Machine Company. (See
1925 - The First Electrical Recording
for this first orchestral electrical recording.)
There were a number of limitations of this new electrical system which the
early recording engineer needed to overcome. If the electrical signal
for the music exceeded certain limits, it could cause the
electromagnet-stylus assembly to move too far and become stuck.
To solve this, a small spring was added to the cutting head to control stylus
Another difficulty was in the conversion by the Bell engineers of an electrical
component into an equivalent mechanical component. As described above,
the Bell Labs design was in part based on the "theory of electrical
filters". Electrical filter theory required that the final section
of such a filter terminate in a resistor. The mechanical equivalent of
the electrical resistor that the Bell engineers developed was a 9 inch
long rubber line or rod. This rubber line was added at the end of
the system to provide mechanical resistance or damping
5,10. The rubber rod was placed up the inside
of the length of the cutting arm. This is the source of the
description of the system by the engineers as being the
"rubber line recorder".
Cutting Arm of the 'Rubber Line' Recorder with the rubber damping rod
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Without the damping resistance of the rubber line, the movement of the
recording stylus at high volume levels, particularly of low frequencies,
would have been excessive. The damping of the rubber line was one of
the essential developments that would make this Bell system linear in its
frequency response 8.
However, this rubber tube also introduced difficulties.
First, it required careful design and repeated testing of the stiffness and
compliance of the rubber column. The coupling of components,
the mass of the components, and characteristics of the rubber each had
to be varied and tested empirically to discover a combination that
gave the disk cutter an essentially flat frequency response. In
fact, in the lab, the Bell system was able to achieve a remarkably
flat response from 250 Hz to about 15,000 Hz 9, although
the total system in use commercially had an upper frequency response
to about 6,000 Hz in the early years (progressively expanded
each year thereafter). It was also found later, from experience,
that the recording engineer would need to carefully pack and adjust
the rubber line in the recording arm, prior to each recording session
to assure good linearity and response in the recorded sound. In
this way, the early Westrex electrical system, like the earlier acoustic
recording system. depended on the craft and resourcefulness of the
recording engineer to gain the best recording results from the system.
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The Western Electric Electrical Recording System
The microphone, plus the amplifier, plus the electromechanical disk
cutting mechanism combined together was the new Western Electric
electrical recording system, commercialized as the
Western Electric engineer George Groves (later a famous Hollywood sound
engineer) at a 1925 Western Electric Cutting Machine.
By early 1924, the Western Electric system was ready for demonstration.
Western Electric's next task was to interest the phonograph recording
companies to license and adopt the Westrex system, to replace the
acoustic recording processes used for the last 50 years. The
fascinating and often surprising story of this is quest to implement
electrical recording of the phonograph disk can be seen by clicking
on the link:
Licensing the Westrex Electrical Recording System to
Victor and Columbia.
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Note: Of course, the Western Electric electrical recording system, while being the
most successful early electrical phonograph recording system was not the
only, or even the first electrical system. Allan
Sutton in his fascinating and meticulously researched book:
Recording the 'Twenties. The Evolution of the American Recording
published in 2009 by Allan Sutton's excellent
describes the history of the development of the various technologies
leading to electrical recording. Well worth a purchase and
Fisher, Marc. Something in the Air: Radio, Rock, and the Revolution That
Shaped a Generation. page xiv. Random House Adult Trade Publishing
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Stephen B. and Butler, Orville R. Manufacturing the Future: A History of
Western Electric. page 115 Cambridge University Press. Cambridge,
UK. 1999 ISBN 0-521-65118-2
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page 68. Rutgers University Press, NJ 2004 ISBN 08-13533-252
Fagen, M.D., ed. A History of Engineering and Science in the Bell System:
The Early Years (1875-1925). New York: Bell Telephone Laboratories,
Frayne, John G. History of Disk Recording Journal of the
Audio Engineering Society, Vol. 33 no 4. page 263 -266. April, 1985
Klapholz, Jesse. The History and Development of Microphones. Sound and
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Measuring Worth website for estimating current values
page 103. Burns, R. W. The Life and Times of A D Blumlein
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94 Burns, R. W. op. cit.
93 Burns, R. W. op. cit.
Joseph P. and Henry C. Harrison. Methods of High Quality Recording and
Reproducing of Music and Speech Based on Telephone Research. Bell
System Technical Journal 5, July, 1926
page 4, 5 Eargle, John. The Microphone
Book. (Second Edition) Focal Press Burlington, MA 2004 ISBN-13
13 Sutton, Allan. Recording the 'Twenties. The
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Colorado 2008. ISBN 978-0-9772735-4-6.
14 page 56. Chanan, Michael. Repeated
Takes - A Short History of Recording and its Effects on Music.
Verso Books. 1995. ISBN 1-85984-012-4
15 page 364. Hoffmann, Frank W. and Ferstler,
Howard. Encyclopedia of Recorded Sound, Second Edition.
Taylor & Francis, Inc. July 2004 ISBN-13 9-78041593835-8
16 Thanks to Christine Rankovic, Ph. D. for this information
on Rogers Harrison Galt.
17 Jones, W. C. Condenser and Carbon Microphones:
Their Construction and Use. Journal of the Society of Motion Picture
Engineers. : January, 1931.
18 pages 110, 111. Adams, Stephen B. and Butler,
Orville R. Manufacturing the Future: A History of Western
Electric. Cambridge University Press. Cambridge,
UK. 1999 ISBN 0-521-65118-2
19 page 92-100. Thompson, Emily.
The Soundscape of Modernity: Architectural Acoustics and the Culture of
Listening in America, 1900-1933. MIT Press. Cambridge,
Massachusetts. 1999 ISBN-13: 9780262701068
20 page 334-348. Maxfield, J. P. and Harrison,
H. C. Methods of High Quality Recording and Reproducing of
Music and Speech Based on Telephone Research. Transaction of the
American Institute of Electrical Engineers. February 1926.
21 Frederick, H. A. The Development of the
Microphone. Journal of the Acoustical Society of America.
July, 1931. New York, New York.
22 pages 116-127. Copeland, Peter.
Manual of Analogue Sound Restoration Techniques.
British Library Sound Archive. London, UK. February, 2001.
23 Moore, Jerrold Northrop. pages 239-251
Sound Revolutions, A Biography of Fred Gaisberg, Founding Father
of Commercial Sound Recordings. Sanctuary Publishing
Ltd. London. 1999. ISBN 1-86074-235-1.