The microphone pervades our daily lives through the sound we hear on
radio, television and recordings, paging in public spaces, and of course
in two-way communications via telephone. In this chapter we will touch
upon some of the highlights of more than 125 years of microphone
development, observing in particular how most of the first 50 of these
years were without the benefits of electronic amplification. The requirements
of telephony, radio broadcast, general communications, and
recording are also discussed, leading to some conjecture on future
requirements.
As children, many of us were fascinated with strings stretched between
the ends of a pair of tin cans or wax paper cups, with their ability to convey
speech over a limited distance. This was a purely mechano-acoustical
arrangement in which vibrations generated at one end were transmitted
along the string to actuate vibrations at the other end.
In 1876, Alexander Graham Bell received US patent 174,465 on the
scheme shown in Figure 1–1. Here, the mechanical string was, in a sense,
replaced by a wire that conducted electrical direct current, with audio
signals generated and received via a moving armature transmitter and its
associated receiver. Like the mechanical version, the system was reciprocal.
Transmission was possible in either direction; however, the
patent also illustrates the acoustical advantage of a horn to increase the
driving pressure at the sending end and a complementary inverted horn
to reinforce output pressure at the ear at the receiving end. Bell’s further
experiments with the transmitting device resulted in the liquid transmitter,
shown in Figure 1–2, which was demonstrated at the Philadelphia
Centennial Exposition of 1876. Here, the variable contact principle
provided a more effective method of electrical signal modulation than
that afforded by the moving armature.
The variable contact principle was extended by Berliner in a patent
application in 1877 in which a steel ball was placed against a stretched
metal diaphragm, as shown in Figure 1–3. Further work in this area was
done by Blake (patents 250, 126 through 250, 129, issued in 1881), who
used a platinum bead impressed against a hard carbon disc as the variable
resistance element, as shown in Figure 1–4. The measured response
of the Blake device spanned some 50 decibels over the frequency range
from 380Hz to 2000Hz, and thus fell far short of the desired response.
However, it provided a more efficient method of modulating telephone
signals than earlier designs and became a standard in the Bell system for
some years.
Another interim step in the development of loose contact modulation
of direct current was developed in 1878 by Hughes and is shown in
Figure 1–5. In this embodiment, very slight changes in the curvature of
the thin wood plate diaphragm, caused by impinging sound waves, gave
rise to a fairly large fluctuation in contact resistance between the carbon
rod and the two mounting points. This microphone was used by Clement
Ader (Scientific American, 1881) in his pioneering two-channel transmissions
from the stage of the Paris Opera to a neighboring space. It was
Hughes, incidentally, who first used the term microphone, as applied to
electroacoustical devices.
The ultimate solution to telephone transmitters came with the development
of loose carbon granule elements as typified by Blake’s transmitter
of 1888, shown in Figure 1–6. Along with the moving armature receiver,
the loose carbon granule transmitter, or microphone, has dominated
telephony up to the present. Quite a testimony to the inventiveness and
resourcefulness of engineers working nearly 130 years ago.
radio, television and recordings, paging in public spaces, and of course
in two-way communications via telephone. In this chapter we will touch
upon some of the highlights of more than 125 years of microphone
development, observing in particular how most of the first 50 of these
years were without the benefits of electronic amplification. The requirements
of telephony, radio broadcast, general communications, and
recording are also discussed, leading to some conjecture on future
requirements.
As children, many of us were fascinated with strings stretched between
the ends of a pair of tin cans or wax paper cups, with their ability to convey
speech over a limited distance. This was a purely mechano-acoustical
arrangement in which vibrations generated at one end were transmitted
along the string to actuate vibrations at the other end.
In 1876, Alexander Graham Bell received US patent 174,465 on the
scheme shown in Figure 1–1. Here, the mechanical string was, in a sense,
replaced by a wire that conducted electrical direct current, with audio
signals generated and received via a moving armature transmitter and its
associated receiver. Like the mechanical version, the system was reciprocal.
Transmission was possible in either direction; however, the
patent also illustrates the acoustical advantage of a horn to increase the
driving pressure at the sending end and a complementary inverted horn
to reinforce output pressure at the ear at the receiving end. Bell’s further
experiments with the transmitting device resulted in the liquid transmitter,
shown in Figure 1–2, which was demonstrated at the Philadelphia
Centennial Exposition of 1876. Here, the variable contact principle
provided a more effective method of electrical signal modulation than
that afforded by the moving armature.
The variable contact principle was extended by Berliner in a patent
application in 1877 in which a steel ball was placed against a stretched
metal diaphragm, as shown in Figure 1–3. Further work in this area was
done by Blake (patents 250, 126 through 250, 129, issued in 1881), who
used a platinum bead impressed against a hard carbon disc as the variable
resistance element, as shown in Figure 1–4. The measured response
of the Blake device spanned some 50 decibels over the frequency range
from 380Hz to 2000Hz, and thus fell far short of the desired response.
However, it provided a more efficient method of modulating telephone
signals than earlier designs and became a standard in the Bell system for
some years.
Another interim step in the development of loose contact modulation
of direct current was developed in 1878 by Hughes and is shown in
Figure 1–5. In this embodiment, very slight changes in the curvature of
the thin wood plate diaphragm, caused by impinging sound waves, gave
rise to a fairly large fluctuation in contact resistance between the carbon
rod and the two mounting points. This microphone was used by Clement
Ader (Scientific American, 1881) in his pioneering two-channel transmissions
from the stage of the Paris Opera to a neighboring space. It was
Hughes, incidentally, who first used the term microphone, as applied to
electroacoustical devices.
The ultimate solution to telephone transmitters came with the development
of loose carbon granule elements as typified by Blake’s transmitter
of 1888, shown in Figure 1–6. Along with the moving armature receiver,
the loose carbon granule transmitter, or microphone, has dominated
telephony up to the present. Quite a testimony to the inventiveness and
resourcefulness of engineers working nearly 130 years ago.
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