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neumann.berlin
the microphone company
KM 64 - The Cardioid
During the 1950’s and 60’s the use of multiple microphones became increasingly common. Ever more microphones were being used simultaneously for the same recording session. While the different sections of an orchestra perform close to each other, microphones for the brass
section, for example, often also pick up the sounds of the
neighbouring woodwinds arriving from a different angle.
The reverse, of course, is equally true. Unfortunately, the
frequency response of a traditional cardioid microphone is
not the same for all directions of sound incidence.
The spectrum of the
woodwinds’ microphone
may be significantly different from the crosstalk of
this section picked up by
the neighbouring section’s
microphone. The solution
would be a cardioid microphone with a frequency response unaffected across the largest possible angle of sound
incidence. Neumann took up the challenge to develop such
a transducer, for which Neumann was issued patent
number 14 37 420.
To achieve this unidirectional pattern, it is necessary that
the sound impinging on the microphone from behind reaches the points at center-front and center-back of the diaphragm with the same amplitude and phase. Theoretically, it would suffice if the sum of the positive and negative
diaphragm movements were zero, and the diaphragm does
not necessarily have to remain still. In practice, however,
useful results are achieved only when the proper phase and
amplitude conditions are fulfilled across the entire diaphragm, allowing it to remain stationary at all points.
This problem had not been solved yet in the construction
of contemporary miniature microphones. The back electrodes commonly used
at the time feature a
large number of holes
that allow sound from
the rear to pass directly to the back side of
the diaphragm. The
distance between the
diaphragm and back
electrode is typically
about 40 fm. The diaphragm is thus stimulated practically point-by-point
through the holes. This led Neumann to re-engineer the
back electrode with only
nine holes, connected by
three comparatively deep
slits, intersecting the nine
holes. With this novel rear
electrode the distance
from the sound exit vents
to the back side of the diaphragm measured more
than 2000 µm, rather than
the traditional 40 µm.
Sound from the rear
would thus expose a larger area on the back side of
the diaphragm instead of
just striking multiple individual points.
Using the newly designed
back electrode, the stimulation of the diaphragm’s
back side corresponds for
the most part that of the
front side, and the design
goal of achieving equal
phase and amplitude levels
on both sides of the diaphragm is almost perfectly achieved. The result is
the KK 64 miniature microphone capsule – with
its nearly ideal cardioid
pattern – introduced in
1964.
The frequency response for a sound incidence angle of 135°
is nearly parallel to the on-axis response. At this angle the
rejection stays at a virtually constant 15 dB between
100 Hz and 18 kHz. With such a microphone sound events
occurring in a three-quarter circle around the microphone
can be recorded, registering only differences in loudness
without any sonic coloration. Besides multi-microphone recordings, practical applications include talents changing
their position on stage during a recording, or recording a
conversation between people in different locations, for example during field reports and roundtable interviews.
The KM 64 is equipped with a Telefunken Triode of the
type AC 701 K. The KM 64’s features and connections
correspond to that of the other miniature microphones
KM 53/54/56 and the accessories of these microphones can
be shared.
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