Handbook for Sound Engineers

Microphones 547

Each channel of the microphone incorporates two
cardioid diaphragms facing 180° of each other (back to
back), Fig. 16-120. Note the 12 Vdc phantom power for
the electronics and the 60 Vdc phantom power for the
lower transducer (1), insuring that transducer 1 is
always biased on. This transducer has a positive output
for a positive pressure. The second or upper transducer
is connected to pin K, which through the S42E, has nine
switchable voltages between 0 Vdc and 120 Vdc. When
the voltage at K is 60 Vdc, the output of transducer 2 is
0 (60 Vdc on either side of it), so the microphone output
is cardioid.
When the voltage at K is 120 Vdc, transducer 2 is
biased with 60 Vdc of an opposite polarity from trans-
ducer 1 so the output is 180° out of polarity, the mixed
output being a figure 8 pattern.
When the voltage at K is 0 Vdc, transducer 2 has a
60 Vdc bias on it with the same polarity as transducer 1.
Because the transducers face in opposite directions,
when these two outputs are combined, an omnidirec-
tional pattern is produced.
By varying the voltage on K between 0 Vdc and
120 Vdc, various patterns between a figure 8 and an
omnidirectional pattern can be produced.
The Shure VP88 stereo microphone, Fig. 16-116, also
employs a switchable pattern. Fig. 16-121 shows the
polar response of the mid capsule and the side capsule.
The left and right stereo signals exhibit their own
equivalent pickup patterns corresponding to, respec-
tively, left-forward-facing and right-forward-facing
microphones. Fig. 16-122 shows the relative levels of
the mid and side microphones and the stereo pickup
pattern of the Shure VP88 microphone in the L position
with the bidirectional side pattern maximum sensitivity
6 dB lower than the maximum mid sensitivity. The
small rear lobes of each pattern are 180° out of polarity
with the main front lobes. For sound sources arriving at
0° the left and right output signals are equal, and a
center image is reproduced between the loudspeakers.
As the sound source is moved off-axis, an amplitude
difference between left and right is created, and the
loudspeaker image is moved smoothly off-center in the
direction of the higher amplitude signal.
When the mid (mono) pattern is fixed as cardioid,
the stereo pickup pattern can be varied by changing the
side level relative to the mid level. Fig. 16-123 shows
an M/S pattern in the M position with the side level
1.9 dB lower than the mid level. Fig. 16-124, position
H, increases the side level to 1.6 dB higher than the mid
level. The three resultant stereo patterns exhibit pickup
angles of 180°, 127°, and 90°, respectively. The
incoming sound angles, which will create left,

Figure 16-121. Polar response of the Shure VP88 M/S

microphone. Courtesy Shure Incorporated.

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