514 Chapter 16
statistics imply that sound pressure signals at the
diaphragm can be evaluated by a precision that
improves linearly with the diameter of the diaphragm.
Thus, larger diaphragms yield better noise performance
than smaller ones.
Another contribution of noise is the frictional effects
in the resistive damping elements of the transducer. The
noise generation from acoustical resistors is based on
the same principles as the noise caused by electrical
resistors so high acoustical damping implies more noise
than low damping.
Noise is also added by the electrical circuit of the
microphone. This noise contribution depends on the
sensitivity of the transducer. High transducer sensitivity
reduces the influence of the circuit noise. The inherent
noise of the circuit itself depends on the operation prin-
ciple and on the technical quality of the electrical
devices.
Noise Reduction. Large-diameter diaphragms improve
noise performance. Unfortunately, a large diameter
increases the directivity at high frequencies. A 1 inch
(25 mm) transducer diameter is usually a good choice.
A further method to improve the noise characteris-
tics is the reduction of the resistive damping of the
transducer. In most directional condenser microphones,
a high amount of resistive damping is used in order to
realize a flat frequency response of the transducer itself.
With this design the electrical circuit of the microphone
is rather simple. However, it creates reduced sensitivity
and increased noise.
Keeping the resistive damping of the transducer
moderate will be a more appropriate method to improve
noise performance, however it leads to the transducer
frequency response that is not flat so equalization has to
be applied by electrical means to produce a flat
frequency response of the complete microphone. This
design technique requires a more sophisticated elec-
trical circuit but produces good noise performance.
The electrical output of a transducer acts as a pure
capacitance. Its impedance decreases as the frequency
increases so the transducer impedance is low in an RF
circuit but high in an AF circuit. Moreover, in an RF
circuit the electrical impedance of the transducer does
not depend on the actual audio frequency but is rather
constant due to the fixed frequency of the RF oscillator.
Contrary to this, in an AF design, the transducer imped-
ance depends on the actual audio frequency, yielding
very high values especially at low frequencies. Resistors
of extremely high values are needed at the circuit input
to prevent loading of the transducer output. These resis-
tors are responsible for additional noise contribution.
The RF circuit features a very low output impedance
which is comparable to that of dynamic-type micro-
phones. The output signal can be applied directly to
bipolar transistors, yielding low noise performance by
impedance matching.
The Sennheiser MKH 20, Fig. 16-55, is a pressure
microphone with omnidirectional characteristics. The
MKH 30 is a pure pressure-gradient microphone with a
highly symmetrical bidirectional pattern due to the
symmetry of the push-pull transducer. The MKH 40,
Fig. 16-56, operates as a combined pressure and pres-
sure-gradient microphone yielding a unidirectional
cardioid pattern.- The microphones are phantom powered by 48 Vdc
and 2 mA. The outputs are transformerless floated,
Fig. 16-57. - The SPLmax is 134 dB at nominal sensitivity and
142 dB at reduced sensitivity. - The equivalent SPL of the microphones range from
10–12 dBA corresponding to CCIR-weighted figures
of 20–22 dB. - The directional microphones incorporate a switchable
bass roll-off to cancel the proximity effect at close-up
micing. The compensation is adjusted to about 5 cm
(2 in) distance.
Figure 16-55. Omnidirectional pressure capacitor micro-
phone. Note the lack of rear entry holes in the case.
Courtesy Sennheiser Electronic Corporation.Figure 16-56. Unidirectional pressure/pressure-gradient
capacitor microphone. Courtesy Sennheiser Electronic
Corporation.