Handbook for Sound Engineers

508 Chapter 16

EO is the output in volts,

Ep is the dc polarizing voltage in volts,

a is the radius of active area of the diaphragm in centi-
meters,
P is the pressure in dynes per square centimeter,

d is the spacing between the back plate and diaphragm
in centimeters,

t is the diaphragm tension in dynes per centimeter.

Many capacitor microphones operate with an equiva-
lent noise level of 15–30 dB SPL. Although a 20–30 dB
SPL is in the range of a well-constructed studio, a
20–30 dB microphone equivalent noise is not masked
by room noise because room noise occurs primarily at
low frequencies and microphone noise at high
frequency as hiss.
In the past the quality of sound recordings was
limited by the characteristics of the analog tape and
record material, apart from losses induced by the
copying and pressing procedures. Tape saturation, for
instance, created additional harmonic and disharmonic
distortion components, which affected the recording
fidelity at high levels, whereas the linearity at low and
medium levels was quite acceptable. The onset of these
distortions was rather soft and extended to a wide level
range that makes it difficult to determine the threshold
of audibility.

The distortion characteristics of the standard studio
condenser microphones is adequate for operation with
analog recording equipment. Although exhibiting a high
degree of technical sophistication, these microphones
show individual variations in the resolution of complex
tonal structures, due to their specific frequency
responses and directivity patterns and nonlinear effects
inherent to these microphones.

These properties were mostly concealed by the
distortions superimposed by the analog recording and
playback processing. But the situation has changed
essentially since the introduction of digital audio. The
conversion of analog signals into digital information
and vice versa is carried out very precisely, especially at
high signal levels. Due to the linear quantization
process the inherent distortions of digital recordings
virtually decrease at increasing recording levels, which
turns former distortion behavior upside down. This new
reality, which is in total contrast to former experience
with analog recording technique, contributes mostly to
the fact that the specific distortion characteristics of the
microphone may become obvious, whereas they have
been masked previously by the more significant distor-
tions of analog recording technique.

Another feature of digital audio is the enlarged

dynamic range and reduced noise floor. Unfortunately,

due to this improvement, the inherent noise of the

microphones may become audible, because it is no

longer covered up by the noise of the recording

medium.

The capacitor microphone has a much faster rise

time than the dynamic microphone because of the

significantly lower mass of the moving parts

(diaphragm versus diaphragm/coil assembly). The

capacitor rise time rises from 10% of its rise time to

90% in approximately 15μs, while the rise time for the

dynamic microphone is in the order of 40μs.

Capacitor microphones generate an output electrical

waveform in step or phase with the acoustical waveform

and can be adapted to measure essentially dc overpres-

sures, Fig. 16-40.

Some advantages of capacitor microphones are:

• Small, low-mass rigid diaphragms that reduce vibra-
  tion pickup.


• Smooth, extended-range frequency response.


• Rugged—capable of measuring very high sound
  pressure levels (rocket launches).


• Low noise (which is partially cancelled by the need
  for electronics).


• Small head size, which provides low diffraction inter-
  ference.

16.3.4.1 Voltage Divider Capacitor Microphone

Voltage divider-type capacitor microphones require a

preamplifier as an integral part of the housing and a

Figure 16-40. Capacitor microphone acoustic wave and

electrical signals. Note the in-phase condition.

B

A

C

D

E A

E F

D

BC

A

B

C

D

E A

B C

D

EF

Acoustical waveform

Electrical waveform