Handbook for Sound Engineers

522 Chapter 16

• Reduced pickup of electrical noise and transients.


• Reduced pickup of electrical signals from adjacent
  wires.

These reductions are realized because the two signal
conductors shown in Fig. 16-63 pick up the same stray
signal with equal intensity and polarity, so the noise is
impressed evenly on each end of the transformer
primary, eliminating a potential across the transformer
and canceling any input noise. Because the balanced
wires are in a shielded cable, the signal to each
conductor is also greatly reduced.

When installing microphones into an unbalanced
system, any noise that gets to the inner unbalanced
conductor is not canceled by the noise in the shield, so
the noise is transmitted into the preamplifier. In fact,
noise impressed on the microphone end of the shield
adds to the signal because of the resistance of the shield
between the noise and the amplifier.
Balanced low-impedance microphone lines can be as
long as 500 ft (150 m) but unbalanced microphone lines
should never exceed 15 ft (4.5 m).

16.5.5 Impedance

Most professional microphones are low impedance,
200 : , and are designed to work into a load of 2000:.
High-impedance microphones are 50,000: and are
designed to work into an impedance of 1–10 M:. The
low-impedance microphone has the following
advantages:

• Less susceptible to noise. A noise source of relatively
  high impedance cannot “drive” into a source of rela-
  tively low impedance (i.e., the microphone cable).


• Capable of being connected to long microphone lines
  without noise pickup and high-frequency loss.

All microphone cable has inductance and capaci-
tance. The capacitance is about 40 pF (40 × 10^12 )/ft
(30 cm). If a cable is 100 ft long (30 m), the capacitance
would be (40 × 10^12 ) × 100 ft or 4 × 10^9 F or

0.004μF. This is equivalent to a 3978.9: impedance at

10,000 Hz and is found with the equation

(16-18)

This has little effect on a microphone with an imped-

ance of 200: as it does not reduce the impedance

appreciably as determined by

(16-19)

For a microphone impedance of 200:, the total

impedance ZT= 190: or less than 0.5 dB.

If this same cable were used with a high-impedance

microphone of 50,000:, 10,000 Hz would be down

more than 20 dB.

Making the load impedance equal to the microphone

impedance will reduce the microphone sensitivity 6 dB,

which reduces the overall SNR by 6 dB. For the best

SNR, the input impedance of low-impedance micro-

phone preamplifiers is always 2000: or greater.

If the load impedance is reduced to less than the

microphone impedance, or the load impedance is not

resistive, the microphone frequency response and output

voltage will be affected.

Changing the load of a high-impedance or ceramic

microphone from 10 M: to 100 k: reduces the output

at 100 Hz by 27 dB.

16.6 Miscellaneous Microphones

16.6.1 Pressure Zone Microphones (PZM)

The pressure zone microphone, referred to as a PZMi-

crophone or PZM, is a miniature condenser microphone

mounted face-down next to a sound-reflecting plate or

boundary. The microphone diaphragm is placed in the

pressure zone just above the boundary where direct and

reflected sounds combine effectively in-phase over the

audible range.

In many recording and reinforcement applications,

the sound engineer is forced to place microphones near

hard reflective surfaces such as when recording an

instrument surrounded by reflective baffles, reinforcing

drama or opera with the microphones near the stage

floor, or recording a piano with the microphone close to

the open lid.

In these situations, sound travels from the source to

the microphone via two paths: directly from the source

Figure 16-64. Noise cancellation on balanced, shielded
microphone cables.

Microphone Preamplifier

+

+

+

+

++

Xc^1

2 SfC

-------------=

ZT

XcZm

Xc+Zm

-------------------=