Handbook for Sound Engineers

502 Chapter 16

diagram of the RE20. By moving the red wire to either
the 250:or 50: tap, the microphone output imped-
ance can be changed. Note the “bass tilt” switch that,
when open, reduces the series inductance and, therefore,
the low-frequency response.

16.2.3.1.6 Two-Way Cardioid Microphones

In a two-way microphone system, the total response
range is divided between a high-frequency and a
low-frequency transducer, each of which is optimally
adjusted to its specific range similar to a two-way loud-
speaker system. The two systems are connected by
means of a crossover network.
The AKG D-222EB schematically shown in
Fig. 16-28 employs two coaxially mounted dynamic
transducers. One is designed for high frequencies and is
placed closest to the front grille and facing forward. The
other is designed for low frequencies and is placed
behind the first and facing rearward. The low-frequency
transducer incorporates a hum-bucking winding to
cancel the effects of stray magnetic fields. Both trans-
ducers are coupled to a 500 Hz inductive-capaci-
tive-resistive crossover network that is electro-
acoustically phase corrected and factory preset for
linear off-axis response. (This is essentially the same
design technique used in a modern two-way loud-
speaker system.)
The two-way microphone has a predominantly
frequency-independent directional pattern, producing
more linear frequency response at the sides of the
microphone and far more constant discrimination at the
rear of the microphone. Proximity effect at working
distances down to 6 in (15 cm) is reduced because the
distance between the microphone windscreen and the
low frequency transducer is large.
The D-222EB incorporates a three-position
bass-roll-off switch that provides 6 dB or 12 dB attenu-

ation at 50 Hz. This feature is especially useful in

speech applications and in acoustically unfavorable

environments with excessive low-frequency ambient

noise, reverberation, or feedback.

16.3 Types of Transducers

16.3.1 Carbon Microphones

One of the earliest types of microphones, the carbon

microphone, is still found in old telephone handsets. It

has very limited frequency response, is very noisy, has

high distortion, and requires a hefty dc power supply. A

carbon microphone^4 is shown in Fig. 16-29 and operates

in the following manner.

Several hundred small carbon granules are held in

close contact in a brass cup called a button that is

attached to the center of a metallic diaphragm. Sound

waves striking the surface of the diaphragm disturb the

carbon granules, changing the contact resistance

between their surfaces. A battery or dc power source is

connected in series with the carbon button and the

primary of an audio impedance-matching transformer.

Figure 16-27. Electro-Voice RE20 cardioid wiring diagram.
Note “bass tilt” switch circuit and output impedance taps.
Courtesy Telex Electro-Voice.

Yel

Blu

Blk

Cartridge

Blk

0.56 MF

150 7

50 7

Red

Grn 140 mH Yel

Bass tilt

Yel

0.2 mH

Red

Case

gnd

Grn

Blu

Red

Shield

Figure 16-28. Schematic of an AKG D222EB two-way

cardioid microphone. Courtesy AKG Acoustics, Inc.

Figure 16-29. Connection and construction of a

single-button carbon microphone.

Replacement transducer/crossover module

Red

Front

facing

HF

transducer

YEL

Black

Hum-bucking

coil

Rear

facing

transducer LF

+

+ Factorypreset for

most linear

response at

90 ooff axis

12dB

6dB

White

Bass-

rolloff

switch

3 2

1

Red (in phase)