Handbook for Sound Engineers

Microphones 515

A special feature of the omnidirectional microphone
is a switchable diffuse field correction that corrects for
both direct and diffuse sound field conditions. The
normal switch position is recommended for a neutral
pickup when closeup micing and the diffuse field posi-
tion is used if larger recording distances are used where
reverberations become significant.
The distinction between both recording situations
arises because omnidirectional microphones tend to
attenuate lateral and reverse impinging sound signals at
high frequencies. Diffuse sound signals with random
incidence cause a lack of treble response, which can be
compensated by treble emphasis at the microphone.
Unfortunately, frontally impinging sounds are empha-
sized also, but this effect is negligible if the reverberant
sound is dominant.

16.3.5 Electret Microphones

An electret microphone is a capacitor microphone in
which the head capacitor is permanently charged, elimi-
nating the need for a high-voltage bias supply.
From a design viewpoint a microphone intended to
be used for critical recording, broadcast, or sound rein-

forcement represents a challenge involving minimal

performance compromise. Early electrets offered the

microphone designer a means of reducing the

complexity of a condenser microphone by eliminating

the high-voltage bias supply, but serious environmental

stability problems negated this advantage.

Well-designed electret microphones can be stored at

50°C (122°F) and 95% relative humidity for years with

a sensitivity loss of only 1 dB. Under normal condi-

tions of temperature and humidity, electret transducers

will demonstrate a much lower charge reduction versus

time than under the severe conditions indicated. Even if

a proper electret material is used, there are many steps

in the fabricating, cleaning, and charging processes that

greatly influence charge stability.

The Shure SM81 cardioid condenser microphone^3

Fig. 16-58, uses an electret material as a means of estab-

lishing a bias voltage on the transducer. The backplate

carries the electret material based upon the physical

properties of halocarbon materials such as Teflon™ and

Aclar, which are excellent electrets, and materials such

as polypropylene and polyester terephthalate (Mylar™),

which are more suitable for diaphragms.

Figure 16-57. Schematic of a Sennheiser MKH 20 P 48 U 3 capacitor microphone. Courtesy Sennheiser Electronic

Corporation.

(^1) 24V
KS20-0
2
3
21
3
24V
5 Mh
100pF
28.5V
-10dB
Bat 83
Bat83
46.5V
A(+)
42V
37.5V
L 1
D 1
D 3
C 5 +MF
D 2
D 4
S 1
C 1 470 pF
B(-)
42V
L 2
26.5VBCB468T 1
0.55V
R 1 470 k 7 C^4 1 nF
R 1
687 1.25V
C (^3) 10 nF
T 2
BC8508B
R 2 0.55V
R 4
R 3 330 7
330 7
330 7
C 2 10nF R 5 1.5 k 7
C 1
L 3
C 7
68 nF
R 7 227
L 5 1 mh
7
3 k 7 39 k^7
390 7
150 nFC^1 6.8 k^7
R 11
S 2
R 10
R 16 8.2 k 7
C 10 C 11 C 12
1 MF 1 MF 27 MF
39 Mh
R 19
L 4
39 k 7
28.5 V
T 4
BC860B 3.3 k^7
R 21
C D^6
68 14 MF
ZMM15
ZMM15 C 13
1 nF
68 MDF 8
C 16

• 68 MF
  ZMM68
  R 20
  R 23
  C 17
  62 7
  39 Mh
  39 Mh
  1 nF
  1 nF
  C 13
  1 nF
  C1 nF 13
  62 7 41 V
  L 7
  1
  32
  XLR 3
  L 4
  C (^15) + R 22
  150 MFD
  7
  150 150 MMFF
  37.5V
  C 13 470 PF
  R 14
  R 17
  R 18
  R 9


• 
  
  
  
  • 
    
    
    
    • DS LL4148
      T 3
      BCB508
      33 V
      3.3 k 7
      41 V
      St