Handbook for Sound Engineers

Microphones 513

for increased distortion levels (up to 1% and more) at

high frequencies.

The measurement results can be extended to higher

signal levels simply by linear extrapolation. This means,

for instance, that 10 times higher sound pressures will

yield 10 times higher distortions, as long as clipping of

the microphone circuit is prevented. Thus, two sounds

of 124 dB SPL will cause more than 10% distortion in

the microphones. Sound pressure levels of this order are

beyond the threshold of pain of human hearing but may

arise at close-up micing. Despite the fact that the audi-

bility of distortions depends significantly on the tonal

structure of the sound signals, distortion figures of this

order will considerably affect the fidelity of the sound

pick-up.

The Cause of Nonlinearity. Fig. 16-52 shows a simpli-

fied sketch of a capacitive transducer. The diaphragm

and backplate form a capacitor, the capacity of which

depends on the width of the air gap. From the acoustical

point of view the air gap acts as a complex impedance.

This impedance is not constant but depends on the

actual position of the diaphragm. Its value is increased

if the diaphragm is moved toward the backplate and it is

decreased at the opposite movement, so the air gap

impedance is varied by the motion of the diaphragm.

This implies a parasitic rectifying effect superimposed

to the flow of volume velocity through the transducer,

resulting in nonlinearity-created distortion.

Solving the Linearity Problem. A push-pull design of

the transducer helps to improve the linearity of

condenser microphones, Fig. 16-53. An additional plate

equal to the backplate is positioned symmetrically in

front of the diaphragm, so two air gaps are formed with

equal acoustical impedances as long as the diaphragm is

in its rest position. If the diaphragm is deflected by the

sound signal, then both air gap impedances are deviated

opposite to each other. The impedance of one side

increases while the other impedance decreases. The

variation effects compensate each other regardless of

the direction of the diaphragm motion, and the total air

gap impedance is kept constant, reducing the distortion

of a capacitive transducer.

Fig. 16-54 shows the distortion characteristics of the

Sennheiser MKH series push-pull element transformer-

less RF condenser microphones. The improvement on

linearity due to the push-pull design can be seen by

comparing Fig. 16-51 to Fig. 16-54.

16.3.4.3.3 Noise Sources

The inherent noise of condenser microphones is caused

partly by the random incidence of the air particles at the

diaphragm due to their thermal movement. The laws of

Figure 16-51. Frequency distortion of eight unidirectional

microphones. Courtesy Sennheiser Electronic Corporation.

Figure 16-52. Conventional capacitor microphone
transducer.

3

2

1

0

2 × 104 dB SPL

¾$f = 70 Hz

Frequency–Hz

200 500 1k 2k 5k 10k 20k

Distortion–%

Diaphragm

Air gap

Backplate

Figure 16-53. Symmetrical capacitor microphone

transducer.

Figure 16-54. Distortion characteristics of the symmetrical

capacitor microphone transducer.

Diaphragm

Backplate

Symmetrical

transducer

Backplate

2 × 104 dB SPL

$f = 70 Hz

Frequency–Hz

Distortion–%

3

2

1

0

(^200500) 1k 2k 5k 10k 20k