Handbook for Sound Engineers

Microphones 505

this reduces the peak at 350 Hz, it does not fix the droop
below 200 Hz. Additional acoustical resonant devices
are used inside the microphone case to correct the
drooping. A cavity behind the unit (analogous to capaci-
tance) helps resonate at the low frequencies with the
mass (inductance) of the diaphragm and voice-coil
assembly.
Another tuned resonant circuit is added to extend the
response down to 35 Hz. This circuit, tuned to about
50 Hz, is often a tube that couples the inside cavity of
the microphone housing to the outside, Fig. 16-34.

The curvature of the diaphragm dome provides stiff-
ness, and the air cavity between it and the dome of the
pole piece form an acoustic capacitance. This capaci-
tance resonates with the mass (inductance) of the
assembly to extend the response up to 20 kHz.
To control the high-frequency resonance, a nonmag-
netic filter is placed in front of the diaphragm, creating
an acoustic resistance, Fig. 16-34. The filter is also an
effective mechanical protection device. The filter
prevents dirt particles, magnetic chips, and moisture
from gravitating to the inside of the unit. Magnetic
chips, if allowed to reach the magnetic gap area, eventu-
ally will accumulate on top of the diaphragm and impair
the frequency response. It is possible for such chips to
pin the diaphragm to the pole piece, rendering the
microphone inoperative.

Fig. 16-35 illustrates the effect of a varying sound

pressure on a moving-coil microphone. For this simpli-

fied explanation, assume that a massless diaphragm

voice-coil assembly is used. The acoustic waveform,

Fig. 16-35A, is one cycle of an acoustic waveform,

where a indicates atmospheric pressure AT; and b repre-

sents atmospheric pressure plus a slight overpressure

increment ' or AT + '.

The electrical waveform output from the

moving-coil microphone, Fig. 16-35B, does not follow

the phase of the acoustic waveform because at

maximum pressure, AT + ' or b, the diaphragm is at rest

(no velocity). Further, the diaphragm and its attached

coil reach maximum velocity, hence maximum elec-

trical amplitude—at point c on the acoustic waveform.

This is of no consequence unless another microphone is

being used along with the moving-coil microphone

where the other microphone does not see the same 90°

displacement. Due to this phase displacement,

condenser microphones should not be mixed with

moving-coil or ribbon microphones when micing the

same source at the same distance. (Sound pressure can

be proportional to velocity in many practical cases.)^6

A steady overpressure which can be considered an

acoustic square wave, Fig. 16-35C, would result in the

output shown in Fig. 16-35D. As the acoustic pressure

rises from a to b, it has velocity, Fig. 16-35, creating a

voltage output from the microphone. Once the

diaphragm reaches its maximum displacement at b, and

stays there during the time interval represented by the

distance between b and c, voice-coil velocity is zero so

electrical output voltage ceases and the output returns to

zero. The same situation repeats itself from c to e and

from e to f on the acoustic waveform. As can be seen, a

moving-coil microphone cannot reproduce a square

wave.

Another interesting theoretical consideration of the

moving-coil microphone mechanism is shown in Fig.

16-36. Assume a sudden transient condition. Starting at

a on the acoustic waveform, the normal atmospheric

pressure is suddenly increased by the first wavefront of

a new signal and proceeds to the first overpressure peak,

AT+' or b. The diaphragm will reach a maximum

velocity halfway to b and then return to zero velocity at

b. This will result in a peak, a', in the electrical wave-

form. From b on, the acoustic waveform and the elec-

trical waveform will proceed as before, cycle for cycle,

but 90° apart.

In this special case, peak a' does not follow the input

precisely so it is something extra. It will probably be

swamped out by other problems (especially mass)

encountered in a practical moving-coil microphone. It

Figure 16-34. Omnidirectional microphone cross-section
view.

Resonator

Sintered bronze filter

Magnet

Case cavity

Tube

Connector

Transformer