Audio Engineering

642 Chapter 22

the reference sound fi eld pressure is 1 Pa or 94 dB. This rating is more complicated as it
involves the microphone output impedance. All microphones, regardless of whether the
construction is moving coil, capacitor, ribbon, etc., have intrinsic output impedance that
in general is complex and frequency dependent. Strictly speaking, in order for such a
device to deliver maximum power, it must work into a load that is matched on a conjugate
basis with the reactance of the load being the negative of the reactance of the source and
the resistance of the load being equal to the resistance of the source.

Suppose then that the real part of the microphone’s output impedance is Ro. This being
the case, the available input power in watts that the microphone can deliver to the input of
a successive device, AIP , is given by

AIP E

o Ro



1

4

⎛ 2 1

⎝

⎜⎜

⎜

⎞

⎠

⎟⎟

⎟⎟( )

⎛

⎝

⎜⎜

⎜⎜

⎞

⎠

⎟⎟

⎟⎟. (22.3)

If AIP is referenced to 1 mW and the microphone is exposed to a sound fi eld of 1 Pa, then

AIP

R

S

o

V

0 001

1

4

10 10

3 10 1

.



⎛

⎝

⎜⎜

⎜

⎞

⎠

⎟⎟

⎟⎟( )

⎛

⎝

⎜⎜

⎜⎜⎜

⎞

⎠

⎟⎟

⎟⎟

⎟

⎛

⎝

⎜⎜

⎜⎜

⎞

⎠

⎟⎟⎟

⎟⎟. (22.4)

This can be converted to a power level by taking the logarithm to the base 10 of Equation
(22.4) and then multiplying by 10 dBm to yield

LSRAIP   ( (^630) V 10 log o)dBm. (22.5)
L AIP expresses the power sensitivity of a microphone in terms of dBm/Pa. If our example
microphone has anRo of 200 Ω along with its voltage sensitivity of  60, then its power
sensitivity would be
    6 30 60 23 59 dBm/Pa.
Another useful way to express the power sensitivity of a microphone would be to
reference the available input power to a sound fi eld of 0.00002 Pa. This would produce a