Introduction to Acoustics

Microphones and Their Calibration 24.8 Overall View on Microphone Calibration 1043
24.7 Frequency Response Measurement with Electrostatic Actuators
Electrostatic actuators were used by Ballantine [24.2]to
investigate microphone calibration and Koidan [24.59]
gave a comprehensive discussion of the uncertainties
between measurements obtained with electrostatic
actuators compared with those measured with the
coupler pressure reciprocity method. The description
and analyses of some commercially available electrostatic
actuators for microphone calibration have
been published [24.60–62] and standardized [24.63]
for frequency response measurement for working
standard (WS) microphones. The electrostatic actuator
method, Fig. 24.16, consists of a metal grid
polarized with a high voltage direct-current (DC)
supply (usually 800 V). An AC time-varying sweep
signal provides the electrostatic pressure on the microphone
diaphragm. With the usual amplification
and a recorder, the frequency response is displayed
graphically.
Madella [24.64] andNedzelnitsky [24.65] pointed
out that with electrostatic actuator measurements the effective
mechanical radiation impedance loading of the
microphone diaphragm in the presence of the electrostatic
actuator is different from that of those used in
a coupler or in a free field. Consequently, at higher frequencies
the absolute values of the difference between
the actuator-determined response of type L microphones
and pressure calibrations determined in couplers by
reciprocity can be as large as about 1.5 dB [24.65].
In practice, the design of the metal grid can be optimized
for a particular microphone type. According to
the IEC standard [24.63], the expanded uncertainty of
the method is 0.1–0.2 dB for frequencies up to 10 kHz
for WS1 (1-inch) and WS2 (half-inch) type microphones.
Since the air gap between the metal grid and
the microphone diaphragm is very small (nominally approximately
0.5 mm) the use of electrostatic actuators
for models of microphones other than those specified is
not recommended. Since the motion of the microphone
diaphragm, caused by electrostatic pressure produce by
the actuator, creates a sound pressure on the outer surface
DC-voltage supply
AC-voltage supply
24.8 Overall View on Microphone Calibration
Primary microphone calibrations, such as those performed
with the reciprocity method, provide uncertainty
estimations. One may wonder what is the contribution
to the calibration uncertainty from two major sources:
10 MΩ
5000pF
Actuator
Microphone
Preamplifier
Measurement
amplifier
Recorder
Fig. 24.16 Frequency response measurement with an electrostatic
actuator (after Frederiksen [24.29], Chap. 15)
of the diaphragm that adds to the electrostatic pressure
and influences the measure response, precautions should
be taken to avoid blocking the actuator space that opens
to the atmosphere.
Apart from being economically attractive, there
are some salient features to the electrostatic actuator
method.
1. The method is ideal for comparing the frequency
responses of microphones, such as those in a production
line, at which only changes from a nominal
value is important.
2. Since the actuator method is unaffected by atmospheric
pressure, humidity and temperature
(assuming the air gap remains constant) [24.63], the
method may be used to measure the change in microphone
responses with respect to variations in the
environment.
Given the fact that the international standard [24.63]
only discusses type WS1 and WS2 working standard
microphones and not the laboratory types LS1 and LS2
microphones, it is obvious that the electrostatic actuator
method still requires further development to reduce
uncertainties.
(a) the stability of the microphones and (b) the stability
of the calibration system that performed the microphone
calibration. It is very difficult to separate these two
sources of uncertainties.
Part H 24.8