Volume 6, Number 4, December, 1998 - Noise News International

Adaptor for
Microphone &
Preamplifier
Fig. 9. Three-port couplerfor matching microphones
(G. Wong, JASA 1991).
Similarly, the three-port two-microphone
cavity 10,1 1 can be used for a precision phase match
of microphones, as illustrated in Fig. 9. Again, the
microphone interchange method is used. Here, the
difference in phase response between the microphones,
and the phase difference between the two
measuring systems can be deduced. The uncertainty
of the method at midband frequencies is approximately
±0.05 degrees, including the uncertainty of
the phase meter.
chanical manipulation, the equivalent volume remains
constant during the measurement.
Precision Determination of Equivalent Volumes
for Reciprocity Calibration of Microphones
The uncertainty in reciprocity calibration of microphones
depends on the ability to determine the
equivalent volumes of the microphones and the coupler
combined. The equivalent volume - which includes
small volumes occupiedby screw threads and
an effective volume caused by nonrigidity of the microphone
diaphragm - is very difficult to measure
preciselj' A method of equivalent volume determination,
1 shown in Fig. 11, that reduces substantially
the uncertainty of reciprocity microphone
calibration has been developed. Three spacers with
precisely known thicknesses in steps of 0.1 em are
included in tum in the microphone cavity. Three
voltage ratios, ~, corresponding to each spacer
thickness are measured. With a precisely known
change in volume, based on known thicknesses and
internal diameters of the spacers, the equivalent volumes
of the microphones, V e , and coupler, V o , can
be calculated. This determination also includes corrections
for capillary tube impedances, heat conduction,
and gas density. For very precise
determination, the spacers are implemented with
optical flats whose thicknesses are measured with
interferometry, and the uncertainty ofthe equivalent
volume determination is approximately 20 ppm.
A Wide-band Piezoelectric Transducer
for Otoacoustic Emission Applications
Otoacoustic emission is a clinical technique based
on sending a small acoustical signal stimulus down
the ear canal and then listening with sensitive equipment
for a response signal generated by the ear. A
deviation from this technique is to send the acoustic
Three-port Coupler for Reciprocity
Calibration of Microphones
The reciprocity microphone calibration as prescribed
by the lEe 1094-2 standard requires voltage
ratio m.easurements for three combinations of microphone
pairs in a coupler. The conventional coupler
for microphone calibration makes it necessary
to remove the microphones and reinstall the microphone-coupler
arrangements. Depending on the design
of the coupler, the repeatability of the
mechanical positions of the microphones and coupler
introduces uncertainty. The three-rort method
for reciprocity microphone calibration, 2 illustrated
in Fig. 10, also eliminates the need to determine the
equivalent volumes for three microphone pairs.
Since the coupler encompasses the three microphones
simultaneously, and without the need of me-
Fig. 10. Three-port couplerfor reciprocity microphone
calibrations (R. Meldrum & S. Thwaites, CSIRO, Australia).
1998 December
NoiselNews International 213