Microphone Technology 671of the center tap as well as exact resistance of the turns on either side of the center tap.
If this is not accomplished, direct current will exist in the transformer winding and the
signal circuit will no longer be exactly balanced.
Finally, a word of caution is in order. Sound systems may employ just a few or a very
large number of microphone cables not only for microphones but also for link circuits. It
is important to maintain correct signal polarity in all microphones, microphone cables,
link circuits, processing electronics, loudspeaker wiring, and loudspeakers. There are
convenient commercial devices called polarity checkers that can be employed to check
individual microphones, cables, and overall system polarity. An investment in such
devices is modest, time saving, and will earn its keep many times over.
22.6 Measurement Microphones ...................................................................................
A collection of measurement microphones, whether residing in sound level meters
or stand-alone devices, is an absolute necessity for sound system installers as well as
acoustical consultants. Such a collection must also be supported by an appropriate
microphone calibrator system that consists of both the calibrator itself and a set of
adapters to accommodate the various individual sizes of the microphones in the
collection.
For many years there were only two suppliers of quality measurement microphones:
Brüel and Kjaer, a Danish fi rm, and GenRad, a domestic fi rm. Brüel and Kjaer still exists,
although not under the original ownership, while GenRad no longer exists. Fortunately,
there are now several new domestic suppliers of quality measurement microphones.
Measurement microphones are dominantly air capacitor or electret capacitor microphones
while ceramic piezoelectric units may still be encountered. The standard sizes in terms
of capsule diameter are 1 inch, ½ inch, ¼ inch, and^1 ⁄ 8 inch. The larger units have higher
sensitivity and lower noise fl oors. The 1-inch unit is favored for making measurements
in quiet environments at frequencies below about 8 kHz. The ½-inch unit is a general
purpose one but has high frequency limitations.
Broad frequency band measurements usually require the ¼- or^1 ⁄ 8 -inch variety,
particularly if high sound levels are to be encountered. All sizes can have low-frequency
responses that extend almost to 0 Hz, with 3 to 5 Hz being typical with even lower values