Microphones 503The change in contact resistance causes the current from
the power supply to vary in amplitude, resulting in a
current waveform similar to the acoustic waveform
striking the diaphragm.
The impedance of the carbon button is low so a
step-up transformer is used to increase the impedance
and voltage output of the microphone and to eliminate
dc from the output circuit.16.3.2 Crystal and Ceramic MicrophonesCrystal and ceramic microphones were once popular
because they were inexpensive and their high-impedance
high-level output allowed them to be connected directly
to the input grid of a tube amplifier. They were most
popular in use with home tape recorders where micro-
phone cables were short and input impedances high.
Crystal and ceramic microphones^5 operate as
follows: piezoelectricity is “pressure electricity” and is
a property of certain crystals such as Rochelle salt, tour-
maline, barium titanate, and quartz. When pressure is
applied to these crystals, electricity is generated.
Present-day commercial materials such as ammonium
dihydrogen phosphate (ADP), lithium sulfate (LN),
dipotassium tartrate (DKT), potassium dihydrogen
phosphate (KDP), lead zirconate, and lead titanate
(PZT) have been developed for their piezoelectric quali-
ties. Ceramics do not have piezoelectric characteristics
in their original state, but the characteristics are intro-
duced in the materials by a polarizing process. In piezo-
electric ceramic materials the direction of the electrical
and mechanical axes depends on the direction of the
original dc polarizing potential. During polarization a
ceramic element experiences a permanent increase in
dimensions between the poling electrodes and a perma-
nent decrease in dimension parallel to the electrodes.
The crystal element can be cut as a bender element
that is only affected by a bending motion or as a twister
element that is only affected by a twisting motion,
Fig. 16-30.
The internal capacitance of a crystal microphone is
about 0.03μF for the diaphragm-actuated type and
0.0005–0.015μF for the sound-cell type.
The ceramic microphone operates like a crystal
microphone except it employs a barium titanate slab in
the form of a ceramic, giving it better temperature and
humidity characteristics.
Crystal and ceramic microphones normally have a
frequency response from 80 to 6500 Hz but can be
made to have a flat response to 16 kHz. Their output
impedance is about 100 k: , and they require aminimum load of 1–5 M: to produce a level of about
30 dB re 1 V/Pa.16.3.3 Dynamic MicrophonesThe dynamic microphone is also referred to as a pres-
sure or moving-coil microphone. It employs a small
diaphragm and a voice coil, moving in a permanent
magnetic field. Sound waves striking the surface of the
diaphragm cause the coil to move in the magnetic field,
generating a voltage proportional to the sound pressure
at the surface of the diaphragm.
In a dynamic pressure unit, Fig. 16-31, the magnet
and its associated parts (magnetic return, pole piece, and
pole plate) produce a concentrated magnetic flux of
approximately 10,000 G in the small gap.
The diaphragm, a key item in the performance of a
microphone, supports the voice coil centrally in the
magnetic gap, with only 0.006 inch clearance.
An omnidirectional diaphragm and voice-coil
assembly is shown in Fig. 16-32. The compliance
section has two hinge points with the section between
them made up of tangential corrugated triangular
sections that stiffen this portion and allow the
diaphragm to move in and out with a slight rotating
motion. The hinge points are designed to permit
high-compliance action. A spacer supports the moving
part of the diaphragm away from the top pole plate toFigure 16-30. Curvatures of bimorphs and multimorph.
Courtesy Clevite Corp., Piezoelectric Division.A. Crystal twister bimorph.B. Ceramic bender bimorph.C. Crystal bender bimorph.
Ceramic stripSilver electrodes
Through holes covered with graphite
D. Multimorph.