Exceptions to the Standard
These last two microphones operate on slightly different principles than the first two. One is a development of the condenser microphone, and the other uses modern piezoelectric materials to effect the conversion.
This is the crystal
microphone. It uses a diaphragm similar
to those in the dynamic microphone, but instead of magnets it places the
pressure on a piezoelectric crystal.
Piezoelectric materials acquire a charge when deformed, and this is used
as the recording charge. Although a
clever use of modern materials, crystal microphones tend to be slightly
fragile, and their reliance on a diaphragm gives them the same limitations on
frequency response as a dynamic microphone.
As a result, these have never been solid competitors for dynamic
microphones except in cases where a permanent magnet is crippling.
This model is the electret microphone. It is based on the technique from condenser microphones, but uses a permanently charged material instead of a battery to provide the charge. Although this is an excellent adaptation and requires much less than the condenser microphones, they tend to have a more limited range than their cousins. However, their extreme ease of production has caused them to be the microphones of choice on small appliances, like walkmans or portable stereos.