Headphones 74125.3.6 High Polymer
High polymer is basically a generic name to cover piezoelectric plastics fi lms such as
polyvinylidene fl uoride fi lm.
Piezo-electric materials have been known for many years. They change their dimensions
when subjected to an electric fi eld or, conversely generate a voltage when subjected to
mechanical strain.
The ceramic barium titanate and crystals such as Rochelle salt and quartz are two
materials that have been used for many years in devices such as crystal phono cartridges,
ultrasonic transducers, and quartz oscillators but their stiffness is too high and mechanical
loss too low for wide-frequency-range audio applications.
High polymer fi lms, however, are very thin, some 8 to 300 μ m, and have very low
mechanical stiffness, which makes them ideal for transducer diaphragms. The basic fi lm
is made piezo electric by stretching it to up to four times its original length, depositing
aluminum on each side for electrodes, and polarizing with a high DC electric fi eld at
80–100ºC for about an hour.
When voltage is later applied across the fi lm, it vibrates in a transverse direction,
becoming alternately longer and shorter. If the material is shaped into an arc, this
lengthening and shortening are translated into a pulsating movement, which will generate
sound waves in sympathy with the electrical input signal.
It is a relatively simple matter to stretch the high polymer diaphragm across a piece of
polyurethane foam pressing against a suspension board to create an arc-shaped diaphragm
and make a very simple form of headphone.
High polymer transducers were fi rst developed by Pioneer. Advantages are claimed to
be a very low moving mass, similar to electrostatics but without the complexity, and, of
course, no power supply. The high polymer headphone is also claimed to be much more
sensitive than the electrostatic type and unaffected by humidity, which can reduce the
sensitivity of electrostatics. Harmonic distortion is also said to be very low at under 1%.
25.4 Basic Headphone Types ........................................................................................
Apart from the many different operating principles described earlier, there are two basic
categories into which headphones will fall, although some designs will include features of
both and will therefore not function purely as one type or the other.