User Manual

Page 55
Figure 64. Audio speaker
elements can also be used for scanning of objects without physically moving the transducer. Each
element within the transducer can be activated sequentially, as with ultrasonic arrays.
AUDIO
Speakers
One of the earliest applications for piezo film was in stereo
tweeters (Figure 64) and headset speakers developed by
Pioneer Electronics. There is strong renewed interest in
these applications as a result of the improvements in the
reliability of the electrodes and lead attachment and
packaging techniques. Gallo Acoustics has developed a high
fidelity omnidirectional tweeter using a cylinder of 52 um
thick piezo film. The tweeter rolls off at frequencies below 2
KHz, and features:
• 330 degrees of horizontal dispersion at high frequencies,
which is as much as ten times the dispersion of
conventional tweeters,
• very wide dynamic range,
• linear frequency response,
• very fast impulse response, faithfully reproducing the highest frequencies.
Novelty audio speakers have also been developed. These devices make use of the thin, light weight,
conformal nature of the piezo film. Examples include speakers for inflatables (like balloons and air
inflatable toys), speakers in apparel (including headgear) and paper thin speakers for magazine
advertising, greeting cards and posters.
Microphones
A diaphragm of piezo film, affixed in a retaining ring or mounted over a hole in a plate, makes an
excellent microphone. Vacuum formed domes on a support can be introduced into the design to take
the membrane slightly out of its neutral axis with a foam backing, a small post, bar or structure to give
the film membrane a slight radius of curvature. A self-supporting, cylindrically curved film also achieves
the mechanical bias. A typical radius of curvature for piezo film microphones which optimizes
sensitivity and electroacoustic efficiency is R
o
= 25 mm, similar to that of an electrostatic microphone
construction.
Sennheiser reports a frequency response for a typical foam backed piezo film microphone of 25 mm
diameter, having R
o
= 25 mm. The free field sensitivity of the device measured at 1 KHz, for sound
pressure incident on the membrane perpendicularly, was -58 dB re 1 V/Pa. Harmonic distortion
approaches 1% only at sound pressure levels exceeding 122 dB, and are not significantly higher for the
range of higher frequencies.
Microphones built with piezo film are low cost, but more importantly, are inherently immune to
moisture, unlike electrostatic types.
Electrostatics dominate the market due to the low cost that has been achieved through very high
volume manufacturing. Nonetheless, piezo film microphones are finding application in designs where