User Manual

Piezoelectric Polymer Speakers Application Note 1242138

1 Aug 98 Rev A

Page 79

ENCLOSURE

PIEZO FILM

STRETCH

Figure 1

INTRODUCTION

A loudspeaker is a transducer which transforms electrical signals into acoustic sound. Conventional speakers

usually consist of a vibrating surface (called a diaphragm) and a driving units which is, in most cases, a voice

coil driven by a magnetomotive force or a piezoelectric ceramic actuator. In piezoelectric polymer speakers,

the active driving element and the air-pushing diaphragm are combined into a single unit. Utilizing the

piezoelectric effect of polyvinylidene fluoride (PVDF) material and the flexibility of the polymer film, a unique

speaker can be formed from PVDF which vibrates under the stimulus of an AC electric field applied to the

surface electrodes on the film.

There are two principal types of loudspeakers. The first uses a vibrating surface to radiate sound directly into

the air. The second uses an acoustic element (a horn, e.g.) which is inserted between the diaphragm and the

air. The direct radiator type is used most often in small radios, residential, and automotive stereo systems.

The horn type is used in high fidelity systems, large sound systems for auditoriums and theaters, and outdoor

music and PA systems. Here we will only discuss piezo polymer speakers for direct radiating systems. The

uses and effects will be discussed in a separate application note.

SPEAKER STRUCTURE

An extremely crude speaker element can be created by simply hanging a sheet of piezo polymer film in air

and applying an AC electrical signal. Sound will emanate from the film and its audio characteristics will vary

as you flex and bend the film in different directions. Although this demonstrates basic operation, the

performance is marginal and unpredictable at best. By controlling the shape of the film and the support

structure of the mounting, a very acceptable speaker can be created.

Please refer to Figure 1. A simple speaker design consists of two parts. The first is the vibrating PVDF

diaphragm and the second is the speaker enclosure and support structure. Because PVDF film is anisotropic

in its surface piezoelectric parameters, it is important to orient the film properly in this design. The maximum

displacement per volt applied is along the machine direction (or “1” direction) of the film. As such, the highest

acoustic output is achieved when the machine direction is oriented in the length L direction. The width W is

perpendicular to the machine direction and the total height of the speaker consists of the enclosure depth H

and the film height H

The function of the enclosure is to isolate the front side of the film from the rear side. The film is curved in

one direction (the direction of highest piezoelectric activity) and clamped rigidly onto each end of the

enclosure. The curvature of the film is maintained by the sides of the enclosure. When a voltage is applied

to the film electrodes, it creates a mechanical strain in the film in both the normal direction and in-plane active

direction. Because the film is very thin, the strain in the normal direction is negligible. However, the larger

displacement caused by in-plane strains is converted

to radial motion along the radius of curvature and

perpendicular to the film surface.