User Manual

Piezoelectric Polymer Speakers Application Note 1242138
Page 80
SPL  20 log
10
P
P
ref
(dB)
f
J
21
R(cm)
kHz

53
R(inch)
kHz
Because of the curvature, the film surface basically moves in and out of the enclosure. The large film area
effectively couples acoustic energy into the surrounding air. This speaker design has very good high
frequency characteristics. The acoustic output is constant for frequencies up to 20 kHz.
Sound pressure levels at a specified distance, and the frequency response curves of a piezoelectric polymer
speaker can be analytically calculated by solving the wave equation for a curved plate in conjunction with the
electro-mechanical equations of piezoelectricity.
The calculations involve the size and shape of the film element, amplitude of the driving signal, piezoelectric
properties and dielectric and mechanical loss properties of PVDF film materials, radiation impedance effects,
and the mass loading effects of the electrode materials, among other parameters. MSI has developed a
computer program to obtain numerical results from a wide variety of parameters. Because of the
complexities, the details of these calculations will not be presented here. Discussed below, however, are the
effects of key structure parameters on the acoustic performance of a typical speaker design.
DESIGN PARAMETERS
PVDF FILM THICKNESS - Piezoelectric film is available in various thickness’. Standard thickness’ from MSI
are 28 µM, 52 µM, and 110 µM. Other thickness’ are available on a special order basis. The sound pressure
level produced by a piezo polymer speaker at a certain distance is directly proportional to the applied electric
field strength. Therefore, for a given voltage, in principle, the thinner the film, the higher the sound pressure
produced. Sound Pressure Level (SPL) is usually used to measure the output performance of a speaker and
is defined by the following equation:
P is sound pressure at a certain distance
from the speaker and P
ref
= 2 x 10
-5
Pascal. Reducing the film thickness by half will generate a 6 dB increase
in SPL. However, reducing film thickness will increase the capacitance of the speaker element, resulting in a
higher current draw from the power amplifier (this will be discussed later). The use of thinner films can also
cause deformation of the film curvature especially for speakers requiring large area of PVDF film. This
deformation of curvature can cause irregularities in the frequency response curve.
CURVATURE OF PIEZOELECTRIC POLYMER FILM - The curvature radius of a film speaker is an important
parameter for determining the frequency range of the speaker. For a rough estimation, the low frequency
cutoff of a speaker is proportional to the square root of the ratio of Young’s modulus of PVDF film to its
density, and inversely proportional to the radius of curvature. Substituting the material property parameters,
this low frequency can be estimated by the following equation:
R is the radius of curvature and f is the low frequency corner. See Figure 2. Note that the above equation
does not take the mass loading effect of the electrodes into consideration. When the PVDF film is thin and
heavy electrode materials such as silver ink are used, the above equation should be modified.