User Manual

Piezoelectric Polymer Speakers Application Note 1242138
Page 82
Figure 4a Figure 4b
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LENGTH AND WIDTH OF SPEAKER FILMS - The acoustic output of a PVDF speaker is directly related to
the length and width dimensions which are, in turn, determined by each different application. The sound
pressure generated by a speaker at a certain distance is linearly proportional to the active area of PVDF film.
Therefore, SPL is a logarithmic function of the PVDF film dimensions if the speaker area is rectangular.
Figures 4a & 4b show the relation between SPL and film length and width respectively at 10 kHz.
POWER CONSIDERATIONS
In order to obtain a significant displacement from PVDF film to radiate sound, a high voltage is usually
required. A step-up audio transformer can be used to increase the output voltage of an audio amplifier. The
designer should give consideration to the electrical properties of the PVDF film speaker and the capabilities of
the audio amplifier when selecting or designing the transformer. The input/output voltage ratio is directly
proportional to the primary/secondary turns ratio of a transformer:
The primary/secondary impedance ratio is proportional to the square of the turns ratio. The transformer must
have an output impedance that is equal to or less than the PVDF film impedance at the highest specified
frequency. If these impedances are not matched, significant distortion may result.
Conventional speakers present a resistive load to the power amplifier output while a PVDF film speaker
presents a capacitive load to the output. Because of this the piezo film speaker impedance changes with
frequency. For a 28 µm thick film with an area of 10 cm X 10 cm, the typical capacitance is about 35 nF. In
the high frequency audio range, the impedance of such a speaker is quite low resulting in a high current draw
from the amplifier. This current is about 90 degrees out of phase with the voltage applied to the PVDF film.
With the high applied voltage and large current at higher frequencies, the apparent power, defined as the
transformer output current times the voltage applied to the PVDF film, is somewhat higher when compared to
conventional speakers. This power issue needs to be considered when choosing amplifiers and transformers.
Because of the capacitive nature of a PVDF film speaker, an electrical resonance is introduced when using a
transformer. This resonance should be avoided within the frequency range of interest or carefully damped.
PRACTICAL DESIGN TIPS FOR SPEAKERS
A reasonably good tweeter can be made using the basic design shown in Figure 1. Keeping the film smooth
and wrinkle free will reduce distortion and provide a flat frequency response. The support should also be as
rigid as possible to maximize the film surface displacement.