MODES OF VIBRATION AND SUPPORTING METHODS FOR THE SOUND ELEMENT Three principal modes of vibration can be created in the element depending on the style of mounting. This is illustrated in Fig. 2MOUNTING(1) Node Support The sound element shown in Fig. 2(a) is node mounted, allowing it to vibrate in a free state. The node, a circumference where no vibration takes place, is created as shown by the broken line in Fig. 1. Mounting at the node causes the least mechanical suppression of vibration, thus allowing the greatest amplitude. Hence this mounting method, as illustrated in Figure 5(a),gives the highest sound pressure output and the most stable oscillation frequency of the three choices. As a result, this is the most appropriate design for high output, self-drive applications.(2) Edge Support Fig. 2(b) shows the mode of vibration when the sound element is supported at the edges. In this mounting configuration, the whole sound plate vibrates up and down as is illustrated by the broken line in the diagram. Hence, the edge method as illustrated in Fig. 5(b), suppresses the fundamental resonant frequency by moving the node. This offers the possibility of a wide frequency response, and is most advantageously used with external drive.(3) Center Support Fig. 2(c) shows the mode of vibration when the sound element is supported at the center. As the main vibration area is forcefully supported, large sound pressure levels are not possible when this method is used. This too is appropriate for external drive but due to design difficulties center support is not useful as an alarm.CIRCUIT DESIGN CONSIERATIONS1. Driving Wave
The piezo elements may be driven by either sinusoidal, pulsed, or square wave, depending upon the particular application. If a sine wave is used, the device will operate at a frequency lower than the resonant frequency (Fo) with a lower sound pressure level. The reason for this is the loss of energy, through the time lag between peak deflections as shown in Fig. 7. It is important that a clean sinusoidal signal be provided, as any clipping of the waveform can result in frequency instability. If square waves or pulsed waves are used to drive the elements, a higher acoustic output will be realized, along with an increase in harmonic levels. A parallel capacitor can reduce these harmonics.2. Driving Frequency
For maximum output, a frequency of between 500Hz and 4KHZ should be used, as recommended by the specific part chosen.
3. DC Precaution
In order to prevent depolarization of the ceramic elements it is necessary that every precaution be taken to prevent them from being subjected to direct current.
4. High Voltage Precautions
Voltage higher than those recommended by specification can damaged the ceramic, even if applied for short durations. Due to the strength of the piezoelectric effect, high voltage can cause the crystals to break the sintered bonds, resulting in permanent damage. Significantly higher sound pressure levels will not be achieved by voltages higher than those recommended by specification.
