The sound producing principle of the piezo electric elements is as follows

The sound source of the piezoelectric element mainly comes from the piezoelectric vibration plate. The piezoelectric vibration plate consists of a piezoelectric ceramic plate with electrodes printed on both sides and a metal plate (brass or stainless steel, etc.). Using adhesive, the piezoelectric vibration plate and the metal sheet are bonded together, which is commonly known as piezo components.

Figure 2 shows an oscillating system of a piezoelectric vibrating plate. When a DC voltage is applied between the two electrodes of a piezoelectric vibration plate, mechanical deformation occurs due to the piezoelectric effect. For piezoelectric elements with distorted shapes, their deformation extends in the radiation direction.

The piezoelectric vibration plate is bent in the direction shown in Figure 2 (a), while the metal sheet bonded to the piezoelectric vibration plate does not extend. On the contrary, when the piezo electric elements contracts, the piezoelectric vibration plate will bend in the direction shown in Figure 2 (b). Therefore, when AC voltage passes through the electrode, as shown in Figure 2 (c), the bending shown in Figure 2 (a) and Figure 2 (b) will alternately repeat, generating sound waves in the air.

Generally speaking, the sound frequency range of a person is between 20 Hz and 20 kHz. The most easily heard audio frequency is from 2 kHz to 4 kHz. Therefore, the vast majority of ultrasonic bender are used in this audio frequency range. At the same time, the resonant frequency (f0) of the buzzer is generally selected within the same range. As shown in Figure 3, the resonant frequency depends on the method used to support the piezoelectric vibration plate. If the shape of the piezo electric elements is the same, its values will decrease in the order of (a), (b), and (c).

However, the sound production effect of exposed piezoelectric ceramic sheets is relatively poor, so there is a need for a sound cavity, which is also known as a resonant cavity. The piezoelectric vibration plate is installed in the resonant cavity to generate high sound pressure (as shown in Figure 4). Using formula (1) (Helmholtz formula), the resonant frequency (fcav) of the resonant chamber in Figure 4 can be calculated. Because piezoelectric vibration plates and resonant chambers have appropriate resonant frequencies, namely (f0) and (fcav), it is possible to increase the sound pressure at a specific frequency and obtain a specific bandwidth by controlling their positions.

Drive circuit of piezo electric elements

Piezo buzzer is a generating device that encapsulates a buzzer in a cavity, divided into active and passive components,

The difference between active and passive drivers is that the driver does not need to generate pulses at an output frequency. Active drivers can emit sound directly at high and low levels, while passive drivers need to output waveforms at a certain frequency to generate sound. Both can be compatible and common on the circuit

Working principle of piezoelectric ceramic element

There are two driving methods for piezo electric elements , namely external driving method and self driving method, as shown in the following figure:

The external driving method uses electrical signals from external oscillation circuits such as a multi frequency oscillator to drive a piezoelectric vibration plate to generate sound. Using this method, the piezo electric elements acts like a speaker. By appropriately suppressing the mechanical oscillation Qm of the vibrating plate of the piezoelectric buzzer, a wider sound pressure frequency band is obtained. This method is usually applied to the switches of various household appliances, the keying sound of office automation equipment, and the alarm sound of digital electronic meters, all using composite sounds. In addition to these, there are also electric bells, telephone receivers, transmitters, card radios, and LCD TV speakers that are used.

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