A typical drive circuit of a piezoelectric sounder is a circuit driven directly by an IC.
Sound is produced by driving the piezoelectric sounder in accordance with the signals input from the IC. Accordingly, the frequency at which sound is produced and the SPL change with the frequency and voltage of the signals input from the IC. Fig. 1 shows an example of a circuit in which the sounder is driven directly by an IC.
Fig. 2 shows an example of circuit that jointly uses a transistor and power supply with the circuit in Fig. 1.
The SPL can be raised by increasing the voltage input to the piezoelectric sounder. Accordingly, if you can separately prepare a power supply voltage higher than the voltage that can be input from the IC, you can obtain an even higher SPL by jointly using a transistor.
There is a way to input signals to each terminal of a piezoelectric sounder to raise the SPL.
Fig. 3 shows an example of a Bridge Tied Load (BTL) circuit that inputs positive-phase and reverse-phase signals from an IC to each terminal of the piezoelectric sounder. As shown in the figure, it is possible to expect an SPL twice (+6 dB) that of the circuit in Fig. 1 by inputting a positive-phase signal from one of the terminals of the IC and a reverse-phase signal from the other terminal into the piezoelectric sounder.
Fig. 4 shows an example of a circuit that jointly uses a transistor and power supply with the circuit in Fig. 3.
In this case as well, if you can separately prepare a power supply voltage higher than the voltage that can be input from the IC, the circuit that jointly uses both a transistor and power supply (Fig. 4) can obtain a higher SPL than the way of making the sounder produce sound with the drive voltage of an IC (Fig. 3)