Application of ceramic speaker in mobile phone design

Today's portable devices require smaller, thinner, more power electronic components. For the design of small mobile phone, loudspeakers, become a manufacturer can produce constraints of ultra-thin mobile phone. In promoting this demand, ceramic or piezoelectric loudspeaker is rising rapidly, become the alternative of moving coil loudspeaker. Ceramic loudspeaker with a slim and compact package provides very competitive sound pressure level (SPL), has replaced the huge potential of the traditional moving coil loudspeaker. The difference between loudspeakers and ceramic speakers are shown in table 1.

Ceramic loudspeaker amplifier circuit drives with output and different driving the traditional moving coil loudspeaker drive requirements. The structure of the ceramic speaker requires the amplifier to drive large capacitive loads, and at higher frequencies the output of a larger current while maintaining high output voltage.

Characteristics of ceramic speakers

Ceramic loudspeaker production process and multi-layer ceramic capacitor is similar to that of, compared with a moving coil type loudspeaker. The manufacturing technology and can make the loudspeaker manufacturers more strictly control the speaker's tolerance. Strict tolerance control is very important for the choice of the speaker, and it also affects the repeatability of the audio characteristics of different production batches.

The equivalent impedance of the ceramic speaker in the driving amplifier can be approximated as a RLC circuit consisting mainly of a large capacitor. In the audio frequency range, the ceramic speakers are usually present in the capacity. The characteristics of the speaker's capacitance determine that the impedance decreases with the increase of the frequency. Figure 2: the relationship between the impedance of the ceramic speaker with the frequency, and the 1micro; F. The impedance has a resonance point, and the acoustic efficiency of the speaker is the highest at this frequency point. The decrease of the impedance curve near the 1kHz frequency indicates the resonant frequency of the speaker.

Relationship between sound pressure and frequency and amplitude

The AC voltage at both ends of the ceramic speaker causes the deformation and vibration of the piezoelectric thin film in the speaker. The displacement is proportional to the amplitude of the input signal. The vibration of the piezoelectric thin film causes the air flow around, thereby making a sound. When the speaker voltage is raised, the deformation of the piezoelectric element is increased, and the sound pressure is formed, thereby increasing the volume.

Ceramic speaker manufacturers typically provide the speaker's maximum driving voltage, the typical value of 15VP-P. Voltage maximum when the offset of the ceramic device reaches its limit. When the applied voltage is greater than the rated voltage, the sound pressure will not increase, but the distortion degree of the output signal is increased. Figure 3 for the maximum voltage when the ceramic speaker output sound pressure (SPL) and the frequency of the relationship curve. By comparing the relationship curves between SPL and frequency, and the relationship curve between impedance and frequency, it can be seen that the efficiency of the high SPL is the highest in the frequency of self excitation.