Research&Experiment On Vibration And Sound Radiation For Diaphragm Of Thin Piezoelectric Ceramic Micro-Loudspeaker

The rapid development of ultra-thin electronic products required the components become thinner and thinner, and better performance inevitably. However, the traditional cone loudspeakers had been unable to adapt the development of electronic products. So the advantages and disadvantages of digital loudspeakers, miniature inductor loudspeaker and piezoelectric ceramic loudspeaker were discussed in this paper, and proved that the piezoelectric loudspeakers have the best developing prospect in thin/miniature loudspeakers application field. But the existing piezoelectric ceramic loudspeakers have some defects, such as bad low-frequency response, non-smooth of the frequency response curves, as well as harmonic distortion. Therefore the theoretic analysis and experiments on the acoustic relationship between the material properties of diaphragm and loudspeaker system were carried out and show in this paper, to explore the method aimed at improving the acoustic performance of piezoelectric ceramic loudspeakers.The mathematical models of diaphragm of traditional loudspeakers, thin plate diaphragm and membrane diaphragm were established in this paper from the basic physical acoustic properties and sound waves equation, and analyzed the radiation characteristics and directivity of sound field of the diaphragm in different working conditions. According to the analysis results, as well as considering the different requirement of diaphragm material in loudspeaker system, the nickel steel thin plate diaphragm with high elastic modulus, the aluminum thin plate diaphragm with low material density, and the aluminum honeycomb sandwich plate diaphragm with high elastic modulus-density ratio were prepared in this paper. And calculated the eigen-frequency matrixes of different diaphragms based on the theoretic analysis.According the national standard GB/T12060on acoustic measurement environment and conditions, a set of micro-loudspeaker measurement system was built based on MATLAB software. The testing accuracy of the micro-loudspeaker measurement system was verified in the paper by compared with the Soundcheck standard acoustic measurement system. And the diaphragms were tested preliminarily using the MATLAB acoustic measurement system.The conclusions were obtained through the measurement results on frequency response characteristics of loudspeakers with different diaphragms, and indicated that the lowest response cutoff frequency of loudspeaker system were determined mainly by the lowest eigen-frequency of diaphragm, however the eigen-frequency of the diaphragm was determined by dimensions of the diaphragm, the boundary condition, the material elastic modulus and material density. The smooth of the frequency response curve was relevant to the eigen-frequency modal density of the diaphragms when the loudspeaker system worked in eigen-frequency mode, the higher modal density a diaphragm had, the smoother the frequency response curve became. The diaphragm had no enough modal density until the vibration frequency of the loudspeaker is twice to lowest eigen-frequency. But generally the lowest response cutoff frequency of the loudspeaker was lower than the lowest eigen-frequency of the diaphragm, due to the influence of the acoustic impedance came from the reaction of the sound field and extra weight.The conclusions were obtained through the measurement results on harmonic distortion characteristics of loudspeakers with different diaphragms, and indicated that the harmonic distortion of the loudspeaker system was determined by the response non-linear of the piezoelectric ceramic vibrator. The harmonic distortion of the loudspeaker system in low-frequency band was determined by the coupling characteristic among diaphragm material and piezoelectric ceramic vibrator, the viscidity characteristic of the glue layer, the uniformity of the diaphragm material. The harmonic distortion of the loudspeaker system in high-frequency band was determined the variety of the eigen-frequency modal of the diaphragm, the more simple the eigen-frequency modal of the diaphragm was, the lower the harmonic distortion of the loudspeaker system was.