Resonance mode and sound pressure produced by circular diaphragms of electrostatic and piezoelectric speakers

To investigate the frequency response characteristics of a push-pull electrostatic speaker (60-mm diameter) and free-edge-like piezoelectric speakers (68-mm and 73-mm diameters), this study was employed optical and acoustic measurements from 20 Hz to 20 kHz. The optical measurements of displacement amplitude and mode shape were used to predict sound pressure levels (SPLs). Comparisons with measured SPL values were used to verify the predictions. When using a mesh with acoustic flow resistance, i.e., unless damped, the electrostatic and piezoelectric speakers both produced numerous resonant frequencies. The two evaluation methods produced SPL values that were in good agreement. The piezoelectric speakers produced jagged SPL curves with peaks steeper than those of the electrostatic speaker. At the first axisymmetric mode, the electrostatic speaker was affected by acoustic resistance, which resulted in the following quality factor (Q) values: without mesh (Q = 6.16) and using a mesh with specific acoustic resistance of 145 rayl (Q = 1.74). By contrast, the piezoelectric speakers at modes (0, 1)-(0, 4) were unaffected by acoustic resistance, which resulted in Q values of 10-11, regardless of whether mesh was applied. Because higher Q performs larger acoustic response and lower Q presents wide broadband, this study concluded that the characteristics make electrostatic speakers suitable for headphones and piezoelectric speakers suitable for audio signaling devices.