压电陶瓷蜂鸣片在仪器结构上共振腔的设计(英).pdfVIP

DESIGN OF A HELMHOLTZ RESONATING CASE FOR A PIEZO BENDER Overview When a piezoelectric element is supported in an Edge or Nodal mode, and has no case or tuned enclosure, the resulting sound pressure level (SPL) produced is very low. This is because the acoustical impedance of the piezoelectric element does not match that of any open air loading. However, by constructing a HELMHOLTZ resonating case and by using proper mounting techniques, the acoustical impedance of the piezo element and the encased air can be more closely matched to that of open air. Mounting The mode of mounting influences the resonant frequency, impedance, bandwidth and resulting sound pressure level. Mounting a piezoelectric bender at its nodal circle results in lowest bender impedance, highest resonating frequency, narrow bandwidth and highest sound pressure level. Mounting a piezoelectric bender at its edge results in higher bender impedance, lower resonating frequency, broader bandwidth and lower sound pressure level. Resonator The highest sound pressure levels are obtained when the piezoelectric element excites a resonator with a resonant frequency equal to the resonant frequency of the piezoelectric element regardless of the mounting method chosen. The following equation can be used to design such a resonator, known as a Helmholtz Resonator. Helmholtz Resonator Where: fo = Resonant frequency of Helmholtz cavity in Hz C = Constant - Velocity of sound @ 344 m/sec @ 24°C h = Resonator cavity in height in meters D = Resonator cavity (support) diameter in meters d = Sound emitting hole diameter in meters t = Sound emitting hole length in meters K = Constant - @ 1.5 N = Number of sound emitting holes p = Constant - @ 3.14 2p = Constant - @ 6.28 a = Constant - @ 4.0 fB = Bender vibrating plate diameter in inches fA = 0.65 (fB) Bender nodal mount diameter in inches fn = Bender nodal mounted resonant frequ