The Schumann resonance frequencies are a set of eigenmodes in the extremely low frequency (ELF) portion of the electromagnetic field spectrum. These frequencies are only present for planetary-like bodies with a substantial ionosphere acting as a perf

超声波发生器内部结构有两个压电晶片和一个共振板。当它的两极外加脉冲信号，其频率等于压电晶片的固有振荡频时，压电晶片将会发生共振，并带动共振板振动，便产生超声波。-Ultrasonic generator internal structure has two piezoelectric chip and a sounding board. When it is applied bipolar pulse signal having a frequency equal to the natural