The natural oscillation rate of a coil's circuit after each pulse fires, similar to how a struck bell rings at its own pitch. For bipolar damped-sine devices like large loop coils driven by capacitor discharge, this ringing appears as a fast tone inside each burst. It's separate from the slower pulse repetition rate. Set by the coil's inductance and parasitic capacitance (f equals 1 divided by 2 pi times the square root of L times C). Typically a few kHz for PEMF loop coils. Reported alongside Frequency in per-setting parameter tables when the waveform is a damped sine.
A real field pulse over time. The height of the pulse is its amplitude, how strong the field gets, separate from how often the pulse repeats. After the pulse the coil rings, a fast shrinking oscillation at the coil's natural frequency; the time from one ring peak to the next is one cycle of that ring frequency. A coil driven at its natural frequency takes up energy most efficiently, which is resonance.
