Cutoff frequency

Full width at half maximum (FWHM). Answer: That is the point where the energy (power) is fallen to the value ½ or 0.5 = 50 percent of the initial power as energy quantity, that is equivalent to (−)3 dB = 10×log(0.5) .

The half-power or 3-dB bandwidth is the width of the range of positive frequencies where a peak value at zero or infinite frequency (low-pass and high-pass signals) or at a center frequency (bandpass signals) is attenuated to 0.707 the value at the peak.

The 3 dB bandwidth of an electronic filter or communication channel is the part of the system’s frequency response that lies within 3 dB of the response at its peak , which, in the passband filter case, is typically at or near its center frequency, and in the low-pass filter is at or near its cutoff frequency.