Report Cutoff frequency and 3dB points in Theory articles Posted October 25, 2006 What is the logic behind the cutoff frequencies being defined at the 3dB points?why do we use the 3dB points only. is there some thing to do with transient time?these are some things we all study in our courses but what's the basis of these concepts. any one who can explain these basic concepts which we use everyday. ???This is a method of calculating bandwidth. The bandwidth is the the difference between the upper and lower 3db points.