I have a filter design problem. I have a 1.5Hz 18 db/octave Butterworth LC digital filter that is applied twice....once forward, and once backward. I then have a 0.5 Hz 6dB/oct analog filter that is applied on top of that. I want to confirm the overall filter being applied. My support tells me it should be 1.53Hz 42 db/oct. I completely understand the 42 db/oct on the slope. (18+18+6) However, I do not mathematically understand why it is 1.53 Hz and not SQRT(1.5^2+1.5^2+0.5^2) = 2.179 Hz. Everybody keeps trying to explain it with a graph of the 3 filters and how they move with respect to db/oct and I get that. What I want to see is the math. Can anybody walk me through the math on this one? If not, can you help me explain WHY math alone cannot provide an explanation?