I am looking to do some frequency response synthesis. I already have the bode plot and would like to get the transfer function of the plot. I know that a single pole zero give +/- 20dB/dec slopes and that a double pole can give 40dB change.

Where I'm stumped is how can I obtain a change of +/- 30 db/dec? Of course I could do an approximation of the bode plot and draw it such that i get to around 2x dB/dec and then use the a single pole/zero approximation, but I'd like to be as accurate as possible with my design. I did a quick google search, and there seems to be no obvious answer to this question.

tia