Consider a single real zero in the left half of the s-plane. This will have the form (s/z+1) ==> (jw/z+1) in time constant form. At low frequencies (w near 0), the magnitude of this term is approximately 1 ==> 0 db. At high frequencies (w/z >> 1), the magnitude of this term is approximately |w/z| ==> 20*log_10(|w/z|) db. At frequency w=w1, the magnitude is |w1/z|, and at frequency w=10*w1, the magnitude is 10*|w1/z|. In decibels, this is an increase by 20*log_10(10) which is 20 db. Therefore, at high frequencies, the magnitude curve for this simple zero will increase by 20 db every time the frequency increases by a factor of 10. When frequency is plotted on a log scale, the curve will be a straight line with a slope of +20 db/decade (decade of frequency = change of frequency by a factor of 10). At low frequencies, the magnitude will be approximately 0 db. The transition between low and high frequencies is at w=z r/s.