I'm reading a book (see figure 5-12) about (among other things) pole zero plots and there is an example I don't understand. For the schematics and formula below:
it adds the following zero pole plots:
and says:
The individual pole zero plots show the dc gain of 1/2 plotting as a straight line from the –6 dB intercept. The two zeros occur at the same break frequency, thus they add to a 40-dB/decade slope. The two poles are plotted at their breakpoints of ω = 0.44/τ and ω = 4.56/τ. The combined amplitude plot intercepts the amplitude axis at –6 dB because of the dc gain, and then breaks down at the first pole.
I understand why the breakpoints are located at the mentioned points and why the slopes go up/down where they do. However, I don't understand why the individual plot starts at 0 dB while the combined one starts at -6 dB.
I have noticed this:
[...] The combined amplitude plot intercepts the amplitude axis at –6 dB because of the dc gain, and then breaks down at the first pole.
First, I don't see why the dc gain is -6 dB for all \$\omega < \frac{0.44}\tau \$. Second, I don't see why this dc gain is applied only to the combined plot and not to the individual one.