Envelope analyis basic question I have a vibration signal sampled at 25.6 kHz, and I want to perform Envelope Analysis on it. 

"Vibration-Based Condition Monitoring of Wind Turbines" by T. Barszcz has the following figure:




I understand why the frequency is changing in the FFT plot during the HP and LP filters, but why is the frequency range changing from (b) -> (c) [0 -> 8 kHz] when performing the abs (rectifier) step?
Cheers! 


Edit:
Here is an FFT for "Gearbox High Speed shaft" from my data.
A high-pass filtered signal has lost its low frequencies, so you can refocus the spectrum on a higher frequency range. 
When you rectify it, the first phenomenon is that the signal becomes positive. Hence, it is not zero-mean anymore, and thus recovers low-frequencies that were attenuated in the high-pass filtered signal.

A second is the presence of higher frequency terms. Typically, when a sinusoidal component has period $T$, its absolute value doubles its period and generates higher-order harmonics.